Formation of enzyme bound carbon dioxide in the reductive carboxylation of alpha-ketoglutarate by isocitrate dehydrogenase.

z* -1socitrate NADP oxidoredmctaae (E.C. 1.1.1.42) catalyses the
reversible oxidative decarboxylation of imcitrate to alpha-ketoglutarate
and carbon dioxide with the specific participation of NADP.
No fme intermediate has been isolated in the overall reaction nor have
attempts to demonstrate the existence of oxalosuecinic acid as a free
intermediate or in the enzyme bound form been successful (Siebert et al.,
1957; Plaut, 1963; Carsiotis, 1958). With large quantities of fairly
purified enzyme and labeled carbon dioxide, we attempted to demonstrate
enzyme bound oxalosuucinate in the overrall reverse reaction by use of the
'chase' technique successfully employed to demonstrate substrate binding
for several enzymes (Krishnaswamy et al., 1962; NishI.mura et al., 1963;
tierson and Moister, 1966). Enzyme bound carbon dioxide was demonstrated
both in the presence and absence of NADP or of alpha-ketoglutarate; the
extent of binding was significantly greater in the presence of NADP or of
alpha-ketoglutarate. Enzyme bound oxalosuccinate was not found under
these conditions. Prior incubation of carbon dioxide with the enzyme
exerted a specific protective effect against inhibition by PCMR of the
oxidation of isocitrate.