Spherosomes of Castor Bean Endosperm: MEMBRANE COMPONENTS, FORMATION, AND DEGRADATION

The membrane components of the castor bean spherosomes were characterized. The storage triacylglycerols of isolated spherosomes were extracted with diethyl ether, and the membrane was isolated by sucrose gradient centrifugation. It had an apparent equilibrium density of 1.12 grams per cubic centimeter, and possessed an antimycin A-insensitive NADH cytochrome c reductase and an acid lipase. Phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol in roughly equal amounts were the major phospholipids. The membrane proteins were resolved into several major and minor protein bands of molecular weights ranging from 10,000 to 70,000 by acrylamide gel electrophoresis, and the protein pattern in the gel was different from those of the endoplasmic reticulum, mitochondrial, and glyoxysomal membranes.The varying amounts of spherosomal components in the seed were followed throughout seed maturation and germination. A striking similarity existed in the developmental pattern of each of the spherosomal components. This finding suggests that the spherosome is synthesized and degraded as one individual unit. The spherosomes isolated from maturing seeds exhibited rapid hydrolysis of the storage lipid in vitro, thus raising the problem of cellular control in preventing in vivo autolysis of the spherosomes during seed maturation.