[Comparative characteristics of tRNA-methyltransferases from rabbit liver and myocardium under normal conditions and...

Comparative studies of the state of aggregation and activity of tRNA-methyltransferases in cytosol (105,000 X g supernatant) from normal and ischemic rabbit liver and myocardium were carried out. The optimal conditions (pH, protein concentration, ionic composition of incubation mixture) for the determination of activity of tRNA-methyltransferases were elaborated. The protein fraction precipitated at 55% saturation of ammonium sulfate was shown to inherit the highest activity of tRNA-methyltransferases. In rabbit liver cytosol, the bulk of the tRNA-methyltransferase activity (approximately 50%) was found to be associated with high molecular weight complexes containing aminoacyl-tRNA-synthetases. The tRNA-methyltransferase activity was increased almost 1.4-fold both in the myocardium cytosol under total ischemia of isolated heart (30 min, 37 degrees C) and in liver cytosol under experimental myocardial infarction (EMI, occlusion of anterior coronary artery for 12 hours). Moreover, the labilization of high molecular weight complexes was observed: up to 80% of the tRNA-methyltransferase activity was localized in the fraction of lower molecular weight complexes and free enzyme fraction. In the total set of eight methylated nucleotides (products of submethylated tRNA methylation by liver enzymes), the decreased m1A content and the increased m7G and m1G contents were observed at EMI. It was assumed that the observed changes in the state of aggregation of tRNA-methyltransferases, in particular, their dissociation from the high molecular weight amino-acyl-tRNA-synthetase complexes are prerequisites for the suppression of protein biosynthesis under ischemic conditions.