Effect of Urea on Ammonium-dependent Synthesis of Carbamyl Phosphate during Spore Germination of Geotrichum candidum.

The specific activities of enzymes catalyzing the ammonium-dependent carbamyl phosphate synthesis (NH(3)-CPS) and the glutamine-dependent carbamyl phosphate synthesis (GLN-CPS) were increased during germination by approximately 5-and 1.7-fold respectively in the presence of 35 mm urea. The increase of NH(3)-CPS and GLN-CPS levels occurred immediately after the onset of germination and prior to the appearance of germ tube. Ammonium also stimulated the NH(3)-CPS activity, but the induction caused by urea was about three times higher than that by ammonium.Both NH(3)-CPS and GLN-CPS were highly labile. NH(3)-CPS was obtained free of GLN-CPS after (NH(4))(2)SO(4) fractionation and diethylaminoethyl cellulose chromatography. The optimum pH for NH(3)-CPS was 8.5 as opposed to broad pH optimum of pH 5.6-8 for the reverse reaction. The Km values obtained for NH(3), glutamine, and carbamyl phosphate were 12 mm, 0.5 mm, and 0.083 mm, respectively.