S-methylation of 2-mercaptopyrazine in rat liver microsomes and cytosol.
1. 2-(Allylthio)pyrazine (2-AP) has been demonstrated to protect the liver against toxicants by inhibiting CYP2E1 activity. Since 2-mercaptopyrazine (2-MP) is presumed to be a metabolite of 2-AP, the experiments were performed to determine whether rat liver microsomal and/or cytosolic preparations could catalyse the S-methylation of 2-MP. 2. It was found that both rat liver microsomes and cytosol could catalyse the S-methylation of 2-MP. The microsomal activity displayed biphasic substrate kinetics, with apparent Km = 8.44+/-2.68 and 417+/-74 microM for the high- and low-affinity activities respectively. The high-affinity activity had an apparent Km for S-adenosyl-L-methionine (Ado-Met) of 3.52 microM. The cytosolic activity also displayed biphasic substrate kinetics, with apparent Km of 3.26+/-0.62 and 91.6+/-23.1 microM for the high- and low-affinity activities respectively. 3. The microsomal S-methylation of 2-MP was inhibited by 2,3-dichloro-alpha-methylbenzylamine (DCMB), SKF-525A and benzylamine, known microsomal thiol methyltransferase (TMT) inhibitors, whereas cytosolic activity was inhibited by anisic acid and 3-chlorobenzoate, which also inhibit cytosolic thiopurine methyltransferase (TPMT). Both activities were inhibited by S-adenosyl-L-homocysteine (Met-Hcy). 4. These results suggest that both TMT and TPMT may be involved in the in vivo methylation of 2-MP.
Lee
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DH
Bioanalysis and Biotransformation Research Center, Korea Institute of Science and Technology, Chungryang, Seoul.
Kim
D H
DH
D000814:Aniline Compounds / Q000494:pharmacology; D000818:Animals; D001596:Benzylamines / Q000494:pharmacology; D002723:Chlorobenzoates / Q000494:pharmacology; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D008297:Male; D008745:Methylation; D008780:Methyltransferases / Q000037:antagonists & inhibitors / Q000378:metabolism; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism*; D011335:Proadifen / Q000494:pharmacology; D011719:Pyrazines / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D014641:Vanillic Acid / Q000031:analogs & derivatives / Q000494:pharmacology
C104935:2-(allylthio)pyrazine; C402065:2-mercaptopyrazine; D000814:Aniline Compounds; D001596:Benzylamines; D002723:Chlorobenzoates; D004791:Enzyme Inhibitors; D011719:Pyrazines; C036427:3-chlorobenzoic acid; C002569:2,3-dichloro-alpha-methylbenzylamine; C030796:benzylamine; D011335:Proadifen; C056267:3-methoxybenzoic acid; D008780:Methyltransferases; C022745:thiopurine methyltransferase; C022742:thiol S-methyltransferase; D014641:Vanillic Acid; C023650:aniline