PURIFICATION AND PROPERTIES OF S-ADENOSYL-L-METHIONINE-L-METHIONINE S-METHYLTRANSFERASE FROM WOLLASTONIA-BIFLORA LEAVES

The plant enzyme S-adenosylmethionine:methionine S-methyltransferase ( EC 2.1.1.12, MMT) catalyzes the synthesis of S-methylmethionine. MMT w as purified 620-fold to apparent homogeneity from leaves of Wollastoni a biflora, The four-step purification included fractionation with poly ethylene glycol, affinity chromatography on adenosine-agarose, anion e xchange chromatography, and gel filtration, Protein yield was about 18 0 mu g/kg of leaves, Estimates of molecular mass from sodium dodecyl s ulfate-polyacrylamide gel electrophoresis and native gel filtration ch romatography were, respectively, 115 and 450 kDa, suggesting a tetrame r of 115-kDa subunits, The 115-kDa subunit was photoaffinity labeled b y S-adenosyl[H-3]methionine. Antibodies raised against W. biflora MMT recognized a 115-kDa polypeptide in partially purified MMT preparation s from leaves of lettuce, cabbage, clover, and maize. The pH optimum o f W. biflora MMT was 7.2, Kinetic analysis of substrate interaction an d product inhibition patterns indicated an Ordered Pi Bi mechanism, wi th S-adenosylmethionine the first reactant to bind and S-adenosylhomoc ysteine the last product to be released, The enzyme catalyzed methylat ion of selenomethionine and ethionine, but not of S-methylcysteine, ho mocysteine, cysteine, or peptidylmethionine. Tests with other substrat e analogs indicated that a free carboxyl group was required for enzyme activity, and that a free amino group was not.