Mouse thiopurine methyltransferase pharmacogenetics: correlation of immunoreactive protein and enzymatic activity

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of aromatic and heterocyclic sulfhydryl compounds including thiopurine drugs such as 6-mercaptopurine. Genetic polymorphisms control levels of TPMT enzymatic activity in both humans and inbred mice. AKR/J (AK) and C57BL/6J (B6) mice have genetically low, whereas DBA/2J (D2) mice have genetically high levels of hepatic TPMT activity. No biochemical differences in TPMT have been detected among these strains of mouse. That observation increased the possibility that inherited variations in TPMT activity in these animals might be correlated with variations in the quantity of enzyme protein, just as they are in human tissue. Polyclonal rabbit antibodies to partially purified mouse liver TPMT were used to develop an immunoprecipitation assay for the enzyme. The volume of antiserum required to titrate 50% of the TPMT activity (antibody dose 50 or AD50) was used as a measure of immunoreactive TPMT protein. When AD50 values were measured in hepatic preparations from AK, B6 and D2 mice, inherited variations in levels of hepatic TPMT activity in these inbred strains were correlated directly with the quantity of TPMT immunoreactive protein. Identical results were obtained with F1 hybrids derived from AK X D2 and B6 X D2 matings. Finally, TPMT enzyme activity and immunoreactive protein levels were also correlated directly in hepatic preparations from six recombinant inbred (RI) strains derived from AK X D2 (AKXD) and six RI strains derived from B6 X D2 (BXD) matings. In neither AKXD nor BXD RI mice were there significant differences between TPMT homospecific activities in animals homozygous for the alleles for high or low TPMT activity.(ABSTRACT TRUNCATED AT 250 WORDS)