Taurine, Glutathione and Bioenergetics - Advances in Experimental Medicine and Biology - page 5

Table 1.1 Approximate midpoint redox potentials E m,pH (mV) at dif f erent pH values
Redox pair pH = 7.0 pH = 7.5 pH = 8.0 pH = 8.5
NADH/NAD + −320 −335 −350 −365
NADPH/NADP + −320 −335 −350 −365
GSH/GSSG −172 −202 −232 −262
Based directly on the values given in Table 3.2 in Bioenergetics 3 (Nicholls and Ferguson 2002 ) .
In their calculations the factor 2.303 RT/F from the Nernst equation was set to 60 mV (correspond-
ing to T = 302 K or 29.2°C)

Please notice that in all the equations and calculations the factor log(10) RT/F
from the Nernst equation has been set to 60 mV as in Bioenergetics 3 (Nicholls and
Ferguson 2002 ). This approximation corresponds to a temperature of T = 302 K or
29.2°C.
The redox potentials for the redox pairs are pH dependent, but the Nernst equa-
tions show that D E /pH » −60 mV/pH for GSH/GSSG, but NADH/NAD + and
NADPH/NADP + has only a dependence of D E /pH » −30 mV/pH.
The thiol group of GSH becomes deprotonated in alkaline pH above the p K a for
the thiol group, and thus leading to a simplified redox equilibrium

+
and an associated Nernst equation without any pH dependence as follows:

GSSG

2e

−

2 GS

−

⎛ ⎞ ⎟

− 2

60 mV [GS ]

⎜

EE =−

0

log ⎟ . (1.5)

⎜

⎟

⎜

⎜ ⎝ ⎟

2 [GSSG]

⎠

The p K a = 9.0 (25°C) for GSH can be found in Table 1.3 , and a more detailed
discussion can be found elsewhere, e.g. (Schafer and Buettner 2001 ) . The pH depen-
dence for the GSH redox potential is indicated as the D E GSH curve in Fig. 1.1 .

1.2.4 Redox Equilibrium NADH/NAD + and GSH/GSSG

From the Nernst equations above, the midpoint potentials can be calculated at rele-
vant pH values in the range from 7.0 to 8.5 as presented in Table 1.1 .
However, the midpoint potentials do not take into account the actual concentra-
tions of the individual species. A better representation is obtained by the use of typi-
cal values for the redox pair concentrations and GSH pool concentration of 10 mM
(Nicholls and Ferguson 2002 ; Wahllander et al. 1979 ). If these actual concentrations
found in the mitochondrial matrix are inserted into Nernst's equation, the actual
redox potentials as presented in Table 1.2 and Fig. 1.2 are obtained.
A surprising consequence when increasing the pH to slightly alkaline conditions at
about pH 8.5 is now the result that the actual redox potentials for the NADH/NAD +