Thiopurine methyltransferase activity in a Korean population sample of children.

Thiopurine methyltransferase (TPMT) is a cytoplasmic enzyme that catalyzes the S-methylation of the cytotoxic drugs 6-mercaptopurine and azathioprine. Red blood cell (RBC) TPMT activity is subject to genetic polymorphism, and we have previously demonstrated an interethnic difference in TPMT activity. To investigate whether there was a race-related difference in RBC TPMT activity, TPMT was measured in a Korean population sample of 309 healthy children. Mean TPMT activity in healthy Korean children was 12.4 +/- 2.4 units/ml RBC, which is similar to the earlier reported TPMT activities in white populations. In contrast to the bimodal or trimodal frequency distributions of RBC TPMT activity in most other population samples, the frequency distribution histogram, the probit plot, and the Shapiro-Wilk test supported a normal distribution of TPMT activity in this Korean population sample of healthy children. Mean RBC TPMT activity showed a tendency to decrease with age, but it was not statistically significant. No gender-related difference in RBC TPMT activity was found.