Following the cleavage reaction xanthoxin moves out of the chloroplast and is converted to ABA by cytoplasmic enzymes. In contrast to the cleavage reaction, the later steps in ABA synthesis have been characterized extensively by feeding potential intermediates to intact plants and by cell-free extracts. For the cell-free extracts, the only cofactor required for the conversion of xanthoxin to ABA was NAD or NADP. A variety of potential intermediates were also fed
to cell-free extracts to determine the likely sequence of reactions (37). If a substrate was not converted to ABA by the cell-free system it was considered an unlikely intermediate in vivo. These experiments indicated that the first step
in the conversion of xanthoxin to ABA is the oxidation of the 4'-hydroxyl to a ketone. The remainder of the ring modifications, i.e. the desaturation of the 2'-3' bond and opening of the epoxide ring, are thought to be non-enzymatic.
Indeed, chemical oxidation of the 4’-hydroxyl is sufficient to convert xanthoxin to AB-aldehyde.