Ethionine inhibits in vivo methylation of nuclear proteins

Nuclear protein methylation was studied in regenerating rat liver by giving [methyl-3H]methionine 45 h after partial hepatectomy. Ethionine, a liver carcinogen, has been shown to alter the methylation patterns in a basic protein (histone) fraction, as well as an acidic protein (non-histone) fraction present in a 0.25 N HCl nuclear extraction. The proteins present in the 0.25 N HCl extraction were separated by chromatography using a Bio-Rex 70 cation exchange column. Polyacrylamide gel electrophoresis and total amino acid analysis showed the first protein fraction contained acidic large molecular weight non-histone proteins, while the second fraction contained basic small molecular weight histone proteins. Both fractions were then hydrolyzed, and the amino acids chromatographed on an Aminex A-5 cation exchange column. The histones were found to contain epsilon-N-mono, di and trimethyllysine derivatives; whereas the non-histone fraction contained these lysine derivatives and additional basic amino acid identified as NG,NG-dimethylarginine. Ethionine (0.5 mg/g body weight) was found to inhibit in vivo methylation of lysine to form epsilon-N-mono, di and trimethyllysine, 46, 52 and 68%, respectively. The formation of NG,NG-dimethylarginine was inhibited by 85%. Ethylation of these proteins was also studied by giving [ethyl-3H]ethionine. After hydrolysis, the non-histones were found to contain a labeled lysine and arginine derivative, but in the histone fraction only labeled lysine was found.