Inhibition of human leukocyte pyrimidine deoxynucleoside synthesis by allopurinol and 6-mercaptopurine

Allopurinol and 6-mercaptopurine, two clinically useful purine antagonists, are inhibitors of deoxythymidine phosphorylase and pyrimidine deoxyribosyltransferase from human leukocytes. The inhibition of these enzyme activities is only in the direction of nucleoside synthesis and is not competitive with either substrate (thymine or deoxyribose-1-phosphate). 6-Mercaptopurine, but not allopurinol, also inhibits uridine phosphorylase.