Studies on the control and properties of ornithine transcarbamylase in germinated spores of geotrichum candidum

The effects of arginine and urea on the levels of ornithine transcarbamylase (OTC) were investigated in relation to the physiological functions of this enzyme in Geotrichum candidum. OTC was repressed in germinated spores to half of its initial level when exogenous arginine exceeded 12 mM. The repression of OTC could not be correlated with intracellular arginine concentration. The addition of urea at the final concentration of 0.035 M increased the specific activity of OTC by 5.5 and 2.5 fold as compared to enzyme levels in arginine-repressed spores and control spores respectively. Simultaneous addition of urea and arginine during germination prevented either arginine repression or urea induction of OTC. The enzyme was partially purified from germinated spores and isolated as a single protein band after disc electrophoresis. Two distinct pH optima for the forward reaction (pH 8.8-9) and backward reaction (pH 7.8) were found. Km values for ornithine and carbamyl phosphate were 5 × 10^-3 M and 6.8 × 10^-4 so respectively. The Km for citrulline in the catabolic direction was 1 × 10^-2 M. Enzymes obtained from cell-free extracts of germinated spores could synthetize ATP from citrulline and ADP under physiological conditions by coupling the phosphorolysis of citrulline with carbamate kinase activity. The initial rate of germination was stimulated in the presence of citrulline as the sole nitrogen source, as compared to arginine, glutamine or yeast extract. These observations suggest that citrulline may be catabolized during germination by means of OTC rather than via the energy-consuming urea cycle.