Isolation und Charakterisierung von Sphärosomen und Glyoxisomen aus Tabakendosperm

Abstract

1. Sphaerosomes (oil droplets) extracted and isolated from tobacco endosperm contain over 90% of the total endosperm lipids. About 99% of the dry matter of these organelles consist of reserve lipids. 2. Upon germination 90% of the lipid reserves are mobilized within 96 hours. The hydrolysis of the neutral fat is achieved by lipolytic activity present in sphaerosomes. This activity could be demonstrated in sphaerosomes of resting seeds. 3. A large fraction of the total activities of proteases, acetyl esterase, phosphatase, RNase and DNase present in the endosperm extract is localized in sphaerosomes. These enzyme activities are present in resting seeds. Protease, acetyl esterase and phosphatase activities increase temporarily in the course of germination. 4. The sphaerosomes of tobacco endosperm are interpreted as lysosomes in which not only the accumulation and mobilization of the reserve lipids take place, but also the breakdown of autophagocytized cytoplasmic material. The morphological and functional relations between sphaerosomes and other types of vacuoles are discussed. 5. Enzymes of the glyoxylic acid cycle present in the endosperm extract from germinating seeds can be sedimented together with mitochondrial enzymes. Upon centrifugation of the mitochondrial fraction in density gradients of sucrose mitochondria can be separated from glyoxisomes. 6. In addition to isocitrate lyase and malate synthetase catalase is associated with glyoxisomes. The former enzymes are present almost exclusively in the fraction of glyoxisomes whereas about 90% of the latter occur in the soluble fraction. 7. The function of glyoxisomes in the conversion of fatty acids into sugar is discussed.