The loss of carbon-20 in C19-gibberellin biosynthesis in a cell-free system from Pisum sativum L.

The fate of the carbon-20 atom in gibberellin (GA) biosynthesis was studied in a cell-free system from Pisum sativum. This carbon atom is lost at the aldehyde stage of oxidation when C20-GAs are converted to C19-GAs. Gibberellin A12 labeled with (14)C at C-20 was prepared from [3'-(14)C]mevalonic acid with a cell-free system from Cucurbita maxima and incubated with the pea system. Analysis of the gas and aqueous phases showed that (14)CO2 was formed at the same rate and in nearly equivalent amounts as (14)C-labeled C19-GAs whereas [(14)C]formic acid and [(14)C]formaldehyde were not detectable. The possibility that C-20 had been lost as formic acid which had then been converted to CO2 was investigated by control incubations with [(14)C]formic acid. The rate of release of (14)CO2 from [(14)C]formic acid was only one fiftieth of the rate of (14)CO2 release from [(14)C]GA12 as the substrate. We conclude that in the formation of C19-GAs from C20-GAs, the C-20 is removed directly as CO2.