Lipoamide dehydrogenase from human liver.

Lipoamide dehydrogenase (reduced nicotinamide adenine dinucleotide :lipoamide oxidoreductase, EC 1.6.4.3) has been isolated from acid-precipitated human liver particles in a highly purified state by a freezing and thawing technique instead of the heat treatment used by other workers. The enzyme was found to have a molecular weight of 138,000, and to contain 2 moles of flavin adenine dinucleotide per mole of protein. In these respects, as in substrate specificity, Michaelis constant, absorption spectrum, and kinetic parameters, the enzyme is similar to preparations isolated from bovine liver, pig heart, and bacteria. This enzyme catalyzed the reduction of nicotinamide adenine dinucleotide by dihydrolipoamide and exhibited diaphorase and transhydrogenase activities. If the enzyme was incubated with the dithiol inhibitors such as arsenite and cadmium chloride in the presence of NADH, lipoamide dehydrogenase and transhydrogenase activities were strongly inhibited, but diaphorase activity was enhanced.