[CANCER RESEARCH (SUPPL.) 42, 3327s-3333s, August1982]
0008-5472/82/0042-0000$02.00
A New Hypothesis Based on Suicide Substrate Inhibitor Studies for the
Mechanism of Action of Aromatase1
Douglas
F. Covey2 and William
Department of Pharmacology,
F. Hood
Washington University School of Medicine, St. Louis, Missouri
Abstract
Recently, it was discovered that 4-hydroxy-4-androstene3,17-dione, 4-androstene-3,6,17-trione,
and 1,4,6-androstatriene-3,17-dione,
compounds previously reported to be com
petitive inhibitors of aromatase, cause a time-dependent loss
of aromatase activity in human placental microsomes. We
report here that 1,4-androstadiene-3,17-dione
(K¡0.32 fiM,
/W) 0.91 x 10~3/sec) and testolactone (K¡35 ¿UM;
kmac,0.36
x 10~3/sec) also cause a similar loss of aromatase activity.
The mechanism which explains the unexpected loss of activity
caused by these five inhibitors is neither established nor ap
parent from current theories of the enzyme mechanism of
action of aromatase. We propose an inactivation mechanism
based on a new hypothesis for estrogen biosynthesis in which
the third enzyme oxidation carried out by aromatase results in
the formation of an enzyme-bound intermediate. This interme
diate is released as an aromatized product via a facile elimi
nation reaction which simultaneously regenerates the unaltered
active enzyme. Various structural modifications made in these
five inhibitors are hypothesized to redirect this elimination
reaction so that the steroid intermediate remains covalently
attached to the enzyme instead of being released as an aro
matized product.
Introduction
Chemically unreactive substrates for an enzyme which are
converted into chemically reactive irreversible inhibitors at the
active site of an enzyme when it carries out its normal catalytic
function are termed "suicide substrates." The rational design
of these inhibitors requires a knowledge of both the substrate
specificity of the target enzyme and its mechanism of action.
Since in the case of aromatase considerable information had
been accumulated in both these areas, we thought that it might
be possible to rationally design a suicide substrate for this
physiologically important enzyme.
Based on the widely accepted conversion of 4-androstene3,17-dione to 3,17-dioxoandrost-4-en-19-al
via the initial 2
successive hydroxylations of the C-19 methyl group (4), we
rationalized that the unreactive steroid RED3 could be enzymatically
converted
into the reactive conjugated
acetylenic
' Presented at the Conference 'Aromatase:
New Perspectives for Breast
Cancer," December 6 to 9, 1981. Key Biscayne, Fla. This research was sup
ported in part by Grant CA-23582 awarded by the National Cancer Institute,
Department of Health and Human Services.
1 To whom requests for reprints should be addressed.
3 The abbreviations and trivial names used are: PED, 10-propargylestr-4-ene3,17-dione; 4-OH-A, 4-hydroxy-4-androstene-3.17-dione;
AT, 4-androstene-3,6,17-trione; ATD, 1,4,6-androstatriene-3,17-dione;
1,4-ADD. 1,4-androstadiene3,17-dione; 4,6-ADD, 4.6-androstadiene-3,17-dione;
testolactone, o-homo-17aoxaandrosta-1,4-diene-3,17-dione;
DHT, 5o-androstan-17/!-ol-3-one;
5a-AD,
5a-androstane-3,17-dione;
testololactone. o-homo-17a-oxaandrost-4-ene-3,17dione.
AUGUST
1982
63110
ketone
10-(1-oxo-2-propynyl)-estr-4-ene-3,17-dione
(10)
which would then covalently modify and inactivate the enzyme
(Chart 1). We were gratified to find that PED is indeed a potent
suicide substrate of aromatase (9), and we are continuing to
study PED as well as attempting to increase its potency through
further chemical modification of the primary PED structure.
Other investigators also have prepared PED as well as addi
tional C-19-substituted analogs of 4-androstene-3,17-dione
for
evaluation as suicide substrates of aromatase (17, 19). For all
these compounds, the potential enzyme-generated
reactive
species is postulated to result from enzymatic oxidation of
either the substituted C-19 carbon atom or the substituent
attached to this atom.
