HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Gallo, R. C. Articles by Breitman, T. R. Search for Related Content PubMed PubMed Citation Articles by Gallo, R. C. Articles by Breitman, T. R. Social Bookmarking                      What's this?

The Enzymatic Mechanisms for Deoxythymidine Synthesis in Human Leukocytes

III. INHIBITION OF DEOXYTHYMIDINE PHOSPHORYLASE BY PURINES

From the ¹ From the Medicine Branch and the Laboratory of Physiology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20014

In a cell-free system prepared from human leukocytes, the syntheses of deoxythymidine and deoxyuridine from pyrimidine base and deoxyribose-1-P, catalyzed by deoxythymidine phosphorylase, are inhibited by purine bases and purine deoxynucleosides. The most effective inhibitors are the 6-oxypurines: hypoxanthine, xanthine, and allopurinol. 6-Mercaptopurine is also a potent inhibitor. The inhibition with a partially purified enzyme is characterized by a decrease in the V_max and a slight increase in K_m for deoxyribose-1-P. Interpretation of the kinetic data with varying concentrations of thymine is complicated by thymine substrate inhibition. However, it is clear that inhibition by purines with varying concentrations of thymine is not primarily due to competition with thymine for the catalytic site but mainly results in a reduction in V_max. Treatment of the enzyme with thymine prior to the addition of purines greatly diminished the inhibitory effects of purines. Attempts to modify the inhibitor sites selectively with urea, mercury, heat, trypsin, or enzyme aging were unsuccessful. In the presence of urea the enzyme is less sensitive to thymine inhibition, but there is no decrease in purine inhibition. In addition, with increasing concentrations of urea there is a corresponding increase in the apparent K_m for thymine. Although both purines and thymine stabilize the enzymes during prolonged dialysis, thymine but not purines also stabilized the enzyme against heat denaturation.

Uridine phosphorylase, catalyzing uridine synthesis from uracil and ribose-1-P, is inhibited by 6-mercaptopurine and hypoxanthine but not by allopurinol. Deoxythymidine synthesis by direct transfer of deoxyribosyl from deoxyuridine to thymine (pyrimidine deoxyribosyltransferase) is also inhibited by purines, but the inhibition is slight compared to the inhibition of deoxythymidine synthesis from thymine and deoxyribose-1-P.

CiteULike    Complore    Connotea    Del.icio.us    Digg    Reddit    Technorati    What's this?