[Regulation of the glyoxylic cycle: effect of the NADPH/NADP ratio].

The utilization of two-carbon sources is carried out through the operation of the glyoxylic acid cycle. The cycle involves several Krebs cycle steps as well as two additional specific enzymes, isocitrate lyase and malate synthase. Kinetic data from the isocitrate transforming enzymes suggest that the flow of isocitrate through the glyoxylic acid cycle depends upon the inhibition of the isocitrate decarboxylating enzymes. Three different mechanisms have been proposed at present that could account for this inhibition: a) Inhibition by concerted action of oxalacetate and glyoxylate; b) NADP-linked isocitrate dehydrogenase phosphorilation in E. coli, and c) competitive inhibition by NADPH with respect to NADP. In the present case the variations of the NADPH/NADP ratio would be channeling the utilization of isocitrate into the glyoxylic acid cycle.