Xanthylic acid

For convenience the group at C—2 of xanthylic acid is represented here as an hydroxyl group, although it actually exists in the carbonyl or lactam form. This liberty is taken with other structures throughout this chapter. 

Inosinic acid dehydrogenase converts inosinic acid to xanthylic acid which in... 

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... 

One of the first demonstrations that acquired resistance could be circumvented was the inhibition of 5. faecalis resistant to 8-azaguanine by 8-azaxanthine. Thus, cells that possess xanthylic acid phosphoribosyltransferase activity could form 8-azaxanthylic acid, which was then converted to 8-azaguanylic acid and incorporated in nucleic acids as such [98]. 

Nucleotides (glutamic acid, guanyl acid, xanthyllic acid)... 

The phosphodiester bonds of xanthylic acid in deaminated RNA were scarcely split by RNase U2 (30). The susceptibility of purine nucleotide residues to RNase U2 decreases in the order of A>G>I X, indicating that the phosphodiester bonds of adenylic acid and inosinic acid without a keto group at the position of purine base are more sensitive to RNase U2 than those of guanylic acid and xanthylic acid. The resistance of TNP-RNA to RNase U2 may be also attributed to the steric hindrance by a larger substituent at 2-amino groups of guanylyl residues, as with RNase T, (SO). 

In 1932 Levene and Harris128 showed that the hydrolysis of xanthylic acid gave rise to the formation of a D-ribose phosphate which was not identical with the known D-ribose 5-phosphate. Since xanthylic acid is the monophosphate derivative of a ribofuranoside of xanthine it followed that the new phosphate was either D-ribose 2-phosphate or the 3-isomer (L). Shortly thereafter the same authors129 succeeded in reducing the new D-ribose phosphate with hydrogen in the presence of platinum oxide to a ribitol phosphoric acid (LI) which was completely... 

Xanthylic and Guanylic Acids. On deamination with nitrous acid, guanylic acid is readily transformed - to xanthylic acid. Consequently, determination of the structure of the latter will serve to show that of guanylic acid also. 

In 1931, Levene - made the remarkable discovery that if an aqueous solution of xanthylic acid (which has pH 1.9) is allowed to stand at 50 the acid will hydrolyze itself, forming xanthine and a phosphoribose with properties quite different from those of the 5-phosphoribose discovered many years earlier by him. - ... 

O, phosphoribonic acid from xanthylic acid x, 5-phosphoribonic acid. (Taken from P A Levene end S. A. Harria, /. Bid. Ckem. 95, 763 (1932)). 

Fia. 3.—Glycoside forniation of phoephoribose from xanthylic acid. 

Hence the phosphosugar is 3 phospho-D-ribose xanthylic acid is 9 -(3-pho8pho-D-ribofuranosyl)-xanthine and guanylic acid is 9 -(3-phospho-D-ribofuranosyl)-guanine. 

Fig. 4. Possible pathways of purine nucleotide anabolism and catabolism. The heavy arrows indicate the normal routes of degradation in man. I = phosphoribosylpyrophosphate, II = phosphoribosylamine, III = inosinic acid, IV = xanthylic acid, V = adenyhc acid, VI = guanyhc acid VII = hypoxanthine, VIII = xanthine, IX — uric acid, and X = adenosine.

It is now known that mild, alkaline hydrolysis of ribonucleic acids yields mixtures of the 2 - and 3 -phosphates of ribonucleosides, although it had earlier been believed that only the 3 -phosphate derivatives are produced. For example, it was reported that guanylic acid (6), obtained from ribonucleic acid hydrolysates, is deaminated with nitrous acid to xanthylic acid (7), which was hydrolyzed at pH 1.9 to a ribose phosphate... 

GMP synthesis The two reactions of GMP synthesis are an NAD -dependent oxidation followed by an amidotransferase reaction. In Step 1, IMP dehydrogenase employs the substrates NAD and H2O in catalyzing oxidation of IMP at C-2. The products are xanthylic acid (XMP or xanthosine monophosphate), NADH, and H. GMP is a competitive inhibitor (with respect to IMP) of IMP dehydrogenase. In Step 2, transfer of the amido-N of glutamine to the C-2 position of XMP yields GMP. This ATP-dependent reaction is catalyzed by GMP synthetase. 

Mild acid hydrolysis of the deaminated purine nucleotides, xanthylic acid and inosinic acid, gives the purine bases (xanthine and hypo-xanthine, respectively) and a reducing sugar phosphate. The same hydrolytic products are obtained by use of a specific pancreatic enzyme. On the other hand, mild alkaline hydrolysis of a nucleotide, or treatment with the appropriate enzyme, liberates free phosphoric acid and a non-reducing compound of base and sugar, known as a nucleoside. (Hydrolysis of ribosenucleic acid with fairly dilute ammonia under pressure, during 3.5 hours at a bath temperature of 175 to 180 , gives an equimolecular mixture of four nucleosides). 

Fio. 3.—Glycoside formation of phosphoribose from xanthylic acid. 

Xanthosine phosphates phosphate esters of xanthosine, or xanthine nucleotides Xanthosine S"-monophosphate, XMP, xandudylU mid, xanthylic acid, M, 364.22 is an intermediate of Pu e biosynthesis (see). 

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