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 presumed to involve O-methylation of 2-hydroxy-3-alkylpyarzine (HP), although this reaction has never been demonstrated in organisms. We detected 2-hydroxy-3-isobutylpyrazine (IBHP) and 2-hydroxy-3-isopropylpyrazine (IPHP) in unripe grapes, and S-adenosyl-L-methionine-dependent O-methyltransferase (OMT) activity toward HP in crude extracts from young shoots and unripe grapes that accumulate MP at different levels. The levels of HP in the berries and stems were estimated by using 2-hydroxy-3-sec-butylpyrazine as an internal standard. The OMT activity observed in the crude extracts from young shoot and berries was extremely low, but no MP-decomposing activity was detected in the solutions. The levels of HP and OMT activity were closely related with the level of MP in the grapes. These results indicate that the predicted final step of MP biosynthesis exists in wine grapes.