Structural Studies on Heart Muscle Malate Dehydrogenases

Often in a single cell there may be several different forms of an enzyme catalyzing the same reaction. In some instances these enzymes may be very closely related, e.g., the carbonic anhydrases (Kannan et al., this volume). The two different forms of the malate dehydrogenases that are present are most readily distinguished by their physical separation in the cell. For example, cardiac muscle cells contain a mitochrondrial malate dehydrogenase (m-MDH) and a soluble (cytoplasmic) malate dehydrogenase (s-MDH) (Englard and Breiger, 1962). There is currently no evidence that these two forms are homologous enzymes. However, judging from their general physical properties and amino acid compositions it would seem that they are not closely related. A brief summary of the chemical properties of pig heart malate dehydrogenases is given in Table I. Note that both forms of the enzyme have approximately the same molecular weight. The m-MDH is a basic protein whereas...