Malate dehydrogenase in leaf peroxisomes

Abstract

The technique of isopycnic centrifugation of leaf homogenates has allowed a separation of chloroplasts, mitochondria, and peroxisomes according to their respective densities. Chlorophyll, cytochrome c oxidase, and glycolate oxidase, respectively, were used as markers for these organeles. Malate dehydrogenase ( l-malate; NAD oxidoreductase, EC 1.1.1.37) showed peaks of activity in the mitochondrial and peroxisomal fractions. The peroxisomal enzyme as assayed by following reduced pyridine nucleotide oxidation had a broad pH optimum from 6.4 to 7.4 and was speciic for NADH. The peroxisomal and mitochondrial forms of malate dehydrogenase were differentiated by their kinetic and electrophoretic behavior. The mitochondrial from had a K m (oxalacetate) of 5.7 · 10 −6 M and was inhibited by oxalacetate concentrations above 7 · 10 −5 M. The peroxisomal form had a k m (oxalacetate) of 1.4 · 10 −5 M and was inhibited by oxalacetate concentrations in excess of 2 · 10 −4 M. The supernatant malate dehydrogenase showed kinetic characteristics intermediate to those of the peroxisomal and mitochondrial forms. Starch-gel electrophoresis of the supernatant fraction showed the presence of both the peroxisoml and mitochondrial forms together with another isozyme.