An immunoglobulin A1 that inhibits lactate dehydrogenase activity, with reversal of inhibition by addition of NADH.

We discovered a patient with low serum lactate dehydrogenase (LD) activity and an abnormal LD isozyme pattern. We analyzed the patient's LD inhibitor using electrophoresis, affinity chromatography, and immunochemical technologies. The LD activity of the patient's serum was inhibited more strongly at 4 degrees C than at 37 degrees C. The decrease of LD activity was more marked in a mixture of the patient's serum with purified LD5 than in that with purified LD1. The immunoglobulin responsible for LD inhibition was an IgA1-lambda. The LD inhibition by the patient's IgA1 was blocked by reduction and alkylation and by NADH. Polymerization of the patient's IgA1 might play an important role in its interaction with LD. Moreover, the possibility exists that part of the patient's IgA1 molecule fits into a pocket of LD in instead of NADH. This is the first report of NADH reversing such LD inhibition.