Organelle Membranes from Germinating Castor Bean Endosperm: II. ENZYMES, CYTOCHROMES, AND PERMEABILITY OF THE GLYOXYSOME MEMBRANE.
Glyoxysome ghosts were isolated from germinating castor bean endosperms using established methods. Electron microscopic examination showed that some matrix material was retained within the glyoxysomal membrane. Two cytochrome reductases and phosphorylcholine glyceride transferase co-sedimented with the alkaline lipase, a known component of the glyoxysome membrane, in sucrose gradient centrifugation of osmotically shocked glyoxysomes. The activities of these enzymes in the glyoxysome membranes were compared to those in the endoplasmic reticulum relative to phospholipid content. On this basis, the phosphorylcholine glyceride transferase was 10-fold more active in the endoplasmic reticulum, whereas the lipase was 50-fold more active in the glyoxysome membrane. The cytochrome reductases were only 2-fold more active in the endoplasmic reticulum, indicating that they are components of the two membranes. Difference spectroscopy of the glyoxysome membrane suspension revealed the presence of a b5-type cytochrome similar to that found in the endoplasmic reticulum. Since the glyoxysome membrane is apparently derived from the endoplasmic reticulum, components of the endoplasmic reticulum such as these are likely to be incorporated into the glyoxysome membrane during biogenesis.Enzyme activities involving the cofactors NADH or CoA were measurable in broken, but not in intact, glyoxysomes. Thus, it appears that cofactors for enzymes within the organelle cannot pass through the membrane.
Donaldson
R P
RP
Department of Biological Sciences, George Washington University, Washington, DC 20052.