Malate Dehydrogenases

Abstract

Malate dehydrogenases (diphosphopyridine nucleotide-linked) have been purified from Bacillus subtilis, Bacillus stearothermophilus, and Escherichia coli. Each enzyme preparation is homogeneous in its ultracentrifugational, electrophoretic, and immunochemical properties. Sephadex gel filtration studies show that the enzymes from the two Bacillus species are equal in size and that the enzymatically active forms are larger than the enzymatically active form of the E. coli malate dehydrogenase. Ultracentrifugal analyses of the native and acid-dissociated enzymes from B. subtilis and E. coli indicate that the former (mol wt 117,000) is composed of 4 subunits while the latter (mol wt 60,000) is composed of 2 subunits. Amino acid analyses and peptide mapping of tryptic digests of the former indicate that the subunits of B. subtilis enzyme are identical. There are some similarities in the amino acid compositions of the B. subtilis and E. coli enzymes, but the former is lacking both tryptophan and cysteine, while both amino acids are present in the latter enzyme. Corrected excitation and emission fluorescence spectra are presented which support the analytical values obtained for the tryptophan contents of these two enzymes. The malate dehydrogenase of B. stearothermophilus is more heat stable and has a higher optimum temperature for catalytic activity than the enzymes from the two mesophilic bacteria.