INOSINE NUCLEOSIDASE

Fig. 29.2 Caffeine is produced from xanthosine derived from four routes (1) inosine-5 -monophosphate (IMP) originating from de novo purine synthesis (de novo route), (2) adenosine released from the 5-adenosyhnethionine (SAM) cycle, (3) the cellular adenosine nucleotide pool (AMP route), and (4) the guanine nucleotide pool (GMP route). Enzymes AMPDA AMP deaminase, APRT adenine phosphorihosyltransferase, AK adenosine kinase, ARN adenosine nucleosidase, GRD guanine deaminase, IMPDH IMP dehydrogenase, 5 NT 5 -nucleotidase...

Investigations on the reversal of nucleosidase activity established that purified nucleosidases from rat liver could synthesize inosine and guanosine from ribose 1-phosphate and the respective purines 89a), This procedure has been extended to the enzymic synthesis of iV-ribosylnicotinamide 89h) and A-2-deoxyribosylhypoxanthine and -azaguanine 89c) from their respective bases and ribose 1-phosphate. Reactions such as these may play a role in the natural synthesis of nucleotides. 

AMP IMP-I-NH3 (Adenylate-deaminase) IMP Inosine - - P (5 — Nucleotidase) Inosine Hypoxanthine-l-Ribose (Nucleosidase)... 

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. 

AMP to 8.3 mAf for UMP and 20 mA/for XMP. Doremus and Blevins have reported some properties for this enzyme from soybean nodules. Purine nucleosidase (EC 3.2.2.1) was described as having a pH optimum of 7 - 8 and a of 0.3 mM for inosine (Christensen and Jochimsen, 1983). Nucleosidase has been purified from cowpea nodules (Atkins et ai, 1989). The native molecular weight was 160,000 with a subunit molecular weight of 30,600. This enzyme was apparently cytoplasmic and was markedly xanthosine specific. The values for xanthosine and inosine were similar (0.80 and 0.83 mM, respectively). Neither of these enzymes has exhibited any specific relationship to ureide formation. 

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