Lactate dehydrogenase
Biochemistry and fwnction of lactate dehydrogenase
Lactate dehydrogenase (EC 1.1.1.27, L-lactate: NAD+ oxidoreductase) is a n ubiquitous enzyme among vertebrate organisms', and carries out a straightforward equilibration of: NAD' + lactate + NADH + pyruvate Lactate dehydrogenase activity is typically measured spectrophotometrically a t 340 nm by following either the oxidation of NADH with pyruvate (decrease in absorbance) or reduction of NADt with lactate (increase in absorbance) a t rates producing a change in absorbance of 0.05 to 0.1 per min. Under standard conditions one unit of enzyme catalyses the oxidation of NADH or reduction of NAD' a t 1 pmol per min. Lactate dehydrogenase (LDH) is also present in many invertebrates2, in plants3, and in many microbes4x5; in a few of these organisms D-lactate serves as the s u b ~t r a t e ~. ~~. ~~ and in yeast, cytochrome C serves in lieu of NAD. The widespread occurrence of LDH stems from the fact that the enzyme plays a critical role in glycolytic metabolism. It permits organisms to generate a temporary oxygen debt in the form of accumulated lactate to be later discharged by the reoxidation of lactate to pyruvate when oxygen becomes available.
Lactate dehydrogenase has received great attention because of its important metabolic role and because of its existence in several isozymic forms6. These isozymes are tetrameres generated by the association of two or three different kinds of subunits7 in all higher vertebrates. Each kind of subunit, A, B, or C, is encoded in a specific gene -Ldh 1, Ldh 2, and Ldh 3, respectively. Two different subunits generate five tetrameric isozymes: A4, A1B3, A2B2. A3B1 and B47, and three subunits can generate 15 isozymes. for isozyme subunits of several species'6. f s 5 . I 7 hut numer-