Synthesis Of Inosine Research Articles

An alternate metabolic route for the synthesis of inosine 5'-phosphate (IMP) in the Lesch-Nyhan erythrocyte.

The cells of the Lesch-Nyhan individual, which are deficient in hypoxanthine-guanine phosphoribosyltransferase (HGPRT) activity, lack the ability to convert the 6-keto purines, hypoxanthine and guanine, to the corresponding nucleotides (1). Mammalian cells, in general, lack appreciable kinase activity for the 6-keto purine nucleosides, inosine and guanosine (2,3). Thus, the major route to IMP and GMP in the Lesch- Nyhan cell is via the overall pathway of de novo purine nucleotide synthesis in which the purine ring of IMP is built up on carbon 1 of a molecule of ribose 5-phosphate (4), The IMP is readily converted to GMP in normal human and Lesch-Nyhan cells.

A new synthesis of inosine from 5-amino-1-beta-D-ribofuranosyl-4-imidazole-carboxamide.

Inosine was prepared (15% yield) by treatment of 5-amino-1-beta-D-ribofuranosyl-4-imidazolecarboxamide (AICA-riboside) with chloroform in the presence of sodium methoxide. This ring closure can be reasonably explained by assuming the formation of dichlorocarbene from chloroform and alkali. Carbon tetrachloride or hexachloroethane as a carbene source was more effective for the ring closure of AICA-riboside, giving inosine in 48% and 51% yields respectively.

Inhibitors of nucleoside transport. A structure-activity study using human erythrocytes.

The passage of nucleosides across the plasma membrane of erythrocytes is a membrane-mediated process which is strongly inhibited by derivatives of 9-beta-D-ribofuranosylpurine (1) with S, O, or N atoms at the purine 6 position bearing variously substituted arylalkyl groups. In this structure-activity study, nucleoside derivatives were compared in respect to their ability to inhibit a transport-dependent aspect of nucleoside metabolism in erythrocytes, the synthesis of inosine from external guanosine and hypoxanthine. 6-Benzylthio, 6-benzylamino, and 6-benzyloxy derivatives of 1 were inhibitory at 10(-5)-10(-6) M and the similarity of their activities suggested that alkylation of the transporter as the mechanism of transport inhibition was unlikely. The hydrophobicity of the 6-position substituents appeared to contribute importantly to inhibitory activity. Although replacement of the ribofuranose moiety by other sugars reduced inhibitory activity, compounds with 9-butyl groups were inhibitory. 6-[(2-Hydroxy-5-nitrobenzyl)thio] derivatives of 1 were the most potent of the inhibitors tested, being active at about 10(-7) M.

Regulatory role of adenine nucleotides in the biosynthesis of guanosine 5'-monophosphate.

Derepression of the synthesis of inosine 5'-monophosphate (IMP) dehydrogenase and of xanthosine 5'-monophosphate (XMP) aminase in pur mutants of Escherichia coli which are blocked in the biosynthesis of adenine nucleotides and guanine nucleotides differs in two ways from derepression in pur mutants blocked exclusively in the biosynthesis of guanine nucleotides. (i) The maximal derepression is lower, and (ii) a sharp decrease in the specific activities of AMP dehydrogenase and XMP aminase occurs, following maximal derepression. From the in vivo and in vitro experiments described, it is shown that the lack of adenine nucleotides in derepressed pur mutants blocked in the biosynthesis of adenine and guanine nucleotides is responsible for these two phenomena. The adenine nucleotides are shown to play an important regulatory role in the biosynthesis of guanosine 5'-monophosphate (GMP). (i) They induce the syntheses of IMP dehydrogenase and XMP aminase. (The mechanism of induction may involve the expression of the gua operon.) (ii) They appear to have an activating function in IMP dehydrogenase and XMP aminase activity. The physiological importance of these regulatory characteristics of adenine nucleotides in the biosynthesis of GMP is discussed.

INHIBITORS OF NUCLEOSIDE METABOLISM

When incubated with hypoxanthine and guanosine, intact human erythrocytes synthesize inosine, which appears in the incubation medium. This system was employed to test 143 variously substituted purines and purine analogues as potential inhibitors of nucleoside metabolism; 26 inhibitory compounds were detected, all of which inhibited inosine synthesis by intact erythrocytes, but failed to inhibit the same reaction in preparations of disrupted erythrocytes. These compounds also inhibited the uridine-supported synthesis of inosine by intact Ehrlich ascites tumor cells in vitro.Most of these inhibitors were 6-mercaptoethers of purine (or 2-aminopurine) ribonucleoside. A wide variety of substituent groups on the sulfur atom, ranging from methyl (but not hydrogen) to p-nitrobenzyl, imparted inhibitory activity. 2-Arnino-6-(p-nitrobenzylmercapto) purine ribonucleoside, the most effective agent found, inhibited inosine synthesis by 74% when the ratio of guanosine concentration to that of inhibitor was 1000.Compounds with important inhibitory activity had particular substituent groups at both positions 6 and 9 on the purine ring; if present separately, the active substituent groups imparted much less inhibitory activity than when present together on the same molecule. The only 6-position substituents found to impart inhibitory activity were mercapto, chloro, and iodo groups. Ribosyl, arabinosyl, tetrahydro-2-pyranyl, and n-butyl substituents at position 9 produced inhibitory compounds when a 6-mercaptoether substituent was present.

Enhancement of ribonucleoside synthesis in Ehrlich ascites tumor cells by arsenate and iodoacetate.

Ehrlich ascites tumor ceils in vitro synthesize ribonucleosides, which appear mainly in the incubation medium, by the transfer of ribose from a donor ribonucleoside to an acceptor base. In the present study, it was found that the rates of synthesis of inosine and uridine in this system were markedly enhanced in the presence of arsenate or iodoacetate. The exchange of isotope between extracellular inosine and hypoxanthine-8-C14was similarly enhanced by arsenate, but the more rapid exchange between uridine and uracil-2-C14was unaffected. Arsenate did not cause changes in the rates of uridine breakdown that would account for the enhanced rate of nucleoside synthesis and did not promote the release of nucleoside-synthesizing enzymes from the tumor cells into the incubation medium. Because lactate formation during the uridine-supported synthesis of inosine was markedly inhibited by arsenate and iodoacetate, the increase in ribonucleoside synthesis appears to be indirect and to be related to inhibition of ribose phosphate catabolism.

Enzymic synthesis of inosine 5′-diphosphate glucose and inosine 5′-diphosphate mannose

Enzymic synthesis of inosine 5′-diphosphate glucose and inosine 5′-diphosphate mannose

Nucleoside metabolism by monkey brain extracts

1. 1. Monkey brain extracts metabolize the ribose moiety from purine ribonucleosides to form a variety of phosphate esters which have been separated and identified by paper chromatography. 2. 2. These products are formed by the successive action of nucleoside phosphorylase, transketolase and the glycolytic enzymes. 3. 3. Phosphoglucomutase was isolated as partially purified fraction, and the presence of phosphoribomutase activity was indicated by the synthesis of inosine from xanthine and ribose-5-phosphate and by the accumulation of ribose-5-phosphate from nucleoside incubations. 4. 4. The conversion of the purine moieties from nucleosides to free bases also has been investigated and the relationship of these changes to the rate of nucleoside metabolism is discussed.