Quantitative aspects of triacylglycerol metabolism and sterol synthesis from 14C-acetate in etiolated seedlings of Euphorbia lambii

Triacylglycerols occur in both the endosperm and embryo of Euphorbia lambii seeds. Upon germination, the amount of these neutral lipids in the endosperm decreased with 1.06 mg fatty acid day^-1. The embryo contained 1.4 mg fatty acids in the triacylglycerols and this value declined slowly to 0.4 mg seedling^-1 during the 8 day period of endosperm depletion. Radioactive acetate was rapidly taken up by the cotyledons of intact seedlings, translocated throughout the entire seedling, and up to 10.5% of the ¹⁴C proceeded to the sterols and latex triterpenols. Maximum uptake values of 1.4 μmol seedling^-1 day^-1 of acetate were measured. Acetate uptake and subsequent incorporation into sterols and triterpenols decreased substantially in the presence of increasing amounts of sucrose (up to 0.3 M). Traces of acetate did not effect [¹⁴C]-sucrose uptake and corresponding synthesis of [¹⁴C]-sterols and triterpenols, but increased concentrations of acetate (0.05 M and up) reduced both uptake of sucrose and its conversion into unsaponifiable lipids.
The uptake capacity of the cotyledons for [¹⁴C]-glycerol exceeded the daily production in the endosperm, but only a small amount of label proceeded to the sterols and triterpenols. [¹⁴C]-Triacylglycerols were never detected in the seedling, regardless of the labeled substrate used. Although acetate is an efficient precursor in triterpenol and sterol synthesis, the uptake capacity of the cotyledons for this metabolite is too small in relation to the daily production of water soluble substrates in the endosperm. If acetate is released by the endosperm, only a marginal contribution towards triterpenol and sterol synthesis in the seedling is to be anticipated from this substrate.