Nucleosidase hydrolytic

Hydrolases are hydrolytic enzymes, biochemical catalysts that use water to cleave chemical bonds, usually dividing a large molecule into two smaller molecules. Examples of common hydrolases include esterases, proteases, gly-cosidases, nucleosidases, and lipases. 

Transition-state structures computed from multiple kinetic isotope-effect, data have provided important information on the catalytic mechanisms of glycosylases. This is exemplified by the refined transition-state structures derived by Schramm and his associates and used to probe the hydrolytic reactions catalyzed by nucleosidases and the ADPR (adenosine diphosphori-bosyl) transferase toxins of Vibrio cholerae and Corynebacterium diphthe-... 

A pyridine ribosidase from Xanthomonas pruni has been described which splits nicotmamide riboside in preference to purine ribosides (18 ). The relative activity towards nicotinamide riboside and inosine is about 30 to 1. In contrast to this, extracts of SaccJiaromyces cereviaiae promote the hydroljrtic cleavage of nicotinamide riboside and inosine at approximately the same rate (183). A hydrolytic nucleoadase from a strain of Lactobacillus delbruckii has been purified and also shown to have equal activity towards the two nucleosides (184). Xanthomonas pyridine nucleosidase appears to be unusual for bacteria its significance is not clear as yet. 

Two classes of MTA nucleosidases have been reported in the Literature a hydrolytic nucleosidase that cleaves MTA into adenine and l ethylthioribose (MTR) has been described in Aerobacter aeroge-nes and in Escherichia coli and a phosphorolytic nucleosidase has been purified from several mammalian tissues i.e. rat ventral prostate, rat lung, human prostate and human placenta28. 

The oxidizing enzymes involved in the conversion of purines to ureides have been well studied and are described in the next subsection. Little attention has, however, been paid to the hydrolytic enzymes. Conversion of nucleotides to ureides by nodule tissue or cell-fiee extracts or organelle preparations thereof implies that an efficient hydrolytic system is present. A study by Christensen and Jochimsen (1983) identified a 50-fold excess in levels of 5 -nucleotidase in soybean nodules over nodules of Pisum sativum. Similar differences were found for levels of the enzyme in other organs of the two plants. Levels of purine nucleosidase were, however, not significantly different between the two species, although levels in soybean were somewhat higher in all organs. 

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