Oxidation of isocitrate and associated phosphorylations

Abstract

Rat-liver mitochondria were incubated with isocitrate (and other substrates) under various conditions. The rates of O2 uptake, substrate removal and incorporation of P32 into ATP were measured. Phosphorylation quotients between 1.4 and 3.4 were found when isocitrate was substrate. Fluoride (0.04 [image]) inhibited O2 uptake in the presence of succinate (up to 40 %) or isocitrate (up to 30%), but did not change the phosphorylation quotient. Fluoride inhibited the exchange of P[beta] and P[gamma] atoms of ATP presumably by inhibition of myokinase. Fluoride reduced the ratio atoms of oxygen consumed/tricarboxylic acid consumed to values less than 1. The tri-carboxylic acid removal was not accounted for by the appearance of an equivalent amount of a-oxoglutarate. Semicarbazide (0.04 [image]) had no effect on phosphorylation when [beta]-hydroxybutyrate was the substrate, but almost completely inhibited phosphorylation when citrate was substrate; O2 uptake was unaffected. In spite of non-phosphorylation the concentration of ATP was maintained. Anaerobically, semicarbazide did not appreciably inhibit ATP activity. Incorporation of P32 into ATP occurred anaerobically when isocitrate was substrate, at about 25% of the aerobic rate. This rate is much higher than that observed in the presence of succinate or [alpha]-oxoglutarate. The anaerobic incorporation was lowered by fluoride and 2:4-dinitrophenol. The P/O ratio associated with the activity of the triphosphopyridine nucleotide-specific isocitric dehydrogenase was 2.36; the ratio was reduced to 1.56 by fluoride (31% inhibition). The P/O ratio associated with the activity of the diphosphopyridine nucleotide-specific isocitric dehydrogenase was 2.83; fluoride reduced this to 2.68 (6% inhibition).