The cyclization of 8′-hydroxy abscisic acid to phaseic acid in vivo

Abstract

The cyclization of 8′-hydroxyabscisic acid to phaseic acid was investigated by feeding RS-[3′,5,7- 2D 6]abscisic acid to tomato ( Lycopersicon esculentum) shoots, the 1′- S-enantiomer was metabolized to dihydrophaseic acid-4′- O-β- d-glucopyranoside (DPAGS) which was isolated as the acetylated methyl ester and subjected to 500 MHz 1H NMR. The 2H atom from C-3′ of abscisic acid (ABA) was confined to the axial 3′- pro-S position of DPAGS (numbered as in ABA) and so the carbanion presumed to be formed at C-3′ during cyclization is protonated from the oxymethylene bridge, β-face. Enolization of the 3′-hydrogen atoms of phaseic acid in D 2O at pH 10.55 exchanged the axial hydrogen atom (α-face) preferentially. Thus the carbanion present at this position during cyclization is protonated from the opposite side. This suggests that the reaction that gives rise to phaseic acid is catalysed enzymically.