Storey B T
Johnson Research Foundation, University of Pennsylvania, Philadelphia, Pennsylvania 19104.
Plant Physiol. 1971 Dec;48(6):694-701. doi: 10.1104/pp.48.6.694.
Energy-linked reverse electron transport from succinate to endogenous NAD in tightly coupled mung bean (Phaseolus aureus) mitochondria may be driven by ATP if the two terminal oxidases of these mitochondria are inhibited, or may be driven by the free energy of succinate oxidation. This reaction is specific to the first site of energy conservation of the respiratory chain; it does not occur in the presence of uncoupler. If mung bean mitochondria become anaerobic during oxidation of succinate, their endogenous NAD becomes reduced in the presence of uncoupler, provided that both inorganic phosphate (P(i)) and ATP are present. No reduction occurs in the absence of P(i), even in the presence of ATP added to provide a high phosphate potential. If fluorooxaloacetate is present in the uncoupled, aerobic steady state, no reduction of endogenous NAD occurs on anaerobiosis; this compound is an inhibitor of malate dehydrogenase. This result implies that endogenous NAD is reduced by malate formed from the fumarate generated during succinate oxidation. The source of free energy is most probably the endogenous energy stores in the form of acetyl CoA, or intermediates convertible to acetyl CoA, which removes the oxaloacetate formed from malate, thus driving the reaction towards reduction of NAD.In the absence of P(i) and presence of oligomycin, oxidation of succinate by the alternative cyanide-insensitive oxidase pathway, in the presence of sulfide to inhibit cytochrome oxidase, does not reduce endogenous NAD, either in the aerobic steady state or in anaerobiosis. Under these conditions, only the reversed electron transport pathway from succinate to endogenous NAD is active and ATP cannot interact with the respiratory chain. The source of energy for NAD reduction must come from the respiratory chain, and this result shows that oxidation of succinate through the alternate pathway does not provide this energy.
在紧密偶联的绿豆（Phaseolus aureus）线粒体中，如果这两个线粒体的末端氧化酶受到抑制，从琥珀酸到内源性NAD的能量偶联逆向电子传递可能由ATP驱动，或者可能由琥珀酸氧化的自由能驱动。该反应特定于呼吸链能量守恒的第一个位点；在解偶联剂存在时不会发生。如果绿豆线粒体在琥珀酸氧化过程中变为厌氧状态，在解偶联剂存在的情况下，其内源性NAD会被还原，前提是同时存在无机磷酸盐（P(i)）和ATP。在没有P(i)的情况下，即使添加ATP以提供高磷酸盐电位，也不会发生还原。如果在解偶联的好氧稳态中存在氟代草酰乙酸，厌氧时内源性NAD不会发生还原；该化合物是苹果酸脱氢酶的抑制剂。这一结果表明，内源性NAD是由琥珀酸氧化过程中产生的延胡索酸形成的苹果酸还原的。自由能的来源很可能是以乙酰辅酶A形式存在的内源性能量储备，或者是可转化为乙酰辅酶A的中间体，它们去除了由苹果酸形成的草酰乙酸，从而推动反应朝着NAD的还原方向进行。在没有P(i)且存在寡霉素的情况下，在存在硫化物以抑制细胞色素氧化酶的情况下，通过替代的氰化物不敏感氧化酶途径进行的琥珀酸氧化，无论是在好氧稳态还是厌氧状态下，都不会还原内源性NAD。在这些条件下，只有从琥珀酸到内源性NAD的逆向电子传递途径是活跃的，并且ATP不能与呼吸链相互作用。NAD还原的能量来源必须来自呼吸链，并且这一结果表明通过替代途径进行的琥珀酸氧化不能提供这种能量。
