SOME KINETIC-STUDIES ON RIBONUCLEOSIDES DEGRADATION BY EXTRACTS OF PENICILLIUM-CITRINUM

Cell-free extracts of 3-4 days old mats of nitrate-grown Penicillium c itrinum catalyze the hydrolytic cleavage of the N-glycosidic bonds of inosine, guanosine and adenosine optimally at pH 4, 0.1 M citrate buff er. The same extracts catalyze the hydrolytic deamination of cytidine at a maximum rate in 0.08 M Tris-acetate buffer pH 6.5, 40-degrees-C a nd 50-degrees-C were the most suitable degrees for purine nucleoside h ydrolysis and cytidine deamination, respectively. The incubation of th e extracts at 60-degrees-C, in the absence of cytidine caused a loss i n the deaminating activity, while freezing and thawing had no effect o n both activities. The deaminating activity seems to be cytidine speci fic as neither cytosine, adenine, adenosine nor guanosine could be dea minated. Uridine competively inhibited this activity, while ammonia ha d no effect. The apparent K(m) value of this enzyme for cytidine was 1 .57 x 10(-3) M and its K(i) value for uridine was 7.8 x 10(-3) M. The apparent K(m) values of the N-glycosidic bond cleaving enzyme for inos ine, guanosine and adenosine were 13.3, 14.2 and 20 x 10(-3) M, respec tively.