Gibberellic acid decomposition and its loss of biological activity in aqueous solutions

In addition to gibberellic acid (GA(3)), significant amounts of iso-GA(3), an isomer of GA(3), accumulated in cultures media of the fungus Gibberella fujikuroi. It has been reported that iso-GA(3) is an intermediate in the decomposition of GA(3) to gibberellenic acid, a dicarboxylic acid lacking the gamma-lactone ring. In this work, we studied the decomposition of GA(3) in buffered aqueous solutions at pH 5 and 7, both by spectrophotometry, measuring the appearance of gibberellenic acid at 254 nm, and using a bioassay by measuring the induction of alpha-amylase in barley endosperm. Results indicate that at both pH values, biological inactivation of GA(3) is more rapid than the appearance of gibberellenic acid, suggesting that inactivation is largely due to conversion to the intermediate, iso-GA(3). Furtheremore, no induction of alpha-amylase activity in barley endosperm was obtained in bioassays with iso-GA(3).