It is now clear, however, that the introduction of a nonreactive
C-19 substituent, which is activated by the previously dis
cussed mechanism, is only one way to design suicide sub
strates for aromatase. We have found, for example, that 4-OHA, AT, and ATD, compounds which contain an unaltered C-19
methyl group and which were previously shown to be compet
itive inhibitors of human placental aromatase (3, 21), are sui
cide substrates of this enzyme (6, 8). It is obvious that the 4hydroxy group of 4-OH-A and the 6-keto group of AT are
necessarily involved in the observed inactivation process since
these are the single structural modifications which distinguish
each of these steroids from 4-androstene-3,17-dione.
For ATD,
however, this inhibitor differs from 4-androstene-3,17-dione
by
the presence of 2 additional double bonds, and the possibility
existed that only one of these double bonds was causally
related to the ability of the inhibitor to decrease aromatase
activity. We report here results which demonstrate that only the
additional C-1 ,C-2 double bond is required for this process.
Thus, 1,4-ADD but not 4,6-ADD causes a time-dependent loss
of aromatase activity. In addition, testolactone, another steroid
containing a 3-keto-1,4-diene system, also was found to dis
play the kinetic profile expected for a suicide substrate. Finally,
we propose a new mechanism for estrogen biosynthesis which
may explain why these steroids should be aromatase suicide
substrates.
Materials and Methods
Materials. The [l^HH-androstene-S,!
7-dione (46.1 Ci/mmol)
was purchased from New England Nuclear, Boston, Mass. The 4androstene-3,17-dione,
DHT, 5a-AD, 1,4-ADD, and 4,6-ADD were
purchased from Steraloids, Inc., Wilton, N. H. Testolactone and testo
lolactone were gifts from E. R. Squibb and Sons, Inc., Princeton, N. J.
All steroids used were chromatographically
pure. NADPH was pur
chased from Sigma Chemical Co., St. Louis, Mo. Protein determinations
were done with a Bio-Rad Protein Assay Kit 1 purchased from Bio-Rad
Laboratories, Richmond, Calif. Liquid scintillation counting of tritium
was done in ScintiVerse purchased from Fisher Scientific Co., Pitts
burgh, Pa.
Enzyme Methods. Human placental microsomes were prepared by
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D. F. Covey and W. F. Hood
for protein determination. The remaining resuspended microsomes (1.5
ml) were spun again at 100,000 x g for 60 min at 4°.The supernatants
were discarded while the pellets (P2) were resuspended in buffer (1.5
ml), and portions (0.5 ml each) were taken for aromatase assay and
protein determination.
Results
Chart 1. A proposed mechanism for the inactivation of aromatase by RED. An
alternate mechanism has been proposed by Metcalf et al. (19).
our previously described method (9). The specific activities of the 2
preparations used were 190 and 370 pmol/min/mg
protein. Timecourse experiments were done in a shaking water bath at 37° in air
with initial incubations (3.0 ml) which contained 100 mM KCI, 10 mw
potassium phosphate buffer (pH 7.5), 1 mM EDTA, 100 ¡IMNADPH
(omitted in appropriate controls), propylene glycol (1 drop per 0.5 ml),
and microsomes (0.039 to 0.083 mg protein per 0.5 ml). At various
times after the addition of inhibitors dissolved in ethanol (0.01 ml/0.5
ml incubation volume), aliquots (0.5 ml) were removed and added to
assay tubes containing [1,2-3H]-4-androstene-3,17-dione
(0.5 nmol;
—¿700,000cpm), and remaining enzyme activity was determined by the
method of Thompson and Siiteri (23). After 5 min, the assay reactions
were stopped by the addition of chloroform (5 ml) and vortexing ~40
sec. After centrifugation at 1470 x g for 5 min, aliquots (0.10 ml) were
removed from the water phase and added to scintillation mixture (10
ml) for determination of 3H¡>O
production. Control experiments dem
onstrated that: (a) the concentration of [1,2-3H]-4-androstene-3,17dione [1 ;IM. which is -22-fold
greater than the K, of this substrate
(14)] added for determination of remaining enzyme activity was suffi
cient to protect the enzyme from further significant loss of activity while
rates of product formation were measured; (t>) complete aromatization
of the radiolabeled substrate released 60% of the tritium into water;
and, (c) the assay was linear over the 5-min assay in either the
presence or absence of inhibitor.
