S-adenosylhomocysteine analogues as inhibitors of specific tRNA methylation

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Of 17 base- or amino acid-modified analogues of S-adenosylhomocysteine, six were found to produce at least 50% inhibition of the activity of an unfractionated tRNA methyltransferase extract at concentrations of 200 μM. The inhibitory effects of these six analogues on five purified rat liver tRNA methyltransferases were examined. The purified enzymes differed greatly in their sensitivity to the analogues. K i values for the inhibitory analogues were determined for the three most highly purified methyltransferases. The kinetic analyses indicated that inhibition is competitive for nearly all enzyme/inhibitor combinations. The K i values for good enzyme/inhibitor pairs were in the range of 0.11–2 μM. Each analogue appears to inhibit one methylation more strongly than others; e.g. the K i values obtained for N 6- methyl-S- adenosyl- l-homocysteine are approx. 0.4 μM for guanine-1 tRNA methyltransferase, 6 μM for adenine-1 tRNA methyltransferase and 100 μM for N 2-guanine tRNA methyltransferase I. Structural features which are important for inhibitory activity are presence of a terminal amino group on the amino acid and the presence of adenosine rather than any other base. Ring nitrogens, a terminal carboxyl group and conformation at the asymmetric carbon appear to be important for some but not all of the tRNA methyltransferases examined.