Abstract

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of 6-mercaptopurine and other heterocyclic and aromatic thiol compounds. In humans, TPMT activity is controlled by a common genetic polymorphism. C57BL/6J (B6) and AKR/J (AK) inbred mice have low hepatic and renal TPMT activities, whereas DBA/2J (D2) mice have high enzyme activities. Low TPMT activity is inherited in these mice as an autosomal recessive trait. The properties of TPMT in liver homogenates from B6, AK and D2 mice were compared in order to study the biochemical basis for inherited differences in TPMT activity among these strains. Biochemical and physical properties of hepatic TPMT were very similar in all three strains. Apparent Michaelis (Km) constants for 6-mercaptopurine were 0.98, 0.75 and 1.1 mM for B6, AK and D2 mice, respectively. Apparent Km values for S-adenosyl-L-methionine, the methyl donor for the reaction, were 2.2, 1.5 and 3.0 microM for B6, AK and D2 mice. IC50 values for inhibition by 3,4-dimethoxy-5-hydroxybenzoic acid were 0.83, 1.0 and 1.2 microM, whereas IC50 values for inhibition by S-adenosyl-L-homocysteine were 5.4, 6.6 and 5.8 microM for B6, AK and D2 mice, respectively. Half-life and slope values for thermal inactivation of hepatic TPMT were similar among B6, AK and D2 mice. No differences among strains in Rf values of the enzyme activity after electrophoresis were detected. Ion exchange chromatography with an NaCl gradient showed a major peak of TPMT activity that eluted with 51 to 56 mM NaCl for all three strains.(ABSTRACT TRUNCATED AT 250 WORDS)