Inhibition of Tritiated Thymidine Incorporation into Dna by Alkaline Phosphatase Preparations.

A factor present in homogenized normal human blood leukocytes containing alka line phosphatase activity was shown to be effective in depressing the incorporation of TDR-H3 into DNA-H* when preincubated with dog bone marrow, lymph, and human chronic granulocytic leukemia cell suspensions. Human donor blood buffy coats were extracted, and an approximately 200-fold purified preparation of leukocyte alkaline phosphatase was prepared by the method of Traubowitz. The purified leukocyte al kaline phosphatase preparation readily dephosphorylated thymidylic acid and was free of detectable thymidine phosphorylase activity. This purified leukocyte enzyme preparation was also seen to be active in depressing the incorporation of TDR-H3 into DNA-H3 when preincubated with the intact proliferative test cells. The active factor present in the purified alkaline phosphatase preparation was heat-labile, nondialyzable, and appeared to be inhibited by 10~3MZn ion—allproperties shared by alkaline phos