Pharmacogenetics of human thiopurine methyltransferase: Kidney-erythrocyte correlation and immunotitration studies

Thiopurine methyltransferase (TPMT) activity in human red blood cells (RBC) is regulated by inheritance. Approximately 89% of subjects are homozygous for an allele for high enzyme activity, TPMT(H), and have high RBC TPMT activity. One in 300 subjects is homozygous for an allele for low activity, TPMT(L), and lacks detectable RBC enzyme activity, and about 11% of subjects are heterozygous for the two alleles and have intermediate activity. When TPMT activity was measured in renal cortical homogenates from 51 kidney samples, there was a bimodal distribution of enzyme activity. Of the subjects studied, 88% (45/51) had high and 12% (6/51) had intermediate enzyme activities. There was a significant correlation between renal and RBC TPMT activities in the 20 subjects in whom both activities were measured (r = 0.665, P < .001). In this sample all three of the subjects with 'intermediate' RBC activity also had intermediate renal activity. These results were compatible with the conclusion that the genetic polymorphism regulating RBC TPMT activity was also involved in the regulation of TPMT activity in the human kidney. The molecular mechanism responsible for the genetic regulation of TPMT activity was examined by immunotitration with antibodies against purified human kidney TPMT. RBC lysates from five subjects with intermediate activity, 6.3 ± 0.5 U/ml of RBC (mean ± S.E.M.), and five subjects with high activity, 13.7 ± 0.7 U/ml RBC, were studied. The average AD₅₀ value, a measure of immunoreactive TPMT protein, was significantly lower for the intermediate activity subgroup, 15.2 ± 0.5 μl/ml RBC, than was the ml RBC (P < .001). Similar results were found with immunotitration of homogenates from three kidney samples with intermediate enzyme activity, 1.55 ± 0.10 U/mg of protein, and five samples with high activity, 4.29 ± 0.12 U/mg of protein. The average AD₅₀ values were 3.7 ± 0.6 and 8.2 ± 0.6 μl/mg of protein, respectively, for these groups of samples (P < .001). Therefore, there was a significant reduction in immunoreactive TPMT protein in the RBCs of subjects with the allele TPMT(L) and in the kidneys of subjects with intermediate levels of enzyme activity.