[Inhibiting effect of S-adenosyl-L-homocysteine and its structural analogs on the process of enzymatic methylation of tRNA].

The action of S-adenosyl-l-homocysteine (S-Ado-Hcy), its four structural analogues S-Ino-Hcy, S-Guo-Hcy, S-Urd-Hcy, S-Cyd-Hcy and the five corresponding sulfoxides on tRNA methylases has been investigated. The data obtained in the study of overall incorporation of 14CH3-groups into an unfractioned tRNA preparation suggested that both the affinity of the inhibitors tested for various methylases and the type of inhibition were different. The experiments performed with unfractioned tRNA preparation permit to get an idea of the average inhibitory potency of each of the compounds. The study of their action on individual tRNA methylases by means of fractionation of minor components produced demonstrated that the affinity of the inhibitors tested for various methylases was really different. Thus, S-Ado-Hcy, S-Ino-Hcy and S-Urd-Hcy practically do not inhibit m1A methylase but have the highest affinity for m5C methylase. In an experiment with tRNAPhe which is a substrate for a single, namely m5C methylase, the type of inhibition of this methylase by S-Cyd-Hcy was revealed; it was found to be non-competitive with respect to S-Ado-Met, and the S-Cyd-Hcy concentration reducing the methylation by 50 percent was 1.2-10(-4) M.