Role of oxalacetate in lipoate effect on frog gastric mucosa.

The mechanism of action of lipoate on frog gastric mucosa was investigated. Oxalacetate (OAA) reversed lipoate-inhibited QO2 and QH+ of chambered mucosas by 70 and 40%, respectively. Pyruvate or glucose produced similar effects. Neither activity was affected by OAA when added after glucose, pyruvate, decanoate, butyrate, or lipoate-propionate-inhibited mucosa. Lipoate-treated or lipoate-propionate-treated mucosa did not respond to histamine; OAA addition prior to histamine restored responsiveness. Tracer and chromatographic techniques showed that lipoate reduced and pyruvate increased OAA formation. Preincubation of mitochondrial extracts of gastric mucosa with 2 mM lipoate increased pyruvic dehydrogenase activity 110%. Pyruvic carboxylase (PC) activity was primarily in the mitochondrial fraction of the gastric mucosa. The PC preparation was shown to have an absolute requirement for CoASAc, contained biotin, was not inhibited by lipoate, and had an apparent Km approximately equal to 3.6 X 10(-4) M for pyruvate. The results suggest that OAA concentration is regulated by PC activity and is one of the factors controlling QO2 and QH+ in the frog gastric mucosa.