Beef-heart malic dehydrogenases III. Comparative studies of some properties of M-malic dehydrogenase and S-malic dehydrogenase

Abstract

The two malic dehydrogenases from beef heart, identified as being supernatant and mitochondrial in origin, have been compared further with regard to kinetic behavior and additional striking differences have been observed. With respect to sulfhydryl groups of mitochondrial malic dehydrogenase and supernantant malic dehydrogenase the following have been established. First, mitochondrial malic dehydrogenase has twice the number of half-cystine residues contained in supernatant malic dehydrogenase (12 as opposed to 6). Secondly, all sulfhydryl groups of undenatured mitochondrial malic dehydrogenase can be titrated with p-mercuribenzoate, although reaction occurs slowly, while only half of the sulfhydryl groups of supernantant malic dehydrogenase can be titrated even in presence of excess reagent. Finally, the titration of mitochondrial malic dehydrogenase with p-mercuribenzoate results in loss of enzymic activity after addition of only three equivalents of reagent, whereas no loss of activity of supernatant-malic dehydrogenase occurs after half its sulfhydryl groups have been titrated. Significant differences in amino acid composition exist between mitochondrial malic dehydrogenase and supernatant malic dehydrogenase. The latter enzyme contains more lysine, arginine, tyrosine, methionine, aspartic acid and tryptophan than does the former. On the other hand, mitochondrial malic dehydrogenase contains more phenylalanine, glycine, proline and threonine.