Interactions of Methionine and Selenomethionine with Methionine Adenosyltransferase and Ethylene-generating Systems

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Abstract
Since selenomethionine appears to be a better precursor of ethylene in senescing flower tissue of Ipomoea tricolor and in IAA-treated pea stem sections than is methionine, the effectiveness of selenomethionine and methionine to participate in reactions which may be connected to ethylene biosynthesis were compared. Evidence is presented that selenomethionine is also a better substrate of methionine adenosyltransferase (ATP: methionine S-adenosyltransferase, EC 2.5.1.6) from I. tricolor, the Vmax for selenomethionine being twice as high as that for methionine. The affinity of the enzyme is higher for methionine than for selenomethionine. Methionine added to flower tissue together with selenomethionine inhibits the enhancement of ethylene synthesis by the seleno analog. Methionine reduces the high, selenomethionine-dependent reaction rates of methionine adenosyltransferase from I. tricolor flower tissue. Selenomethionine is less effective as an ethylene precursor than is methionine in model systems involving oxidation by free radicals. Apparently activation of methionine by methionine adenosyltransferase and formation of S-adenosylmethionine are more likely to be involved in ethylene biosynthesis than is oxidation of methionine by free radicals.