Competition experiments were carried out at 37°in 0.5-ml incubation
volumes containing [1,2-3H]-4-androstene-3,17-dione
(6 different con
centrations from 0.01 to 1 /IM) alone or with inhibitors dissolved in
ethanol (0.01 ml)-100 mM KCI-10 mM potassium phosphate buffer, pH
7.5-1 m« EDTA-propylene glycol (1 drop) in 10-ml screw cap glass
test tubes. The reaction was started by the addition of microsomes
[0.039 mg protein in 0.10 ml solution containing 0.5 mM NADPH, 100
mM KCI, 10 mM potassium phosphate buffer (pH 7.5), and 1 mM EDTA]
to the incubations. Reactions were stopped by the addition of chloro
form (5 ml), and the 3H2O content was determined as described
previously.
Concentrations
Initially, 6 steroids known to inhibit the aromatization of 4androstene-3,17-dione
(21, 22) were tested for their ability to
cause a time-dependent loss of aromatase activity (Table 1).
As expected, each steroid diminished the initially observed
aromatase activity relative to control (no steroid added) be
cause of competition between inhibitor and radiolabeled sub
strate during the assay step. Microsomes appeared to increase
in activity when incubated with either DHT or 5a-AD for 30 min.
This probably was due to the metabolism of these steroids by
other enzymes in the microsomes to products which had a
decreased affinity for aromatase. Three concentrations of 4,6ADD were evaluated and, while minor losses of initial activity
occurred, they were not considered to be significant. Since in
a competition experiment we found that 4,6-ADD was a tightly
bound competitive inhibitor (apparent K, 42 nM) of the aroma
tization of 4-androstene-3,17-dione
(apparent Km 21 nM), it is
clear that we tested this steroid at concentrations sufficient to
saturate aromatase and therefore maximize any time-depend
ent loss of activity that could be caused by this compound.
Testololactone caused a small loss of aromatase activity that
was somewhat greater in the presence of NADPH cofactor,
and in a competition experiment it was found to be a competi
tive inhibitor (apparent K¡14 ¿IM)of the aromatization of 4androstene-3,17-dione.
No further experiments were done with
testololactone to clarify the reason for the small time-dependent
loss of activity.
By contrast, substantial losses of activity were found for 1,4ADD and testolactone in the presence of NADPH. Failure to
observe this loss of activity when NADPH cofactor was omitted
Time-dependent
Initial and remaining aromatase activity of microsomes exposed to inhibitors
in the presence (+) or absence (-) of 100 fiM NADPH for 30 min at 37° in air
was measured as described in "Materials and Methods." Activities are expressed
as relative percentage of the corresponding initial or remaining control (NADPH
present but no inhibitor) activity after 30 min. Controls routinely lost 16% activity
during the 30-min incubations.
of 4,6-ADD evaluated were 0.2, 0.5, and 1
UM, and concentrations of testololactone tested were 50 and 200 ¡JIM.
Kinetic results were analyzed by Lineweaver-Burk plots (16). Regres
sion lines for all kinetic data were drawn according to a least-squares
Relative % of time-matched
control activity
Inhibitor1
Initial75
M)4,6-ADD1
,4-ADD (1 .-,
fit.
