S-adenosyl-L-methionine-dependent O-methylation of 2-hydroxy-3-alkylpyrazine in wine grapes: A putative final step of methoxypyrazine biosynthesis

The final step of 2-methoxy-3-alkylpyrazine (MP) biosynthesis has been pres umed to involve O-methylation of 2-hydroxy-3-alklylpyarzine (HB), although this reaction has never been demonstrated in organisms. We detected 2-hydro xy-3-isobutylpyrazine (IBHP) and ? 2-hydroxy-3-isopropylpyrazine (IPHP) in unripe grapes, and S-adenosyl-L-methionine-dependent O-methyltransferase (O MT) activity toward HP in crude e?;tracts from young shoots and unripe grap es that accumulate II-IF at different levels, The levels of HP in the berri es :Ind stems were estimated by using 2-hydroxy-3-sec-butylpyrazine as an i nternal standard. The OMT activity observed in the crude extracts from youn g shoot and berries was extremely low, but no MP-decomposing activity was d etected in the solutions. The Levels of HP and OMT activity were closely re lated with the level of MP in the grapes. These results indicate that the p redicted final step of MP biosynthesis exists in wine grapes.