Regulation of the enzymes of tyrosine catabolism in Tetrahymena pyriformis

1. 1. A tyrosine-catabolizing system similar to that found in rat liver was found to be present in Tetrahymena pyriformis , strain W. The five constituent enzymes of the system, i.e. tyrosine aminotransferase, p -hydroxyphenylpyruvate oxidase, homogentisate oxidase, maleylacetoacetate isomerase and fumarylacetoacetate hydrolase were assayed in extracts of the cells. Fumarylacetoacetic acid was prepared enzymatically from homogentisic acid with a protein fraction of the cells. 2. 2. All the tyrosine-catabolizing enzymes and phosphoenolpyruvate (PEP) carboxykinase were repressed by glucose and by acetate. The first enzyme of the catabolic sequence, tyrosine aminotransferase, and PEP carboxylase were repressed by glucose in both logarithmic and stationary phases of growth while the four subsequenct catabolis enzymes were repressed essentially only in stationary phase. A close metabolis relationship was found to exist between tyrosine aminotransferase and PEP carboxykinase. 3. 3. The repression of enzymes by glucose and acetate was accompanied by a repression of carbon flow from tyrosine to glycogen. 4. 4. An inverse relationship exists between the activities of tyrosine aminotransferase and PEP carboxykinase on the one hand and isocitrate lyase on the other. This relationship and the repression of the catabolic enzymes by acetate, a glucogenic precursor in Tetrahymena , suggests that the regulation of the tyrosine-catabolizing enzymes and particularly of tyrosine aminotransferase is geared to the demand for gluconeogenesis from tyrosine in these cells.