Xanthosine-5 -phosphate

The second step in the conversion of inosinic acid to guanylic acid is the aminolysis of xanthylic acid with glutamine by xanthosine-5 -phosphate aminase [65]. This aminase, isolated fromf. coli B, is inhibited allosterically by adenosine and adenylic acid [320], and it is also inhibited by psicofuranine (9-0-D-psicofuranosyladenine) (LXXIV) [284, 321-326], which apparently is not... 

The related antibiotic decoyinine (angustmycin A, LXXIII) [285] also inhibits xanthosine-5 -phosphate aminase [329], presumably in the same manner as psicofuranine (LXXIV), but less effectively. There is evidence that psicofuranine (LXXIV) and decoyinine (LXXIII) may be interconvertible in cells [329]. 

GMP synthetase [EC 6.3.4.1], also known as xanthosine-5 -phosphate ammonia ligase, catalyzes the reaction of ATP with xanthosine 5 -phosphate and ammonia to produce GMP, AMP, and pyrophosphate (or, diphosphate). GMP synthetase (glutamine-utilizing) [EC 6.3.5.2] catalyzes the reaction of ATP with xanthosine 5 -phosphate, L-glutamine, and water to produce GMP, AMP, L-glutamate, and pyrophosphate. 

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). 

The aminase could not be detected in the guanine-less mutant of Aerobacter aerogenes that accumulates xanthosine. The accumulation of xanthosine, instead of xanthosine 5-phosphate, is most probably caused by the action of phosphatases. 

A3. Abrams, R., and Bentley, M., Biosynthesis of nucleic acid pinrines. III. Guanosine 5 -phosphate formation from xanthosine 5 -phosphate and L-glutamine. Arch. Biochem. Biophys. 79, 91-110 (1959). 

L2. Lagerkvist, U., Biosynthesis of guanosine 5 -phosphate. II. Amination of xanthosine 5 -phosphate by purified enzyme from pigeon liver. J. Biol Chem. 233. 143-149 (1958). 

M17. Moyed, H. S., and Magasanik, B., Enzymes essential for the biosynthesis of nucleic acid guanine xanthosine 5 -phosphate aminaae of Aerobacter aerogenes. J. Biol. Chem. 226, 351-363 (1957). 

Xao XEDS XES XMP XPS XRD XRDF XRF Xyl xanthosine X-ray energy-dispersive spectrometry X-ray emission spectroscopy xanthosine 5 -monophosphate, xanthosine 5 -phosphate X-ray photoelectron spectroscopy X-ray diffraction X-ray radial distance function X-ray fluorescence xylose... 

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). 

The biosynthesis of caffeine takes the same path up to xanthosine-5 -phosphate, which is here dephosphorylated and methylated at N-7. However, there are also reports that xanthosine-5 -phosphate can be methylated directly. [506] Afterthe sugar residue is cleaved off, two additional methylation steps take place via the intermediates 7-methykanthine and theobromine. [507]... 

Xanthosine-5 -phosphate i/-glutamine amido-ligase (AMP) GMP synthetase... 

However, as shown in Scheme 14.3, some of which has been seen earlier as Scheme 12.111 in Section H of Chapter 12) inosine phosphate, also can undergo hydration at C2. Oxidation of the resulting carbinolamine in the presence of inosine monophosphate dehydrogenase (EC 1.1.1.205) produces xanthosine 5 -phosphate. The xanthosine 5 -phosphate is then converted to guanosine 5 -monophosphate (GMP). 

GMP. Xanthosine 5 -phosphate (xanthyHc acid, XMP) has similar properties as IMP and GMP. 

The aminating enzyme, xanthosine 5 -phosphate aminase, has been purified partially from Aerobader 168, 169), pigeon liver 170), and calf thymus 171). The liver and thymus enzymes required glutamine, but utilized ammonia to a small extent as an aminating agent. Glutamic acid and aspart ne were ineffective. 

The nucleotides that are constituents of nucleic acids, guanylic acid and adenylic acid, were formed from inosinic acid. Inosinic acid was oxidized by a DPN-enz3une to xanthosine 5 -phosphate, which was then ami-nated by glutamine to form guanosine 5 -phosphate. Adenylic acid was produced by the cleavage of adenylosuccinic acid which was formed by the reaction of inosinic acid with aspartic acid. 

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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