Some kinetic studies on ribonucleosides degradation by extracts of penicillium citrinum

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Abstract
Cell‐free extracts of 3–4 days old mats of nitrate‐grown Penicillium citrinum catalyze the hydrolytic cleavage of the N‐glycosidic bonds of inosine, guanosine and adenosine optimally at pH 4, 0.1 M citrate buffer. The same extracts catalyze the hydrolytic deamination of cytidine at a maximum rate in 0.08 M Tris‐acetate buffer pH 6.5, 40°C and 50°C were the most suitable degrees for purine nucleoside hydrolysis and cytidine deamination, respectively. The incubation of the extracts at 60°C, in the absence of cytidine caused a loss in the deaminating activity, while freezing and thawing had no effect on both activities. The deaminating activity seems to be cytidine specific as neither cytosine, adenine, adenosine nor guanosine could be deaminated. Uridine competively inhibited this activity, while ammonia had no effect. The apparent Km value of this enzyme for cytidine was 1.57×10−3M and its Ki value for uridine was 7.8×10−3M. The apparent Km values of the N‐glycosidic bond cleaving enzyme for inosine, guanosine and adenosine were 13.3, 14.2 and 20×10−3 M, respectively.