Evidence for multiple forms of isocitrate lyase in Neurospora crassa.

The effect of carbon source on isocitrate lyase formation was studied in a wildtype strain of Neurospora crassa and in a uridine-deficient mutant. A constitutive level of the enzyme was produced in a casein hydrolysate medium. The enzyme was repressed by glucose, although the two strains varied with respect to the degree of glucose repression. Acetate strongly stimulated isocitrate lyase formation. The enzyme formed in the presence of acetate differed in several respects from that formed in glucose-grown cells. Differences were found in pH-activity curves, K(m) values, and in sensitivity to phosphoenolpyruvate inhibition. Diethylaminoethyl cellulose chromatography allowed separation of two enzymatically active components which showed different rates of heat inactivation. These data indicate the presence of multiple forms of isocitrate lyase in Neurospora.