Guanylate deamination

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). 

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. 

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... 

These and other observations pointed to the direct formation of inosi-nate from a derivative of guanine, and an enzyme was discovered which catalyzes the irreversible reductive deamination of guanylate to inosinate. 

Guanylate reductase, which deaminates this nucleotide, catalyzes a reductive, rather than hydrolytic, deamination and has been discussed in Chapter 9. Like adenylate deaminase, it has a catabolic role and also functions in purine nucleotide interconversion. A guanosine deaminase has recently been identified in a pseudomonad (13), but it is not known to occur in animal cells. 

L-Cysteine is transformed to L-cysteine sulfinic acid and L-cysteic acid. Cysteamine (D 11) yields hypotaurine and taurine (Fig. 189). The latter compounds may be transformed to other secondary products by deamination, thiolation, guanylation, and methylation. L-Cysteine sulfinic acid may be degraded to L-alanine and sulfurous acid, which is oxidized to sulfuric acid. 

Evidence exists for specific enzymes which will deaminate 3-adenylic acid, 5-adenylic acid, guanylic acid, adenosine, guanosine, and cytidine. Little is known concerning their distribution. 

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