Biosynthesis of the Methyl Donors S-adenosylmethionine and S-methylnlethionine in Barley Seedlings (Hordeum vulgare L.)

Abstract

Summary Methionine adenosyltransferase was partially purified from 2-day-old shoots of barley seedlings. The specific activity of methionine adenosyltransferase was highest in shoots of 2-day-old seedlingsand then the activity rapidly declined. The total activity was also at maximum during germination in the shoots after 4 to 6 days. The specific activity of methionine adenosyltransferase was lower in the roots than in shoots. Barley methionine adenosyltransferase required divalent cations for activity. The greatest activity was found with Mgp2+ or Mn{2+}. K + or NHf4 + increased the enzyme activity. The pH optimum of the enzyme was 8 to 9. The Kfm value of ATP was found to be 0.2 mM, but in the case of methionine non-linear kinetics were obtained, the double reciprocal plots curving downwards. The Kfm value for methionine was estimated to be 0.2 mM. The products in the reaction, S-adenosylmethionine and tripolyphosphate inhibited the enzyme activity. S-adenosylmethionine and tripolyphosphate exhibits non-competitive inhibition patterns versus methionine with Ki values 1.5 mM and 0.1 mM, respectively. S-adenosylmethionine and tripolyphosphate also exhibits non-competitive inhibition patterns versus ATP with Kfi values 1.0 and 0.05 mM, respectively. Methionine S-methyltransferase was partially purified from shoots of 3-day-old barley seedlings. The enzyme does not require Mnp2 + or Znp2+for activity. The Kfm values for methionine and S-adenosylmethionine were both 0.2 mM. The pH optimum for the enzyme activity was 6 to 7. Product inhibition studies showed competitive inhibition between methionine and S-methylmethionine, and between S-adenosylmethionine and S-adenosylhomocysteine with Kfi values 0.3 mM and 0.3 mM, respectively. S-adenosylhomocysteine showed non-competitive inhibition versus methionine with a Kfi of 1.0 mM. The occurrence of methionine S-methyltransferase during germination of the barley seedlings were also investigated, and the enzyme was found in shoot and root tissue and in embryos.