Abstract
Tetrahymena pyriformis. strain E, cells grown in proteose-peptone media contained malate synthase in high activity but were low in isocitrate lyase. The formation of malate synthase was repressed by glucose; this repression was partially relieved by acetate. The formation of isocitrate lyase was stimulated by acetate in the presence or in the absence of glucose. Cells grown in chemically defined media containing acetate contained both isocitrate lyase and malate synthase in high activity; cells grown in such media containing glucose were virtually devoid of both enzymes. These findings show that isocitrate lyase and malate synthase, though operationally linked in the glyoxylate cycle, are not co-ordinately repressible. Tetrahymena capable of converting fats into glycogen contained both enzymes in higher activity than those incapable of glyconeogenesis. In the former type of organism, the enzymes were found in only one of the two mitochondrial fractions obtained after disruption of the cells; in the latter type, the enzymes were not localized in any single cellular component. These results indicate that the conversion of fat into glycogen by Tetrahvmena necessitates both the presence of high activities of glyoxylate-cycle enzymes and their incorporation into an organized intracellular structure.