Abstract

Cell-free extracts of nitrate-grown Penicillium viridicatum could catalyze the hydrolytic cleavage of N-glycosidic bond of adenosine, guanosine and inosine to the corresponding base and ribose by a ribonucleoside hydrolase; however, there is no evidence for the degradation of these compounds through phosphorylation. The rate of hydrolysis of the three ribonucleosides was in the order inosine> guanosine> adenosine. It was proven that adenosine hydrolyzing enzyme is not associated with the cell membrane. Maximum enzyme activity was observed at pH 4 and 50°C. Heat inactivation kinetics and the effect of the nature of the buffer on enzyme activity revealed that, the cleavage of the three purine ribonucleosides is affected by one intracellular nucleoside hydrolase. It was proven experimentally that, all the metal ions tested had a remarkable inhibitory effect on the activity of the ribonucleoside hydrolase. Results obtained indicate that extracts of P. viridicatum catalyzed the conversion of guanosine into guanine and ribose by a nucleoside hydrolase and the resulting guanine was then deaminated to xanthine by an inducible guanine deaminase. In addition, xanthosine was not split into ribose and xanthine by the same extracts under the same experimental conditions and even at different pH values of the reaction mixture.

Key words: Purine ribonucleosides, adenosine, guanosine, inosine, hydrolase, Penicillium viridicatum.