Electron transport, decreased reversal

In resting muscle the high concentration of ADP does not decrease the proton gradient effectively and the high membrane potential slows electron transport. ADP, formed when ATP is hydrolyzed by myosin ATPase during contraction, may stimulate electron transport. However, the concentration of ATP (largely as its Mg salt) is buffered by its readily reversible formation from creatine phosphate catalyzed in the intermembrane space, and in other cell compartments, by the various isoenzymes of creatine kinase (reviewed by Walliman et al., 1992). 

Pretreatment with the Type I substrate, ethylmorphine, resulted in 100% mortality in both rats and mice, and aminopyrine pretreatment resulted in 100% and 64% mortality in rats and mice, respectively, exposed to disulfoton (Pawar and Fawade 1978). Nickel chloride, cobalt chloride, or cycloheximide decreased the levels of cytochrome bs, cytochrome c reductase, and total heme in rats (Fawade and Pawar 1983). These electron transport components were further decreased in rats pretreated with these inhibitors and given a single dose of disulfoton. Data from this study suggests an additive effect, since disulfoton also decreases the activities of these components. Evidence of an additive effect between disulfoton and these metabolic inhibitors was suggested by the decrease in ethylmorphine N-demethylase and acetanilide hydroxylase activities when rats were given an inhibitor followed by disulfoton. In another experiment, these inhibitors decreased the activity of delta-aminolevulinic acid synthetase, but this decrease was reversed when disulfoton was administered. 

Figure 5. Reversal of decreased electron transport components in vitamin C deficient guinea pigs with ascorbic acid (15).

Ascorbic acid deficiency also reduces the activity of several dehydrogenases involved in the Krebs cycle. The mechanism by which the vitamin leads to such alteration is not clear, but the effect is reversed by insulin administration. Thus, miscellaneous observations on the effect of ascorbic acid on carbohydrate metabolism have been made, but they are difficult to interpret because no specific coenzyme effect of ascorbic acid has been demonstrated. Again, ascorbic acid is assumed to be directly involved in an electron transport chain that involves cytochrome and NAD. The vitamin may also affect the electron transport chain indirectly because a decrease in NADH concentration has been observed in vitamin C deficiency. But this decrease may also result from an interference with the insulin production because it is... 

Many investigators have shown that nonspecific reagents as diverse as calcium phosphate gel, EDTA, histidine, and nonspecific proteins activate succinoxidase preparations in otherwise unfavorable environments. The mechanism of the activation is not established, but it has been repeatedly suggested that the activators in some manner influence the steric orientation of components of the particulate succinoxidase. Another component of electron-transport systems has been implicated by Nason and Lehman. DPNH oxidation by a particulate fraction of rat muscle was found to be decreased by extraction of 10 per cent of the lipid with isooctane the activity was restored by addition of a-tocopherol (vitamin E) or the lipid extracted from muscle or bovine serum albumin. These lipids are able to reverse the inhibition of cytochrome c reduction caused by antimycin A. It has not been determined whether the tocoph-... 

Fig. 5.15. The dependence of the magnitude on the reversible light-induced decrease of the high field component of the EPR signal from spin label on TEMPOamine concentration in the suspension of bean chloroplasts functioning under conditions of cyclic electron transport mediated by phenasine metosulphate, photosystem 2 was inhibited by diuron (after [70]).

We ascribe the field-induced quenching observed when both Q and the oxygen evolving apparatus were in the reduced state to reversed electron transport from Q to the intermediary acceptor, Pheophytin. The fluorescence is low when Phe is in the reduced state (Klimov et al., 1977), probably due to excitation transfer towards Phe, followed by rapid internal conversion to the ground state (if the electron is actually pushed on towards P680, the same quenching mechanism is expected). By lack of a positive charge to recombine with, the electron returns to Q as soon as the membrane potential decreases. 

Another possible two-electron mechanism involves the direct transport of two electrons from a mononuclear transition metal complex to a substrate (S). Such a transport alters sharply the electrostatic states of the systems and obviously requires a substantial rearrangement of the nuclear configuration of ligands and polar solvent molecules. For instance, the estimation of the synchronization factor (asyn) for an octahedral complex, with Eq. 2.44 shows a very low value of asyn = 10 7to 10 8 and, therefore, a very low rate of reaction. The probability of two-electron processes, however, increases sharply if they take place in the coordination sphere of a transition metal, where the reverse compensating electronic shift from the substrate to metal occurs. Involvement of bi- and, especially, polynuclear transition metal complexes and clusters and synchronous proton transfer in the redox processes may essentially decrease the environment reorganization, and, therefore, provide a high rate for the two- electron reactions. 

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