Spectrophotometric method for selective assay of the five isoenzymes of human lactate dehydrogenase, based on their different stabilities at alkaline pH.

After preincubation of human lactate dehydrogenase (EC 1.1.1.27) isoenzymes 1 through 5 with lactate at pH 9.4, 9.8, 10.25, and 10.6 at 30 degrees C for 10 min, the reduction of NAD+ was measured at the same pH values and temperature during the interval 1-2 min after adding the coenzyme. Relative to the reference activities measured at pH 8.7 by the method of Buhl et al. (Clin Chem 23: 1289-1295, 1977), the respective activities of isoenzymes 1 through 5 thus measured were 111, 104, 96, 68, and 0% at pH 9.4; 123, 108, 88, 0, and 0% at pH 9.8; 140, 100, 0, 0, and 0% at pH 10.25; and 138, 0, 0, 0, and 0% at pH 10.6. These relations allow selective assay of the respective isoenzymes in samples containing mixtures of them. Optimal conditions for such selective assay at 37 degrees C are reported.