Membrane lipid metabolism in germinating castor bean endosperm.

Castor bean (Ricinus communis L. var. Hale) endosperms, excised after 2 days germination at 30 C, were incubated 5 min to 8 hr with (14)C-acetate and (3)H-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 (3)H-diglycerides containing (14)C-fatty acids of more than 20 carbons. In contrast, the endoplasmic reticulum produced (3)H-phospholipids as well as (3)H-diglycerides rich in (14)C-linoleate. The phospholipids synthesized and their acyl contents were of the types known to be the major components of organelle membranes in this tissue. Phospholipids and diglycerides containing (14)C and (3)H were found in the glyoxysomes and mitochondria subsequent to their appearance in the endoplasmic reticulum. The results show that germinating castor bean endosperm synthesizes membrane lipids de novo from acetate rather than reutilizing stored lipid components directly. It is also apparent that the endoplasmic reticulum is responsible for several steps in membrane lipid production.