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 in 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 (K(m) constants for 6-mercaptopurine were 0.98, 0.75 and 1.1 mM for B6, AK and D2 mice, respectively. Apparent K(m) values of S-adenosyl-L-methionine, the methyl donor for the reaction, were 2.2, 1.5 and 3.0 μM for B6, AK and D2 mice. IC50 values for inhibition by 3,4-dimethyoxy-5-hydroxybenzoic acid were 0.83, 1.0 and 1.2 μM, whereas ICV50 values for inhibition by S-adenosyl-L-homocysteine were 5.4, 6.6 and 5.8 μM 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 R(f) 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. Studies were than performed with 25 recombinant inbred (RI) strains derived from B6 and D2 mice (BXD) and 23 RI strains derived from AK and D2 mice (AKXD). Eleven BXD and 12 AKXD strains had low hepatic TPMT activities, whereas 14 BXD and 11 AKXD strains had high hepatic TPMT activities. These results supported the conclusion that the level of hepatic TPMT activity in these mice is inherited as a monogenic trait. Studies of RI animals also showed that the genetic locus regulating level of TPMT activity in these mice, Tpmt, was closely linked to a locus for a lymphocyte alloantigen, Ly-28. Unfortunately, the chromosomal location of the Ly-28 locus, and thus of Tpmt, is unknown. These RI animals will be very useful in future pharmacologic and toxicologic studies of the TPMT genetic polymorphism.
|Original language||English (US)|
|Number of pages||7|
|Journal||Journal of Pharmacology and Experimental Therapeutics|
|State||Published - 1987|
ASJC Scopus subject areas
- Molecular Medicine