Isozyme-specific enzyme inhibitors. 12. C- and N-methylmethionines as substrates and inhibitors of methionine adenosyltransferases of normal and hepatoma rat tissues.

The 2-, 3-, and 4-mono-C-methyl derivatives of D,L-methionine (Met) have been resolved into the 10 possible enantiomeric forms having the configurations 2-Me-D, 2-Me-L, 3(alpha or beta)-Me-D, 3(alpha or beta)-Me-L, 4(alpha or beta)-Me-D, and 4(alpha or beta)-Me-L (the alpha designation was given to enantiomeric pairs that had higher Rf values on silica gel chromatograms than their diastereomeric counterparts). All compounds were weak, poorly selective inhibitors of the rat M-2 (normal tissue) and M-T (Novikoff ascitic hepatoma) variants of Met adenosyltransferase. Kinetic analysis of the three most effective showed them to be competitive inhibitors with respect to Met with both variants; the strongest inhibition (KM(Met)/Ki = 0.03) was that of M-T by 3 beta-Me-L-Met. The Me-Met enantiomers had low substrate efficiencies (Vmax/KM) in the range (0.5-2.2) X 10(-4) that of L-Met with M-2 (0.2-1.3) X 10(-3) with M-T among seven compounds studied. At a 4 mM level, seven of the enantiomers were converted to adenosylmethionine derivatives more rapidly by M-T than by M-2. Among these, 2-Me-L-Met, 3 alpha-Me-L-Met, 3 alpha-Me-D-Met, and 4 beta-Me-D-Met had little or no substrate activity with M-2. These differences in substrate specificity are potentially exploitable in the design of compounds with selective toxicity for rat tumor tissue. N-Me- and N-(n-Bu)-Met, and the Met analogue in which NH is substituted for S, were weak inhibitors of M-T and M-2 and showed no substrate activity at a level of 4 mM.