6+9+
Experiments done to investigate the reversibility of the inhibition
caused by 1,4-ADD, testolactone, and 4,6-ADD were done at 37° in
air with initial incubations (3.0 ml) that contained microsomes (0.058
mg protein per 0.5 ml) with or without added 100 /tM NADPH in the
buffer described earlier. Steroids were added in ethanol (60 ,»l)
to yield
final concentrations
of 10 /ÕM,400 ¡IM,and 10 MM 'or 1,4-ADD,
testolactone, and 4,6-ADD, respectively. Controls received ethanol (60
/il) only. Sixty min after the addition of inhibitors, aliquots (0.5 ml) were
removed and assayed for remaining activity as described earlier. The
remaining incubation volumes (2.5 ml) were spun at 100,000 x g for
60 min at 4°.The supernatants were discarded while the pellets (Pi)
were resuspended in buffer (2.5 ml). Portions (0.5 ml each) were
removed for the assay of aromatase activity as described earlier and
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Table 1
effects of inhibitors on aromatase activity
1659
64
UM2?MTestolactone
53+
+
441573
1968
UM)Testololactone
(400
+
6871
3069
/IM)DHTdO/iM)5<>-AD(10
(400
68+
+
588337
78+
UM)NADPH
2430min72
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Suicide Substrates
from the incubations suggests that aromatase enzyme catalysis
is involved in the inactivation process. Protection experiments
also support this conclusion. Thus, the inclusion of 0.5 /¿M
4androstene-3,17-dione
in incubations containing NADPH and
1 put 1,4-ADD increased the half-life of inactivation from 15.6
to 38.3 min. The half-life of inactivation caused by 400 JUM
testolactone (in the presence of NADPH) was increased from
33.5 to 315 min when 0.5 JUM4-androstene-3,17-dione
was
included in that incubation. The presence of 10 rriM dithiothreitol, added as a scavenger of reactive electrophilic compounds,
had no major effect on the rates of inactivation induced by
either of these inhibitors, indicating that an enzyme-generated
reactive intermediate did not escape from the aromatase active
site and later return to covalently modify and inactivate the
enzyme.
For 1,4-ADD and testolactone, additional time-course exper
iments (Charts 2 and 3) were carried out at several inhibitor
and the Aromatase
Mechanism
70
100
90
60
80
I
50
-^
70 -_
E 40
60 %_
Õ?,
o
6
C
50 2
30
40 a
E
in
(V
o
30 i
20
12
18
24
30
36
Incubation Time (minutes)
42
20
Chart 3. Time course for the decrease in aromatase activity by testolactone
in the presence of NADPH. •¿
control: testolactone: D. 25 /IM; A, 33.3 JIM;O, SO
»M:•¿.
' 00 ¡M:A, 400 IM. Duplicate experiments were done with 25 \M inhibitor,
but assays for remaining activity were taken at different times in each experiment
O.e., there are no duplicate measurements at any one time point). The data
shown for the remaining concentrations of inhibitor are the average and range of
duplicate experiments assayed at identical times.
concentrations, and the loss in activity was found to follow
pseudo-first-order kinetics. After a correction was made for the
loss of control enzyme activity during the experiments, the
kinetic data of Charts 2 and 3 were analyzed using double
reciprocal plots of the apparent rate constants for inactivation
(kapp = In 2/t,/2) versus the inhibitor concentration (15). For
1,4-ADD, the plot was linear (r = 0.979) and yielded values for
the apparent K, and pseudo-first-order overall rate constant for
decrease in activity of 0.32 ¡IMand 0.91 x 10~3/sec, respec
tively. For testolactone, the linear plot (r = 0.992) gave corre
sponding values of 35 fiM and 0.36 x 10"3/sec.
Finally, Table 2 summarizes the results of centrifugation
experiments done to investigate the irreversibility of the inac
tivation caused by 1,4-ADD and testolactone. The competitive
inhibitor 4,6-ADD was included in these experiments to assess
the difficulty involved in removing a high concentration (~238fold its K,) of a reversibly bound inhibitor from the microsomes.
