Amino acid sequence of dogfish muscle lactate dehydrogenase.

Lactate dehydrogenase isolated from vertebrate tissues catalyzes the reversible oxidation of L-lactic acid to pyruvic acid. The catalysis requires nicotinamide adenine dinucleotide (NAD’) as a coenzyme and is specific for the A-hydrogen of the nicotinamide ring (1) (Fig. 1). Although the enzyme is tetrameric, no cooperative effects have been observed between subunits (2). Each identical subunit, having a molecular weight of 36,000, binds 1 molecule of substrate and 1 molecule of coenzyme (3, 4). Using a variety of chemical modifications, several amino acid residues have been identified which participate in this oxidation-reduction process catalyzed by lactate dehydrogenase. Each polypeptide chain contains a single histidine residue which is essential for catalytic activity (5). Modification of 1 cysteine residue per subunit also inhibits the enzymatic activity, and a dodecapeptide containing this cysteine has been characterized from several lactate dehydrogenases (6-8). Several arginine residues have been implicated in the catalysis, but the exact number and identification of these arginines has not been ascertained by active site labeling (9, 10). Evidence also exists which indicates that 1 tyrosine residue per subunit is essential for enzymatic activity (11). The crystal structure is known for the apoenzyme (12, 13) and for the ternary complex of lactate dehydrogenase NAD+ . pyruvate (13), and these two structures are compared in detail by White et al. (14). In most cases, the crystallography has confirmed the earlier chemical modification data (15). The structure and function of lactate dehydrogenases have been comprehensively reviewed by Holbrook et al. (16). X-ray crystallography has also yielded extensive information regarding the secondary structure of the molecule and how this secondary structure might be related to the overall evolution of dehydrogenases and other nucleotide-binding proteins (17).