Multiple species of mammalian S-adenosylmethionine synthetase. Partial purification and characterization

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Abstract
Two species of S-adenosylmethionine (S-Ado-Met) synthetase (EC 2.5.1.6) exist in rat liver cytosol and a distinct species of the enzyme exists in kidney cytosol. S-Ado-Met synthetases .alpha. and .beta. in rat liver cytosol were partially purified about 200- and 80-fold, respectively. The apparent MW estimated by gel filtration and the sedimentation coefficient are 210,000 and 9 S for S-Ado-Met synthetase .alpha. and 160,000 and 5.5 S for S-Ado-Met synthetase .beta.. Both enzymes absolutely require Mg2+ and K+ for the activity and are completely inhibited by p-(chloromercuri)benzoate. Kinetic studies indicate that S-Ado-Met synthetases .alpha. and .beta. exhibit negative cooperativity with low S0.5 (ligand concentration required for half-maximal velocity) for L-methionine (17 .mu.M) and ATP (0.5 mM) and positive cooperativity with much higher S0.5 values (S0.5 (L-methionine) = 0.5 mM, S0.5 (ATP) = 2 mM), respectively. The cryoprotectants dimethyl sulfoxide and glycerol markedly lower the S0.5 values of S-Ado-Met synthetase .beta. without significant effect on Vmax. A single species of S-Ado-Met synthetase was purified about 1000-fold from rat kidney cytosol. The kidney enzyme, termed S-Ado-Met synthetase .gamma., has an apparent MW of 190,000 and a sedimentation coefficient of 7.5 S and is resistant to the inhibition by p-(chloromercuri)benzoate. S-Ado-Met synthetase .gamma. exhibits slightly negative cooperativity with an apparent S0.5 value for L-methionine of 6 .mu.M and for ATP of 70 .mu.M.