Twice resuspended microsomes initially treated with 1,4-ADD
or testolactone in the presence of NADPH had 12.7 and 33.1 %,
respectively, of the original activity of control microsomes
incubated without added steroid. Based on the half-lives for
loss of aromatase activity found in the previously presented
time-course experiments, the expected remaining activity
would be 4.4 and 28.9%, respectively. Thus, in each case, the
agreement between the calculated and observed remaining
activity after excess reversibly bound inhibitor was removed is
quite good, and this demonstrates that the time-dependent
portion of the loss of activity caused by these inhibitors is not
readily reversible. As expected, twice-resuspended
micro
somes treated with these inhibitors in the absence of NADPH,
or with the competitive inhibitor 4,6-ADD in the presence of
NADPH, had activities comparable to those of twice resus
pended control microsomes.
IO
I4
I8
22
26
Incubation Time (minutes)
30
34
Chart 2. Time course for the decrease in aromatase activity by 1,4-ADD in
the presence of NADPH. The data shown for experiments carried out with 0.67
and 5 /*M are each from single experiments. The data shown for the remaining
concentrations of inhibitor are the average and the range (bars; not shown when
this value is smaller than the symbol) of duplicate experiments.
AUGUST
1982
Discussion
Six steroids were evaluated to obtain new information on the
molecular structure of aromatase suicide substrates. The cri
teria that we used to establish suicide substrate inhibition
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D. F. Covey and W. F. Hood
Table 2
Evidence for irreversible time-dependent inactivation
Microsomes (0.058 mg protein per 0.5 ml) in the presence (+) or absence (-) of 100 /UMNADPH were incubated with inhibitors for 60 min
at 37°in air. An aliquot was removed for aromatase activity determination, and the remaining microsomes were pelleted at 100,000 X g for
60 min at 4°.The supernatant was discarded and the pellet (Pi) was resuspended for aromatase activity and protein content determination.
A portion of the resuspended microsomes were put through the centrifugation procedure again. The supernatant was discarded, and the pellet
(P2) was resuspended for aromatase activity and protein content determination. Activities are expressed as percentage of initial control
microsome activity.
1,4-ADD
(10 (iM)Testolactone
fiM)Control
+Initial
microsomes
Resuspended pellet (P,)
Resuspended pellet (P2)100a
4,6-ADD(10/iM)
95.1 (0.028)6
69.6 (0.020)9.4
(400
+
42.2 (0.037)
(0.030)
59.2 (0.033)
65.3(0.017)0.83.8 12.7(0.019)14.9 65.9(0.015)16.9
31.3(0.030)
73.9 (0.028)
33.1 (0.020)58.8 55.5(0.018)
Specific activity, 65.2 pmol estrogen per 5 min per 0.058 mg protein.
' Numbers in parentheses, actual protein concentration in 0.5-ml assay.
included: (a) a requirement for enzyme catalysis in the initiation
of a time-dependent pseudo-first-order loss of aromatase ac
tivity; (b) substrate protection against this inactivation; and (c)
failure to completely reverse the inhibition by twice pelleting
and resuspending the initially treated microsomes in steroidfree buffer. Accordingly, 1,4-ADD and testolactone, but not
4,6-ADD, testololactone, DHT, or 5a-AD, were considered to
be aromatase suicide substrates. The slow inactivation caused
by testolactone may explain how this compound, which has
such a poor affinity for aromatase, successfully lowers circu
lating estrogen levels in humans (2).
These results and those of our earlier studies (6, 8) demon
strate that 4-androstene-3,17-dione
can be converted from a
substrate into a suicide substrate by the introduction of any
one of the following substituents: a 4-hydroxy group, a 6-keto
group, or a C-1 ,C-2 double bond. The reason why these
modifications in structure have produced this class of inhibitors
is unknown, and it continues to intrigue those of us interested
in the mechanisms of drug action. Our current experiments
with these inhibitors are guided by an inactivation mechanism
based on a new hypothesis for estrogen biosynthesis which is
outlined in Chart 4.
