BIOCHEMICAL SOCIETY TRANSACTIONS
554
species contributing to the total fluorescence. This suggests contributions from tyrosine
residues in addition to tryptophan residues. The well defined tyrosine shoulder at
308nm indicates that in the extracted basic protein the factors that contribute to the
internal quenching of tyrosine in other proteins (Teal, 1960)are largely absent. The fluorescence spectrum from intact myelin preparations also show a definite tyrosine component and this may represent a contribution from the basic protein providing that the
tyrosine in this protein is largely unquenched in the intact membrane structure.
Financial support from The Multiple Sclerosis Society of Great Britain (to A. J. C.) and the
Wellcome Trust (to A. J. S. J.) is gratefully acknowledged.
Avruch, J., Carter, J. R. & Martin, D. B. (1972) Biochim.Biophys. Acta 288,27-42
Banik, N. L. & Davison, A. N. (1973) J. Newrochem. 21,489-494
Caspar, D. L. D. & Kirschner, D. A. (1971) Nature (London) New Biol. 231, 46-52
Mokrasch, L. C. (1969) in Handbook ofNeirrochemistry (Lajtha, A., ed.), vol. I , pp. 171-193,
Plenum Press, New York
Radda, G. K. & Vanderkooi, J. (1972) Biochim. Biophys. Acta 265, 509-549
Rumsby, M. G . , Riekkinen, P. J. & Arstila, A. V. (1970) Brain Res. 24, 495-516
Shooter, E. M. & Einstein, E. R. (1971) Annu. Rev. Biochem. 40, 635-652
Teal, F. W. J. (1960) Biochem. J. 76, 381-388
Analogues for Experiments on Energy Coupling and Ion Movement
MARTIN DAVIES
Department of Biology, University of York,York YO1 5DD, U.K.
A primary electrogenic process may be defined as a spontaneous electrochemical
reaction, i.e. one capable of being maintained in a metabolic steady state, in which is
inherent an anisotropic redistribution of charge. If the catalyst of such a reaction is
orientated in a membrane, the process may generate or contribute to a membrane potential Em.This use of the word electrogenic is intended to distinguish such a primary process from the Donnan equilibrium, which results in a membrane potential, or from other,
steady-state or transient, situations in which diffusion potentials or bi-ionic potentials
occur. Movements of ions that occur in an applied electric field, which may itself be due to
(bJ
M+
c
ATP
Fig. 1. (a) Inside-out vesicle and (b) eqirivalent circuit for vesicle
4, Electrogenic processes; -- +, electrophoretic movements or passive fluxes.
Currents ( I ) , e.m.f. values ( V ) , and resistances (R),have subscripts which indicate
their analogy with the vesicle. M+ is univalent cation; A- is univalent anion.
1974
.
546th MEETING, YORK
555
Table 1. Calculated characteristics of the analogue
Symbols are defined in the text. Vp = 1V, and a = 1 throughout.
Va (V)
m
E m (V)
4
0.80
20
0.88
0.64
0.80 0.50 0.50 0.50 0.50 0.30 0.30 0.20 0.20
10
10
20
20
10
20
10
58
28
0.86 0.74 0.74 0.73 0.72 0.64 0.62 0.57 0.57
0.40 0.95 0.90 0.87 0.75 0.92 0.84 0.93 0.86
a primary electrogenic process, are termed electrophoretic. The prediction of Em for a
chemiosmotic system is a difficult, perhaps intractable, physicochemical problem ;it may
be helpful to consider instead how closely a simple electrical analogue resembles the real
system. The theory for the following analogue can be transcribed in chemical terms, but
this has two serious disadvantages: simplified elements of the analogue tend to be confused with specific postulates from experiments, and the notation for fluxes is more cumbersome than that for currents.
An inside-out vesicle, e.g. a submitochondrial particle, is compared in Fig. 1 with an
equivalent circuit. In practice Rm%Ro,and since in theory KO approx. equals K1,it is
taken that Ro = R1 = 0. The system is considered in a steady state of chemiosmotic ATP
production, in which Vp> V,. Variables R, and R, are thought of as dependent on the
maximal velocity of the ATPase (adenosine triphosphatase) and pump respectively, but
may also depend on their poise, whereas V values depend on poise only.
Equations are written by using Kirchoff's laws, and further parameters are defined as
follows: a = RJR,, m = R,,/R,, q = ( Vp- Va)/Va,the membrane potential Em= I,,, R ,
and, to represent efficiency, 4 = IJI,. The characteristics of the model, shown in Table 1,
are calculated from eqns. 1 and 2:
qm- 1
4 = a(l+q) +qm
Table 1 indicates that in order to achieve high efficiencies, m will probably be in the range
10-100, and may vary widely under many conditions with little change in Em.It has been
shown graphically that at constant a and q, 4 increases linearly as l/m decreases.
The model can be related to experimental data (Mitchell & Moyle, 1967a,b; Papa et
al., 1972) as follows. Consider 1mg of mitochondria1 protein contained in about 10l2
vesicles of total area 400cm2. At a respiration rate of 100ng-atoms of oxygen/min, if
H+/O= 4, then the pump rate is 400ng-atoms of H+/min, giving Z,, = 0.64 x
A . If V,
is taken as 0.8 V, then the calculated R, is 1250Q. If the poise of the ATPase is represented
by a AG of 40 kJ/mol, this gives V, = 0.21 V for a two electron reaction. An ATPase rate
of 200nmol/min leads to R, = 660Q. A specific conductance of the membrane to protons of 0.45/*0hm-'.cm-~ is equivalent to 5 . 5 103Q.
~
This may be considered as in
parallel with another resistance of comparable value, representing other ions, thus giving
R, = 2.5 x 1 0 3 ~ .
By using the above figures, the calculated 4 = approx. 0.6 and m = approx. 2.0 lend
credence to the chemiosmotic hypothesis. (However if V, is taken as 3 (O.SO)V, which
may be more realistic, 4 falIs to a very low vaIue.) An uncoupler or ionophore could
decrease the appropriate parallel resistance 50-fold, giving R, approximately equal to
IOOQ, which would markedly decrease 4, in accordance with experimental findings.
In a more sophisticated model based on the same principle, the composite nature of the
membrane is considered more carefully, and the membrane capacitance is included, thus
providing an analogue for the study of transient responses, for example to step changes.
Mitchell, P. & Moyle, J. (1967~)Biochem. J. 104, 588-600
Mitchell, P. & Moyle, J. (19676) Biochem. J. 105, 1147-1162
Papa, S., Scarpa, A., Lee, C. P. & Chance, B. (1972) Biochemistry 11, 3091-3098
Vol. 2