NAD(P)H dehydrogenase and its role in the vitamin K (2-methyl-3-phytyl-1,4-naphthaquinone)-dependent carboxylation reaction

Abstract

A simple 3-step method was established for purification of NAD(P)H dehydrogenase (quinone) (''DT-diaphorase'', EC 1.6.99.2) from rat liver by affinity chromatography with a recovery of above 50%. The final enzyme preparation was purified about 750-fold and was electrophoretically homogeneous. Gel filtration showed that the enzyme had a MW of about 55,000, and 1 molecule of FAD was found/55,000 MW. Sodium dodecyl sulfate/polyacrylamide-gel electrophoresis gave a MW of about 27,000. Two N-terminal amino acids, asparagine/aspartic acid and glutamine/glutamic acid, were found in about equal yield, suggesting the presence of 2 non-identical polypeptide chains in the enzyme. NAD(P)H dehydrogenase was selectively removed by this affinity-chromatographic method from a microsomal carboxylation system. The system, which was solubilized by detergent and is dependent on vitamin K (2-methyl-3-phytyl-1,4-naphthaquinone or analogs with other side chains) lost its activity on removal of the enzyme. The activity can be completely restored to the system by adding purified cytoplasmic NAD(P)H dehydrogenase or by using the quinol form of vitamin K1 (2-methyl-3-phytyl-1,4-naphthaquinol).