Five strains of Streptomyces were screened for the abilities of
their extracts to catalyze the hydrolytic or deaminating activities of
purine and pyrimidine ribonucleosides and their bases. These studies are
rare in Streptomyces. No hydrolytic cleavage for N-glycosidic
bond of nucleosides was observed in all screened strains. Hydrolytic deamination
was the only degradative activity occurred with cytidine (as substrate)
from the ribonucleosides and their bases tested. Streptomyces hygroscopicus
NRRL B-1476 gave the highest level of the hydrolytic deamination of cytidine
to uridine. Uridine was chromatographically identified in cell-free extracts.
Optimum pH and temperature of the enzyme activity were determined at 7.0
and 50Â°C, respectively. Thermal stability experiments indicated that
the enzyme completely restored its activity at 50Â°C for 30 min, however
a complete loss in enzyme activity was recorded when the enzyme was incubated
at 80 and 90Â°C for 20 and 5 min, respectively. Dialyzed extract caused
an increase in enzyme activity of about 55%. Results obtained indicate
the involvement of sulfhydryl group(s) in the catalytic site of the enzyme.
HgCl2, CuSO4 and FeCl3 (10-2
M) caused a complete inhibition of enzyme activity, whereas, little enzyme
activity was retained in presence of AgCl2, MgSO4,
BaCl2 and NaCl. Inhibition by uridine was of the competitive
type and the enzyme exhibited classic Michaelis Menten saturation kinetics.
Its apparent Km and Ki values were found to be 4.16
and 21.9 mM, respectively.