The origin and turnover of organelle membranes in castor bean endosperm

The origin and turnover of organelle membranes in castor bean (Ricinus communis L. var. Hale) endosperm was examined using choline-(14)C as a phospholipid precursor. On sucrose gradients three major particulate fractions were separated; a light membranous fraction (density 1.11-1.13 gram per cm(3)), the mitochondria (1.18 gram per cm(3)), and the glyoxysomes (1.24 gram per cm(3)). Choline-(14)C was readily incorporated into lecithin in all three particulate fractions, but the light membranous fraction became labeled first. Incorporation continued into all three fractions for 6 hours, at which time the available choline-(14)C had been completely used. Subsequently, (14)C was lost from the three components at distinctly different rates. When an excess of unlabeled choline was added after 1 hour (pulse-chase experiment), incorporation of choline-(14)C into glyoxysomes and mitochondria continued for three hours, but at a diminishing rate. This was followed by a period in which the (14)C content of the mitochondria declined at a rate expected, if the half life of lecithin in the membrane were about 50 hours and that of the glyoxysomes 10 hours. These values are close to those calculated from the experiments in which no chase was used. The labeling in the light membrane fraction behaved differently from that of the mitochondria and glyoxysomes following the chase of unlabeled choline. Incorporation continued for only 1 additional hour, and then the (14)C content declined sharply in the subsequent 4 hours. The early kinetics and subsequent interrelationships are those expected if the lecithin in the membranes of mitochondria and glyoxysomes originates in components of the light membrane fraction.