Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:1.17.3.2 (xanthine oxidase)
 8,383 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The enzymatic inosine 5'-monophosphate assay described by Grassl [in, Methods of Enzymatic Analysis (H. U. Bergman, ed.), pp. 2168-2171, Academic Press, New York (1974)] is highly nonspecific, as ITP, ATP, ADP, AMP, and adenosine react stoichiometrically. The reactivity with the adenine derivatives is due to the tri- and diphosphatase activity of alkaline phosphatase (AP), coupled with adenosine deaminase (and possibly AMP deaminase) contamination of commercially available preparations of AP, purine-nucleoside phosphorylase, and/or xanthine oxidase. The inclusion of coformycin (0.05 microgram/ml), a potent inhibitor of these deaminases, completely eliminated the cross-reactivity. ITP, however, still reacted stoichiometrically due to the tri- and diphosphatase activity of AP. Meyer and Terjung [Amer. J. Physiol. 237 C111-C118 (1979)] introduced a modification of Grassl's procedure, substituting 5'-nucleotidase for AP. It has been found that this disallows reactivity with ATP, ADP, and ITP but that AMP and adenosine still react completely. Coformycin prevents this cross-reactivity. It is therefore recommended that the assay be carried out with 5'-nucleotidase (instead of AP) and coformycin, in order to achieve a more specific assay, and one more suitable for use with whole tissue extracts.
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PMID:An enzymatic inosine 5'-monophosphate assay of increased specificity. 298 81

The purine nucleoside cycle is a cyclic pathway composed of three cytosolic enzymes, hypoxanthine-guanine phosphoribosyltransferase, IMP-GMP specific 5'-nucleotidase, and purine-nucleoside phosphorylase. It may be considered a 'futile cycle', whose net reaction is the hydrolysis of 5-phosphoribosyl-1-pyrophosphate to inorganic pyrophosphate and ribose 1-phosphate. The availability of a highly purified preparation of cytosolic 5'-nucleotidase prompted us to reconstitute the purine nucleoside cycle. Its kinetics were strikingly similar to those observed when dialyzed extracts of rat brain were used. Thus, when the cycle is started by addition of inorganic phospate (Pi) and hypoxanthine or inosine (the 'inosine cycle'), steady-state levels of the intermediates are observed and the cycle 'turns over' as far as 5-phosphoribosyl-1-pyrophosphate is being consumed. In the presence of ATP, which acts both as an activator of IMP-GMP-specific 5'-nucleotidase and as substrate of nucleoside mono- and di-phosphokinases, no IDP and ITP are formed. The inosine cycle is further favored by the extremely low xanthine oxidase activity. Evidence is presented that ribose 1-phosphate needed to salvage pyrimidine bases in rat brain may arise, at least in part, from the 5-phosphoribosyl-1-pyrophosphate hydrolysis as catalyzed by the inosine cycle, showing that it may function as a link between purine and pyrimidine salvage. When the cycle is started by addition of Pi and guanine (the 'guanosine cycle'), xanthine and xanthosine are formed, in addition to GMP and guanosine, showing that the guanosine cycle 'turns over' in conjunction with the recycling of ribose 1-phosphate for nucleoside interconversion. In the presence of ATP, GDP and GTP are also formed, and the velocity of the cycle is drastically reduced, suggesting that it might metabolically modulate the salvage synthesis of guanyl nucleotides.
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PMID:The purine nucleoside cycle in cell-free extracts of rat brain: evidence for the occurrence of an inosine and a guanosine cycle with distinct metabolic roles. 1278 25