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

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 S-adenosyl-L-methionine-dependent arylamine N-methyltransferase 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 < or = 0.05 to 0.001), and between the 1st through the 28th day (P X or = 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 < or = 0.05 to 0.01 and P < or = 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 < or = 0.01 to 0.001), and between the 0.5th through the 42nd day (P < or = 0.05 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 (P < or = 0.01 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.

Arylamine Acetyltransferase Activity from a Regenerating Liver after Partial Hepatectomy and from a Cholestatic Liver after Common Bile Duct Ligation in Rats

A study was made of the change in arylamine acetyltransferase(AAT) activity in regenerating and/or cholestatic rat livers. Cytosolic, mitochondrial and microsomal AAT activities were determined over a period of 10 days in rat livers which were regenerating after 70%(median and left lateral lobes) partial hepatectomy and over a period of 42 days in rat livers with cholestasis induced by a common bile duct ligation. The values of Km and Vmax in these hepatic enzymes were measured. Both the cytosolic and the microsomal AAT activities in the regenerating rat livers showed significant increases from the first day to the third day after the partial hepatectomy. However, the mitochondrial AAT activity did not change. The cytosolic and the microsomal AAT activities in the cholestatic rat livers showed a significant increase on the first day and from the first day to the second day, respectively after the ligation; Both the cytosolic and the microsomal AAT activities showed significant decreases from the fourteenth day to the forty-second day after the ligation. However, the mitochondrial AAT activity did not change. The Vmax values of both the cytosolic and the microsomal AAT activity in the regenerating and/or cholestatic rat livers showed significant increases on the first day after the partial hepatectomy and/or the ligation. However, the Vmax values of both the cytosolic and the microsomal AAT activities in the cholestatic rat livers showed significant decreases on the twenty-eighth day after the ligation. On the other hand, the Km values of the above enzymes did not change. In view of the above results, the AAT activity in the regenerating rat liver appears to be due to the enzyme increasing its biosynthesis in the regenerating stage. The AAT activity in the cholestatic rat liver suggests that the enzymes is increasing its biosynthesis in the severe necrotizing stage, but decreasing its biosynthesis severe hepatic dysfunction stage.

Thiosulfate sulfurtransferase and UDP-glucuronosyltransferase activities in cholestatic rat liver induced by common bile duct ligation

We have investigated the effect of cholestasis on the hepatic thiosulfate sulfurtransferase (rhodanese) and UDP-glucuronosyltransferase (UDP-GT) activities in rats. Rhodanese activities in the liver cytosol, mitochondria and microsomal fractions as well as in the rat serum, and UDP-GT activity in the microsome have been investigated for a period of 42 days after common bile duct (CBD) ligation. The cytosolic rhodanese activity showed a significant decrease between the first through the 42nd day, and the mitochondrial activity showed a significant decrease between the 7th through the 42nd day after CBD ligation compared to the activities from the sham operated control, respectively. In the case of microsomal preparation, both rhodanese and UDP-GT also showed significant decrease in their activities after the ligation for the former enzyme between the 14th and the 42nd days, and for the latter enzyme between the third and 42nd days, respectively. On the other hand, the serum rhodanese activity increased markedly soon after the ligation, exhibiting the peak activity after 1 day of CBD ligation with about 4.6-fold increment. The activity subsequently decreased gradually reaching to the control level at the 42nd day post-ligation. Enzyme kinetic parameters of hepatic rhodanese and UDP-GT were analyzed using sodium thiosulfate and p-nitrophenol as substrates, respectively, with the preparations from the 28th day post-ligation. The results indicated that although the K-m values of these enzymes were about the same as the sham-operated control, the V-max values of the both enzymes decreased significantly. These results, therefore, suggest that the biosynthesis of rhodanese and UDP-GT have been reduced in response to cholestasis, and that the elevation of rhodanese activity in the serum is most likely due to leakage from the liver subsequent to CBD ligation.