Arylamine N-methyltransferase and thiol methyltransferase activities in cholestatic rat liver induced by common bile duct ligation

Abstract

Methylation catalyzed by methyltransferases is a major metabolic pathway for an inactivation of some catecholamines, niacinamide as well as aliphatic sulfhydryl drugs and toxic hydrogen sulfides. To investigate the effects of obstructive jaundice in an animal model, common bile duct ligation (CBDL) was performed in the rat and enzyme activities of Sadenosyl- L-methionine-dependent arylamine Nmethyltransferase and thiol methyltransferase were examined in liver cell fractions and serum for a period of 42 d after CBDL. Both mitochondrial and microsomal arylamine N-methyltransferase showed significant increases in their activities between the 1st through the 7th day (P ú 0.05 to 0.001), and between the 1st through the 28th day (P ú 0.01 to 0.001) post-ligation, although the cytosolic arylamine N-methyltransferase activity did not show a significant change compared to the activities from the sham-operated control. The mitochondrial as well as microsomal thiol methyltransferase showed significant increases in their activities between the 1st through the 28th day (P ú 0.05 to 0.01 and P ú 0.01 to 0.001, respectively) post-ligation, although the cytosolic thiol methyltransferase activity did not show a significant change compared to the activities from the sham-operated control. Arylamine N-methyltransferase and thiol methyltransferase in the serum from cholestatic rats also showed significant increases in their activities between the 1st through 28th day (P ú 0.01 to 0.001), and between the 0.5th through the 42nd day (Pú0.05 to 0.001) post-ligation compared to the sham-operated control, respectively. Enzyme kinetic parameters (Km and Vmax) of hepatic membrane-bound arylamine N-methyltransferase and thiol methyltransferase were analyzed with the preparation from the 7th day post-ligation, using tryptamine or 4-chlorothiophenol as substrates and S-Adenosyl-L-[methyl-3H]methionine as co-substrate. The results indicate that although the Km values were about the same as the sham-operated control, the Vmax values of both enzymes increased significantly (Pú0.01 and 0.001, respectively). These results suggest that the biosynthesis of arylamine N-methyltransferase and thiol methyltransferase have been induced in response to obstructive jaundice. Keywords: arylamine N-methyltransferase, cholestatic rat liver, enzyme induction, thiol methyltransferase