Barbiturates and oxidative phosphorylation.

It has been claimed that barbiturates uncouple oxidative phosphorylations (Brody & Bain, 1954; Brody, 1955). These authors have demonstrated that during the oxidation of pyruvate by liver and brain mitochondria, phosphate uptake was lowered proportionately more than oxygen uptake. Support for the uncoupling theory was derived from certain similarities between the barbiturates and 2:4dinitrophenol (Brody & Bain, 1954). 'The slopes of the inhibition curves of these compounds are remarkably similar although dinitrophenol is the more potent agent. Both depress fatty acid oxidation and stimulate oxidative rate in a phosphatedeficient system. The addition of excess magnesium ion does not reverse the uncoupling action of either the barbiturate or dinitrophenol' (Brody, 1955). Further, barbiturate hypnosis is potentiated by 2:4-dinitrophenol (Killam, Brody & Bain, 1958). None of these arguments is conclusive evidence for uncoupling, and they ignore the major difference that, in vitro, barbiturates inhibit (Bain, 1952) whereas 2:4-dinitrophenol stimulates respiration (Loomis & Lipmann, 1948; Parker, 1958). This difference is readily demonstrated in vivo, for after administration of barbiturates oxygen consumption (Costa & Bonnycastle, 1955) and body temperature (Birnie & Grayson, 1952; Lessin & Parkes, 1957) fall, whereas 2:4-dinitrophenol causes a rise of both oxygen consumption (Cameron, 1958) and body temperature (Stoner, 1956). Other differences in behaviour in isolated systems have also been demonstrated (Johnson & Quastel, 1953; Jalling, Low, Ernster & Lindberg, 1957; Messer, 1958). Although the supporting evidence for the uncoupling theory is not conclusive, Brody & Bain (1954) have nevertheless demonstrated a lowering of the phosphorylation quotient (P/O ratio). Respiration, with pyruvate as substrate, of the liver mitochondria used in this paper can be inhibited 50% without any decrease of the P/O ratio (Aldridge, 1957). With such preparations we have re-examined the effect ofoxyand thio-barbiturates upon oxidative phosphorylation. Moreover, by dissociating the process of oxidation from that of oxidative phosphorylation by the intervention of 2:4-dinitrophenol (Loomis & Lipmann, 1948; Parker, 1958), we have studied the effects of barbiturates upon these processes separately. A preliminary report of these results has already appeared (Aldridge & Parker, 1958).