Mitochondria and Glyoxysomes from Castor Bean Endosperm: Enzyme Constituents and Catalytic Capacity

Abstract

The recent isolation of the glyoxysome, a cellular organelle containing the enzymes of the glyoxylate cycle, raised the possibility that this metabolic sequence may be compartmentalized in plant tissues actively converting fat to carbohydrate. To sustain this view it is necessary to demonstrate that all of the pertinent enzymes are present in quantities consistent with the conversion of fat to sucrose in the intact tissue. To this end we have assayed each of the enzymes of the citric acid and glyoxylate cycles in preparations of mitochondria and glyoxysomes from 5-day-old castor bean endosperm. This survey shows that the glyoxysome contains more than 85% of the total isocitratase and malate synthetase activities, and the mitochondria less than 5%. With the exception of aconitase, which appears to be easily solubilized, all enzymatic activities observed in the glyoxysome are sufficient to support rates of acetate metabolism that are greater than those known to be occurring in the intact tissue. In addition to those enzymes which have been previously reported to be compartmented in the glyoxysome, a specific glutamate: oxalacetate transaminase has been found in very high concentrations in both the glyoxysomes and the mitochondria. Our inability to detect succinic dehydrogenase and fumarase in the glyoxysome suggests that the succinate produced from isocitrate cleavage must be transported to the mitochondria (where these activities are high) before undergoing oxidation to malate. With the use of isolated particle preparations, isotopic evidence supporting this conclusion has been obtained.