Thiopurine methyltransferase: mouse kidney and liver assay conditions, biochemical properties and strain variation

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of aromatic and heterocyclic thiol compounds including drugs such as 6-mercaptopurine (6-MP) and 6-thioguanine. In humans, the level of TPMT activity is inherited in a monogenic fashion. It would be important to develop an experimental animal model in which the genetic regulation of TPMT could be studied. Therefore, TPMT activity was measured in kidney and liver homogenates from A/J inbred mice. Apparent Michaelis (Km) constants for the two cosubstrates for the reaction, 6-MP and S-adenosyl-L-methionine (Ado-Met), in mouse kidney were 7.0 X 10(-4) M and 2.4 X 10(-6) M respectively. Apparent Km constants for 6-MP and Ado-Met in mouse liver were 5.4 X 10(-4) M and 2.1 X 10(-6) M respectively. The pH optimum for the reaction was 6.7 in both tissues, and over 95% of the TPMT activity in both mouse liver and kidney was "soluble" after centrifugation at 100,000 g for 1 hr. 3,4-Dimethoxy-5-hydroxybenzoic acid, an inhibitor of human kidney TPMT, decreased mouse kidney and liver enzyme activities by more than 95% at a concentration of 1 mM. TPMT activities were then measured in liver and kidney tissue from nine additional inbred strains of mice aged 7-8 weeks. Six of the nine inbred strains had TPMT activities very similar to those found in A/J animals. However, three strains, the C57BL/6J, C57BL/6ByJ and AKR/J, had significantly lower levels of activity in both liver and kidney than did any of the seven other strains. Liver TPMT activities in these three strains were only 23-32% of the average activity in A/J mouse liver. Kidney enzyme activities in the same three strains averaged 49-62% of the average activity in A/J mouse kidneys. These striking differences in TPMT activity among inbred mouse strains will make it possible to test the hypothesis that inheritance regulates variations in TPMT activity in this experimental animal.