الفهرس 
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Abstract
Thirteen filamentous fungi were screened for the abilities of their extracts to catalyze
the hydrolytic and I or the deaminating activities of some purine and pyrimidine
nucleosides and bases. Penicillium viridicatum was selected for further studies.
2- Ribose and adenine, guanine or hypoxanthine were chromatographically identified
from the degradation of adenosine, guanosine or inosine respectively as a result of
hydrolytic activity in cell-free extraCts of P. viridicatum.
3- The rate of hydrolytic cleavage of N-glycosidic bond of purine ribonucleosides by
extracts of P. viridicatum was in the order Inosine> Guanosine> Adenosine.
4- The degradation of guanosine by extracts of P. viridicatum was suggested to be
affected by a hydrolase to give guanine and ribose. The resulting base was then
deaminated to give xanthine by deaminase.
5- The only activity observed against pyrimidine ribonucleosides was that of cytidine
deaminase. Uridine produced rrom cytidine was chromatographi.<.. ;ally identified .
6- Studies on the properties of the purine nucleoside hydrolase or cytidine deaminase
indicated that optimal activity was obtained at pH 4.0 and 50°C or pH 6.0 and
40°C, respectively.
7- Cell-free extracts of P. viridicatum which was grown on xanthine and uric acid
contained the enzymes xanthine dehydrogenase, uricase, allantoinase, allantoicase and
urease respectively. These enzymes catalyzed the oxidation of xanthine to uric acid, the
oxidative decarboxylation of uric acid to allantoin, the hydrolysis of allantoin to
allantoic acid, the degradation of allantoic acid to glyoxylic acid and urea and finally
urease hydrolyzed urea to ammonia and carbon dioxide