Oxidation of 8'-hydroxy abscisic acid in Black Mexican Sweet maize cell suspension cultures

In a biotransformation study to prepare deuterium labelled phaseic acid (PA) from deuterated abscisic acid (ABA), the product contained fewer deuterium atoms than expected. Thus, spectroscopic data of isolated deuterated PA prepared from biotransformation of (+)-5,8',8',8'-d4-ABA in maize (Zea mays L. cv. Black Mexican Sweet) cell suspension cultures showed 83% deuterium incorporation at the 8'-exo position. Also, metabolism studies of (+)-4,5-d2-ABA in maize resulted in the isolation of deuterium labelled ABA derivatives, namely PA, dihydrophaseic acid (DPA), 4'-O-beta-D-glucopyranosylDPA, 8'-hydroxyPA, 8'-hydroxyDPA and 8'-oxoDPA, as deduced from spectroscopic methods. These combined results suggested the presence of an aldehyde intermediate which is either: (a) reduced to 8'-hydroxyABA and cyclized to PA, or (b) is hydrated and cyclized to 8'-hydroxyPA or (c) is further oxidized to the acid and cyclized to 8'-oxoPA. The chemical synthesis of this intermediate, as well as its biotransformation in maize cell cultures is presented.