Lipoprotein secretion by rat liver Golgi apparatus. Lipoprotein particles and lipase activity

Lipoprotein particles of the size range of very low density lipoproteins in smooth endoplasmic reticulum, peripheral elements of the Golgi apparatus, and secretory vesicles of the immature Golgi apparatus face are 55 to 80 nm in diameter. Particles in mature secretory vesicles are smaller (45 nm). Concomitant with the change in particle size, the lumina of mature vesicles increase in electron density. A technique to fractionate immature and mature secretory vesicles was based on precipitation of a cupric-ferrocyanide complex (Hatchett's brown) through the action of a NADH-ferricyanide oxido-reductase resistant to glutaraldehyde which is characteristic of the membranes of mature secretory vesicles and of the plasma membrane of liver. Mature secretory vesicle fractions so isolated were enriched in cholesterol and depleted in triglycerides relative to immature vesicles on a phospholipid basis. Lipase activity was present in secretory vesicle fractions of the Golgi apparatus as shown by biochemical analysis and by cytochemistry. Cytochemical studies showed lipase to be present in both mature and immature vesicles but most evident in immature vesicles. The findings suggest that some very low density lipoprotein particles are converted to particles of smaller diameter during transit through Golgi apparatus. A lipase-mediated hydrolysis of triglycerides may relate to the transformation.