We have retained the proposal by Osawa (20) that the
direction of aromatase monooxygenations is trans to the C5.C-10 bond. After the first oxidation, rotation of the C-10.C19 bond orients the newly introduced hydroxyl group over Ring
A. This is the conformation of the alcohol observed in the
crystalline state by X-ray diffraction analysis (11 ). In the active
site of the enzyme, however, we propose that the hydroxyl
group is held over Ring A by virtue of hemiketal formation with
the C-3 carbonyl group. We envision that the second monooxygenation occurs on this hemiketal to give a product which
loses water to become the aldehyde. By invoking the hemiketal
intermediate, we not only continue to account for the known
ability of the enzyme to distinguish between the C-19 prof? and
proS hydrogens (20), but also we readily explain why the first
and not the second oxygen atom introduced into the steroid is
found ultimately in formic acid and not in water (1 ). In the third
oxidation, we propose that the addition of oxygen to the C-19
carbon is accompanied by simultaneous bridging of the alde
hyde to the C-3 carbonyl group so that the steroid remains
covalently bound to the enzyme through a peroxy linkage. A
separate nucleophilic group of the enzyme then attacks C-4 of
the steroid which results in concurrent cleavage of the peroxy
link to the heme of the cytochrome and transfer of the resulting
C-19 carbon fragment to C-3. The transferred C-19 formate
3330s
NADPH,02
| HT
hemiketol
formation
rotation
D NAOPH.O,
ENZ-O-Cn
ENZ-0J
*X—ENZYME
HO
Chart 4. New hypothesis for the mechanism of action of aromatase.
moiety should be rapidly lost as formic acid, and the C-3
carbonyl group would be regenerated. Notice that the formic
acid still contains the hydrogen present in the original aldehyde
group and the oxygen atom introduced in the third oxygénation
step as required by the known isotopie labeling data (1). All
that remains is for the newly formed covalent link that holds
the steroid to the enzyme (thus explaining why no satisfactory
third intermediate has yet been isolated) to be broken. This
could be accomplished by an elimination reaction which frees
the unaltered enzyme and forms a keto-diene which rapidly
enolizes to the phenol, thus forming the estrone product. We
have to further postulate that stereospecific loss of the 1/?hydrogen would occur in the elimination step and that enoli
zation of the keto-diene occurs with stereospecific loss of the
2/î-hydrogen to accommodate the remaining known isotopie
labeling data (5, 24). We realize that this mechanism is pres
ently more imaginative than substantive; however, it is not in
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Suicide Substrates
ENZYME
ENZYME
C.
ENZYME
(""BASE
ENZYME
ENZYME
Chart 5. Proposed mechanisms for the ¡nactivation of aromatase by: a, AT;
b, analogs of 4-androstene-3,17-dione
containing a C-1.C-2 double bond; c,
4-OH-A.
conflict with known isotopie labeling data, and its heuristic
value will become apparent in the following discussion.
With this new mechanism as a hypothesis, we can now
rationalize why AT is an aromatase suicide substrate. The initial
2 hydroxylations of AT apparently occur without inactivation of
the enzyme as shown by our studies (7) in which the interme
diate 6-keto,19-alcohol
and 6-keto,19-aldehyde
were unable
to inactivate aromatase without further NADPH-dependent ca
talysis. We postulate that the third oxidation occurs as outlined
in our newly proposed mechanism to liberate formic acid. The
problem arises only when the enzyme has to release the steroid
in the final elimination step. Because of the 6-keto group, the
elimination reaction is redirected so that enolization of the 6keto group occurs instead of ejection of the enzyme nucleophile (Chart 5a). Hence, the enzyme remains attached to the
steroid and the enzyme is inactivated.
Our mechanism also allows one to rationalize why several
steroids which contain a C-1 ,C-2 double bond (1,4-ADD, ATD,
and testolactone) inactivate aromatase. Once again, the inac
tivation occurs after all 3 oxidations have taken place when the
enzyme has to release the steroid product. In the presence of
the C-1 ,C-2 double bond, the steroid A-ring will aromatize
AUGUST
1982
and the Aromatase
Mechanism
without undergoing the final elimination reaction (Chart 5b).
