Formation and properties of lactate dehydrogenase inhibitors in NADH

We describe some characteristics of the mode of formation of inhibitors of lactate dehydrogenase from commercial NADH. Inhibitor formation is time- and concentration-dependent and also varies with the commercial source of NADH. At least two inhibitory components can form in concentrated NADH solutions. One of these can be separated from NADH by chromatography on either diethylaminoethyl-celluose or diethylaminoethyl-Sephadex; the second cannot. The NADH-associated inhibitor appeared to be present in each of the three commercial NADH preparations studied. The 260 nm/340 nm absorbance ratio was of no help in locating this inhibitor during chromatography.