Purine Xanthosine-5-phosphate

Xanthine, Xan 2,6-dihydroxypurine, a purine and the starting point for Purine degradation (see). 152.1, m.p. >400°C (d.). Xan was discovered in 1817 in renal stones. It occiu in the free form, accompanied by other purines. Some derivatives are physiologically important, in particular xanthosine phosphates and Methylated xanthines (see). 

Xanthosine 9-P-D-ribofuranosybtanthine, a P-gly-cosidic nucleotide of D-ribose and the purine base xanthine. M, 284.23, carbonizes at >300°C, [a)p - 51.2° (c = 1 in 0.1 M NaOH). It is formed by the deamination of guanosine. Xanthosine S -monopbos-phate is metabolically important (see Xanthosine phosphates). 

Extracts of Aerobacter aerogenes (the wild, the purine-less mutant, and the guanine-less mutant), as well as a guanine-less mutant of Salmonella typhimurium which accumulates xanthosine in its culture medium, have been shown to contain an enzyme which catalyzes the oxidation of inosine 5-phosphate (XXVIII) to xanthosine 5-phosphate (XXIX). 

Purine nucleosides are cleaved by the action of purine nucleoside phosphorylase with the liberation of ribose 1-phosphate (Kl, PI). The enzyme is apparently specific for purines. The material from erythrocytes catalyzes the phosphorolysis of purine but not pyrimidine nucleosides (T6.) Purine phosphorylase activity is found widespread in nature and in many animal tissues (FIO). Friedkin and Kalckar investigated an enzyme capable of cleaving purine deoxynucleosides to the aglycone and deoxy-ribose 1-phosphate. They concluded that the enzyme was identical to that which splits purine ribonucleosides (F8, F9). This enzyme is capable of degrading inosine, xanthosine, and guanosine to forms readily attacked by other enzymes. In so doing, it permits living cells to retain the ribose and deoxyribose moieties. 

K17. Kuramitsu, H. K., Vdaka, S., and Moyed, H. S., Induction of inosine 5 -phos-phate dehydrogenase and xanthosine 5 -phosphate aniinase by ribosyl-4-amino-S-amindozalecarboxamide in purine requiring mutants of Escherichia cdi B. /. Biol. Chem. 239, 3425 (1964). 

Only the main features of the biosynthesis of the three purine alkaloids mentioned are known. It is likely that the starting material is, again, inosine-5 -phosphate which is first converted into xanthosine-5 -phosphate. This nucleotide contains the purine base xanthine. It is possible that the base is first liberated aad then methylated to give the purine alkaloids. 

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