Metabolite Oxidation and Transport in Mitochondria of Endosperm From Germinating Castor Bean

Abstract

Mitochondria from germinating castor bean endosperm were purified on linear sucrose-density gradients. It is shown that succinate and malate plus glutamate are oxidized rapidly; other substrates including malate (via malic enzyme) and external NADH are oxidized slowly. It is proposed that the oxidation of NADH produced during β-oxidation and the glyoxylate cycle occurs intramitochondrially via malate. Castor bean mitochondria are relatively impermeable to oxaloacetate and a malate-aspartate type shuttle is required. Metabolite transport and exchange properties support the operation of malate_in (or succinate_in)/2-oxoglutarate_out and glutamate_in/aspartate_out shuttles. Not all the exchange systems were reversible. The results support proposed schemes for metabolite transfer between glyoxysomes and mitochondria of castor bean endosperm during germination.