Metabolism of [3H]Gibberellin $\text{A}_{5}$ by Immature Seeds of Apricot (Prunus armeniaca L.)

Immature seeds of apricot (Prunus armeniaca L.) were fed the native gibberellin $\text{A}_{5}$ ($\text{GA}_{5}$) as 1- and 1,2-$[{}^{3}\text{H}]\text{GA}_{5}$ (5.3 Curies per millimole to 16 milliCuries per millimole) at doses (42 nanograms to 10.6 micrograms per seed) 2 to 530 times the expected endogenous level. After 4 days of incubation, seeds were extracted and free [3H]GA-like metabolites were separated from the highly H2O-soluble [3H]metabolites. For high specific activity feeds the retention times (Rts) of radioactive peaks were compared with Rts of authentic GAs on sequential gradient-eluted → isocratic eluted reversed-phase C18 high performance liquid chromatography (HPLC) -radiocounting (RC). From high substrate feeds (530 and 230 × expected endogenous levels) HPLC-RC peak groupings were subjected to capillary gas chromatography-selected ion monitoring (GC-SIM), usually six characteristic ions. The major free GA metabolites of $[{}^{3}\text{H}]\text{GA}_{5}$ were identified as $\text{GA}_{1}$, GA3, and $\text{GA}_{6}$ by GC-SIM. The major highly water soluble metabolite of $[{}^{3}\text{H}]\text{GA}_{5}$ at all levels of substrate $\text{GA}_{5}$ had chromatographic characteristics similar to authentic $\text{GA}_{1}$-glucosyl ester. Expressed as a percentage of recovered radioactivity, low substrate $[{}^{3}\text{H}]\text{GA}_{5}$ feeds (2 × expected endogenous level) yielded a broad spectrum of metabolites eluting at the Rts where $\text{GA}_{1}$, GA3, $\text{GA}_{5}$ methyl ester, $\text{GA}_{6}$, $\text{GA}_{22}$, $\text{GA}_{29}$ (17, 14, 1.6, 7, 1.1, 0.5%, respectively) and GA glucosyl conjugates of $\text{GA}_{1}$, GA3, $\text{GA}_{5}$, and $\text{GA}_{8}$ (33, 11, 1, 0.1%, respectively) elute. Metabolites were also present at Rts where GA glucosyl conjugates of $\text{GA}_{6}$ and $\text{GA}_{29}$ would be expected to elute (8 and 0.1%, respectively). Only 5% of the radioactivity remained as $\text{GA}_{5}$. Increasing substrate $\text{GA}_{5}$ levels increased the proportion of metabolites with HPLC Rts similar to $\text{GA}_{1}$, $\text{GA}_{6}$, and especially $\text{GA}_{1}$ glucosyl ester, primarily at the expense of metabolites with HPLC Rts similar to GA3, GA3-glucosyl ester, and a postulated conjugate of $\text{GA}_{6}$. There was evidence that high doses of substrate $\text{GA}_{5}$ induced new metabolites which often, but not always, differed from $\text{GA}_{1}$, GA3, and $\text{GA}_{6}$ in HPLC Rt. These same metabolites, whe