Abstract

Cell-free extracts of nitrate-grown mycelia of Aspergillus terreus could catalyze the hydrolytic deamination of cytidine to uridine and ammonia followed by the hydrolytic cleavage of the N-glycosidic bond of the produced uridine to the corresponding base (uracil) and ribose. The same extracts could not catalyze the hydrolytic deamination of cytosine. Addition of inorganic arsenate to the reaction mixture containing cytidine or uridine did not affect the amount of ribose liberated indicating the absence of pyrimidine ribonucleosides phosphorylase in the extracts. Cytidine deaminase showed an optimum activity at pH 7.0 and 60°C and stability to high degrees of temperature. Uridine hydrolase activity was optimized at pH 8.0 and 55°C. Incubation of the extracts at 55°C for 60 min showed no effect on uridine hydrolase activity whereas incubation of the extracts at 60 and 70°C for different interval times caused a gradual decrease in activity and the enzyme lost its activity completely by incubation at 80°C for 15 min. Dialyzing the extracts showed no effect on cytidine deaminase activity and a decrease in uridine hydrolase activity. Addition of EDTA at a concentration of 5 x 10^-3 M and 10^-2 M caused an inhibition to the two enzymes activities. The presence of MgSO₄ in the reaction mixture seems to activate greatly both enzymatic cytidine deamination (225 and 128% increases) and uridine hydrolysis (22 and 77% increases) at final concentrations of 5 x 10^-3 M and 10^-2 M respectively. However HgCl₂ and CuSO₄ were found to be potent inhibitors for both activities at the two concentrations.

Keywords: Pyrimidine ribonucleosides, cytidine, uridine, cytidine deaminase, uridine hydrolase, Aspergillus terreus.