Uncoupling rate-limiting step

The maturation of fat body mitochondria is affected by the neuroendocrine balance in adult male B. discoidalis. Removal of the CC arrests the maturation of the respiratory enzymes at their partly developed, 5-day level (58, Figure 1). Injections of CC extracts on days 0 to 5 result in precocious, 10-day levels of respiratory activity by 5 days of age. Administration of the uncoupling agent 2,4-dinitrophenol indicates that electron transport, rather than phosphorylation, is the rate-limiting step for respiration in fat bcxiy mitochondria (59). A situation that differs from vertebrate mitochondria where phosphorylation is limiting. This indicates that the levels of cytochrome enzymes available for electron transport in fat body mitochondria determine the respiratory and ATP synthesis capacities of the tissue and influence its biosynthetic potential. 

The belief that alcoholics are more susceptible to the toxicity of 2,4-DNP during occupational exposure (Perkins 1919) may indicate an interaction with ethanol (and possibly other alcohols) or it may simply be a function of the compromised physiological state of alcoholics. 2,4-DNP appears to markedly increase the rate of ethanol metabolism in rat liver slices by 100-160% (Videla and Israel 1970) and in rats in vivo by 20-30% (Israel et al. 1970). Because 2,4-DNP uncouples mitochondrial electron transport from oxidative phosphorylation, the oxidation of NADH to NAD is accelerated in the mitochondria. Reoxidation of NADH rather than the activity of alcohol dehydrogenase is the rate-limiting step in the metabolism of ethanol, and, therefore, the metabolic effect of 2,4-DNP enhances the clearance of ethanol (Eriksson et al. 1974). Because 2,4-DNP is known to augment the rate of respiration and perspiration, 2.7-8.2% of the initial dose of ethanol was also eliminated by expiration and cutaneous evaporation in the rat (Israel et al. 1970). 

Both these objections have been met by Lardy, who proposed that thyroxine and its analogs act at a specific locus of the oxidative pathway. At low concentrations of the hormone, a rate-limiting step in the chain of reactions that yields chemical energy is uncoupled, and all the steps of the reaction subsequent to the rate-limiting step are accelerated without a marked loss of efficiency in oxidative phosphorylation. In larger doses, the hormone uncouples tightly coupled reactions, and the efficiency of oxidative phosphorylation is lost. 

To obtain the overall uncoupling rate constant, ku, we can again make use of the capture probability y [see Eq. (35)], such that kc = yk+. Recall that y quantifies the extent to which receptor/ligand association is rate-limited by the reaction step. As y approaches 0, association is severely reaction-limited, while as y nears 1, binding is almost purely diffusion-limited. ku is thus given by... 

The reason for the lower activities of the common 2 and 3 variants has been considered. One report has attributed the effect to changes in uncoupling [720]. Our own work, using arachidonic acid as a ligand, indicates that the difference can be explained simply by rates of reduction of P450 2C9, the step which appears to be rate limiting [721]. 

The action of uncouplers is to dissociate oxidation in the respiratory chain from phosphorylation. These compounds are toxic in vivo, causing respiration to become uncontrolled, since the rate is no longer limited by the concentration of ADP or Pj. The uncoupler that has been used most frequently is 2,4-dinitrophenol, but other compounds act in a similar manner. The antibiotic oligomycin completely blocks oxidation and phosphorylation by acting on a step in phosphorylation (Figures 12-7 and 12-8). 

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