MEMBRANE LIPID-METABOLISM IN GERMINATING CASTOR BEAN ENDOSPERM

Castor bean (Ricinus communis L. cv. ''Hale'') endosperms, excised after 2 days germination at 30.degree. C, were incubated 5 min to 8 h with 14C-acetate and 3H-glycerol. Homogenates were fractionated by sucrose gradient centrifugation. Organelles found to be active in lipid synthesis were the lipid bodies and the endoplasmic reticulum. The products of incorporation in the lipid bodies were 3H-diglycerides containing 14C-fatty acids of more than 20 carbons. In contrast, the endoplasmic reticulum produced 3H-phospholipids as well as 3H-diglycerides rich in 14C-linoleate. The phospholipids synthesized and their acyl contents were of types known to be the major components of organelle membranes in this tissue. Phospholipids and diglycerides containing 14C and 3H were found in the glyoxysomes and mitochondria subsequent to their appearance in the endoplasmic reticulum. Germinating castor bean endosperm synthesizes membrane lipids de novo from acetate rather than reutilising stored lipid components directly. It is also apparent that the endoplasmic reticulum is responsible for several steps in membrane lipid production.