This is disastrous for the enzyme since it needs to perform this
reaction to eject the steroid. Thus, by precluding the elimination
reaction from occurring, the steroid must stay attached, and
the enzyme is inactivated.
The apparent conflict between our 1,4-ADD inactivation
mechanism and the earlier finding that this steroid is enzymatically converted to estrone (13, 18) can be resolved if one
assumes that substantial quantities of the aldehyde intermedi
ate formed after the second hydroxylation step diffuse from the
active site and nonenzymatically break down to estrone. Inter
estingly, the gem-diol equivalent of this aldehyde has been
proposed as a transient intermediate in the nonenzymatic aromatization of 2/8-hydroxy-3,17-dioxoandrost-4-en-19-al
(12).
By contrast, enzyme inactivation would result from that portion
of the aldehyde which remains at the active site and undergoes
the third oxidation step.
Finally, the inactivation of aromatase by 4-OH-A also can be
envisioned to result from the redirection of this final elimination
reaction. We propose that the 4-hydroxyl group (following rapid
protonation) leaves as water, instead of the normally departing
enzyme nucleophile. Enolization then occurs to aromatize the
steroid A-ring, and once again inactivation results because the
ability of the enzyme to kick off the steroid product has been
precluded (Chart 5c).
It should be stressed that our hypothesis for the mechanism
of aromatase was formulated to explain how several steroids
of diverse structure could all be suicide substrates of this
enzyme. Other investigators interested in the mechanism of
action of these inhibitors will no doubt propose alternative
explanations.
Regardless of which mechanism eventually
emerges as the one which best describes this inactivation
phenomenon, any hypothesis which provides the basis for welldesigned experiments will have served its purpose. If, however,
our hypothesis is correct insofar as that all the suicide sub
strates discussed here inactivate after the third aromatase
oxidation step while other suicide substrates like RED inactivate
after the second oxidation step, then the combination of struc
tural features from both classes of inactivators into one mole
cule could lead to the first known examples of "double-jeop
ardy suicide substrates" (i.e., one inactivator would provide
the enzyme with 2 successive opportunities to catalyze its own
demise). We are currently pursuing this possibility.
To our knowledge, of the compounds which we and others
have found to be suicide substrates of aromatase, only testo
lactone has been used to treat breast cancer in women. If
aromatase inactivation is the reason for the observed decrease
in serum estrogen levels caused by testolactone, then it is
possible that these levels could be diminished even further by
the more potent aromatase suicide substrates which are cur
rently available. Whether or not these other inhibitors have
additional undesirable pharmacological
properties that pre
clude their use in humans also remains to be established.
References
1. Akhtar, M., Calder, M. R., Corina, D. L., and Wright. J. N. The status of
oxygen atoms in the removal of C-19 in estrogen biosynthesis. J. Churn
Soc. Chem. Commun., 129-130, 1981.
2. Barone, R. M., Shamonki, l. M., Siiteri, P. K., and Jude), H. L. Inhibition of
peripheral aromatization of androstenedione to estrone in postmenopausal
women with breast cancer using A'-testololactone.
J. Clin. Endocrino!.
Metab.. 49: 672-676, 1979.
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Discussion
3. Brodie. A. M. H.. Schwarzel, W. C., Shaikh, A. A., and Brodie. H. J. The
effect of an aromatase inhibitor, 4-hydroxy-4-androstene-3,17-dione,
on
estrogen-dependent processes in reproduction and breast cancer. Endocri
nology, 700. 1684-1695,
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Discussion
Dr. Zumoff: I have a question for Dr. Longcope concerning the
concept he presented of the relative validity of the blood and the urine
methods of measuring conversion. I think that if one is concerned about
how much androstenedione
is converted to estrone, which is then
capable of affecting an organ distant from the area in which it is formed,
thereby having to travel through blood, then it is the blood conversion
which is the valid one from the point of view of the distant biological
effect, rather than the overall conversion which is measured in the
urine. As he pointed out from the ratios between the urine and the
blood conversion rate and the overall increase in the urine conversion
rate, I would estimate that very obese women should have virtually no
increase in blood conversion rate. This would account for the fact that
these very obese women who have such a high increase in urinary
conversion rate may have no detectable increase in blood estrone
levels.
