Hydrolytic cleavage of purine ribonucleosides in Aspergillus phoenicis

Cell-free extracts of nitrate-grown Aspergillus phoenicis could catalyze the hydrolytic cleavage of the N-glycosidic bond of inosine, guanosine and adenosine to the corresponding base and ribose by the nucleoside hydrolase. No evidence was obtained concerning the hydrolytic degradation of N-glycosidic bond of pyrimidine ribonucleosides namely cytidine and uridine by the same extracts. Optimum pH and temperature for adenosine, guanosine and inosine hydrolysis were the same at pH 3.5 and 55 degrees C, respectively. Citrate buffer showed the highest hydrolase activity when compared to the analogous activity obtained with the other buffers used. The rate of hydrolysis of the three nucleosides was in the order inosine > guanosine > adenosine. Incubation of extracts at 55 degrees C for 15 minutes caused about 85%, 75% and 62% loss of activity with adenosine, guanosine and inosine respectively. Dialyzing the extract caused a decrease in enzyme activity. Addition of inorganic arsenate to the reaction mixture (containing adenosine, guanosine or inosine) did not affect the amount of ribose liberated. Addition of EDTA at a concentration of 5 x 10(-3) M caused an inhibition of about 50%, however a complete inhibition for enzyme activity was obtained at 10(-2) M EDTA. MgSO4, CoSO4 and ZnSO4 at a final concentration of 5 x 10(-3) M showed activation of ribonucleoside hydrolase.