Membrane-bound malate dehydrogenase in mitochondria from the alga Cyanidium caldarium

Abstract

Although in some organisms part of the mitochondrial malate dehydrogenase (EC 1.1.1.37) is membrane-bound, this enzyme is generally regarded as a matrix protein. The present paper describes a malate dehydrogenase from the thermo- and acidophilic alga Cyanidium caldarium, that is tightly bound to the mitochondrial membrane. Data obtained from organelle separations on sucrose density gradients, as well as polyacrylamide gel electrophoresis of crude and purified enzyme preparations, indicate that no malate dehydrogenase is present in the mitochondrial matrix. Part of the malate dehydrogenase on the mitochondrial membranes could be solubilized by treatment with deoxycholate. The solubilized enzyme showed a thermostability and electrophoretic mobility different from that of the soluble (cytosolic) malate dehydrogenase.