Redox potential within cells

The redox potential within the cells is substantially lower than in the plasma (May and Williams, 1980), and may vary depending upon particular cellular biochemical activities and a myriad of potential redox couples such as reduced and oxidized glutathione species. In general, greater levels of reduced glutathione in the intracellular fluids than in the plasma may provide an indication of an overall lower oxidation condition within the cells. However, it is interesting to note that intracellular fluids have relatively high concentrations of dissolved sulfate (Table 4), in spite of the more reduced conditions inferred to be present. 

The effect of these ferrocene-based additives on overcharge protection is shown in Figure 44, where AA cells based on lithium, LhMn02, and electrolytes with or without additives were overcharged. In the absence of these redox shuttles (A), the cell voltage continues to rise, indicating the occurrence of major irreversible decompositions within the cell whereas the presence of shuttle agents (B—E) locks the cell potential in the vicinity of their redox potentials... 

Why are there two pyridine nucleotides, NAD+ and NADP+, differing only in the presence or absence of an extra phosphate group One important answer is that they are members of two different oxidation-reduction systems, both based on nicotinamide but functionally independent. The experimentally measured ratio [NAD+] / [NADH] is much higher than the ratio [NADP+] / [NADPH]. Thus, these two coenzyme systems also can operate within a cell at different redox potentials. A related generalization that holds much of the time is that NAD+ is usually involved in pathways of catabolism, where it functions as an oxidant, while NADPH is more often used as a reducing agent in biosynthetic processes. See Chapter 17, Section I for further discussion. 

A quantitative description of oxidative phosphorylation within the cellular environment can be obtained on the basis of nonequilibrium thermodynamics. For this we consider the simple and purely phenomenological scheme depicted in Fig. 1. The input potential X0 applied to the converter is the redox potential of the respiratory substrates produced in intermediary metabolism. The input flow J0 conjugate to the input force X0 is the net rate of oxygen consumption. The input potential is converted into the output potential Xp which is the phosphate potential Xp = -[AG hoS -I- RT ln(ATP/ADP P,)]. The output flow Jp conjugate to the output force Xp is the net rate of ATP synthesis. The ATP produced by the converter is used to drive the ATP-utilizing reactions in the cell which are summarized by the load conductance L,. Since the net flows of ATP are large in comparison to the total adenine nucleotide pool to be turned over in the cell, the flow Jp is essentially conservative. 

As an example we use mitochondria (Fig. 17.6). These are small corpuscles that exist in large quantities within cells. They possess an exterior and an interior membrane where the enzymes cytochrome by c, Ci, a and a3, ATPase, and NADH are located. The interior membrane, of non-repetitive structure, contains 80 per cent protein and 20 per cent lipid. The Gibbs free energy variation of the conjugated redox pairs is given by the formal potential, according to... 

One probable mechanism for the release of iron from siderophores to the agents which are directly involved in cell metabolism is enzymatic reduction to the ferrous state. Due to the very low affinity of hydrdxamate and catecholate siderophores for Fe(II), the reduction converts the tightly bound ferric ion to the ferrous complex, which is unstable with respect to protonation and dissociation at neutral pH or below. Therefore comparison of siderophore complex redox potentials with those of physiological reductants can be very useful for the clarification of the mechanism of iron metabolism. Table IV shows the redox potentials [obtained by cyclic voltammetry (see Fig. 18)) of the siderophores tested so far. The values of all of the hydroxamates are within the... 

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