Dr. Longcope: If you remember my diagram (Chart 1), Step 1 is the
overall aromatization in the tissues; then Step 2 is the estrogen going
into the pool of free estrogen in the blood. This is a measurement we
make when we determine the conversion in blood. This will also be the
measurement we make for urine if Step 3, the direct conjugation of
estrogen in the tissue, does not occur. From the data of all groups, in
normal-weight individuals the p values measured in blood and urine are
essentially identical, indicating that no conjugation of estrogen occurs
in the tissue without the estrogen first entering the blood pool. This is
different in obese individuals. In their case, the data of McDonald
indicate that it is not the conjugation in the tissue of origin but a very
slow release or entry of estrogen formed in the tissues into the blood
pool. There is no evidence that I am aware of for the necessity for
conjugation first. So if you continue infusion for 48 hr, then blood p
and urine p will be similar. This is an infusion period which I find a little
impractical, and I think most people would. Under those conditions, it
is easier to measure the urinary p, which will be higher, as I indicated,
by a factor in our groups of 4 times. It is not the conjugation in the
tissue of origin that presents a problem here. This is the problem that
occurs when you measure androgen interconversions in the urine, as
opposed to in the blood, where there is conjugation and further metab
olism in the tissue where the conversion occurs. This does not appear
3332s
to be necessarily the case with the aromatization. Therefore, I think
that increased aromatization does occur in people who are obese
since, as I mentioned, adipose tissue is the site of aromatization. The
more adipose tissue, the more activity.
Dr. Osawa: I have a comment and question to Dr. Covey in relation
to his mechanism that suggests that, for an inhibitor to be suicide
active, it requires first and second hydroxylations before actual etioglycosamine inhibition occurs. We have recently found that 19-norethisterone compared to the 17a-ethinyl-19-nortestosterone
is an effective
suicide inhibitor the action of which is time and dose dependent and
requires NADPH. It is certainly a better suicide inhibitor than A'testololactone. This is the major ingredient of contraceptive pills, so
the pill-taking woman may have in effect an inhibited aromatase system!
Compared to that, 17a-ethinyltestosterone
is not an inhibitor at all. This
steroid has an angular methyl group as a requirement in its mechanism
and also a conjugated acetylenic ketone in this case, which is required
in the mechanism. But in this steroid you have to open the D ring up to
make it ketoacetylene. Whether this is true or not has to be tested. Yet
at least there is no hydrogen to be oxidized by the monooxygenase in
the usual sense. Still, they are active suicide inhibitors. How do you
accommodate that in your mechanism?
Dr. Covey: I don't! I'll let Dr. Robinson respond after I do. As you
know, Dr. Osawa, there is some disparity between the way Dr. Metcalf
and his group believe that propargylestradiene
works as a suicide
substrate and the way I believe that it works. None of the experiments
which either he or I or my group has done would in fact unambiguously
distinguish one of those mechanisms from the other. So I certainly
could not say for sure that oxygen is not inserted directly into the
acetylene of propargylestrenedione
(RED) and that inactivation in some
part might not result from that. In the end, there are 2 ways to resolve
that. You could make more chemical modifications, hoping to subtract
out one of the 2 mechanisms. The drawbacks to that approach are that
you have made a chemical modification and you must accept that, in
attempting to redesign the system to answer one question, you will
necessarily introduce new structural differences which may raise other
questions simultaneously. The second way would be to start to study
the metabolism of the steroid itself by the enzyme. Both of those
CANCER
RESEARCH
VOL. 42
Downloaded from cancerres.aacrjournals.org on June 17, 2017. © 1982 American Association for Cancer Research.
A New Hypothesis Based on Suicide Substrate Inhibitor Studies
for the Mechanism of Action of Aromatase
Douglas F. Covey and William F. Hood
Cancer Res 1982;42:3327s-3332s.
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