Thiopurine methyltransferase activity in a sample population of black subjects in Florida.

Thiopurine methyltransferase (TPMT), is an enzyme detected in the human red blood cell that catalyzes the S-methylation of thiopurine drugs and is known to exist as a genetic polymorphism in white subjects. Investigations in this laboratory of red blood cell TPMT showed interethnic differences also existed in North American black subjects. A sample group of black subjects in Florida had a mean activity of 8.64 +/- 3.47 U/ml red blood cells and an antimode of 6.5 units, which represented values significantly lower than those obtained for both the mean activity and the antimode in other populations. The findings of this study suggest the possibility that TPMT activity may be under genetic control in North American black subjects and that this ethnic group may be at greater risk of experiencing thiopurine-induced toxicity caused by the lower overall mean activity of the enzyme.