Crithidia fasciculata: Regulation of aerobic fermentation by malic enzyme

Abstract

Aerobic fermentation, described in many protozoan organisms, was investigated using the nonpathogenic protozoan Crithidia fasciculata. The major end-products of this process were organic acids, particularly succinic acid. Inhibition of malic enzyme (EC 1.1.1.40) (l-malate: NADP oxidoreductase (decarboxylating) appeared to be integral to the aerobic fermentative process; the enzyme was inhibited in a cumulative manner by oxalacetate, acetyl-coenzyme A and oxalate. Inhibition by oxalate was noncompetitive with respect to both substrate and coenzyme. The inhibition of malic enzyme by oxalacetate and acetyl-coenzyme A appeared to provide a mechanism for the diversion of carbon from oxalacetate to succinate via the fumarase, thereby avoiding recycling to pyruvate. This was corroborated by the demonstration of an inverse relationship between the concentrations of pyruvate and succinate elaborated by this organism. © 1973.