OXIDATION OF MALATE BY ISOLATED TURNIP (BRASSICA-NAPUS L) MITOCHONDRIA .2. MALATE-OXIDIZING ENZYMES, NUMBER AND LOCATION

The enzymes of malate oxidation in turnip mitochondria have been partially purified and some of their properties studied. Turnips contain a cytoplasmic malate dehydrogenase and two mito-chondrial malate dehydrogenases. These are all distinct isoenzymes with different immunblogical properties but similar molecular weights. The Km for malate is relatively high (8.3 mM) in the mito-chondrial enzymes. One of the mitochondrial enzymes is located in the matrix while the other is membrane-bound but within the matrix compartment. This latter enzyme, which retains its NAD and activity when submitochondrial particles are prepared, is responsible for the first phase of malate oxidation in submitochondrial particles.Two malic enzymes were isolated: One, an NADP enzyme, is a minor component and was not studied further; the other, immunologically distinct from the malate dehydrogenases, is probably located in the matrix compartment. The Km for malate oxidation (1.4 mM) is relatively low. This malic enzyme which apparently lacks OAA decarboxylase activity is NAD-specific but is unstable. The possibility of multiple malic enzymes is discussed. The malic enzymes are responsible for the second NAD-requiring phase of malate oxidation in submitochondrial particles. © 1979 Oxford University Press.