Reduced thioredoxin: a possible physiological cofactor for vitamin K epoxide reductase. Further support for an active site disulfide.

Vitamin K 2,3-epoxide reductase activity from liver microsomes requires only a thiol cofactor, particularly dithiothreitol (DTT). In order to identify a likely physiological cofactor, reduced lipoic acid and reduced thioredoxin were tested as cofactors in beef and rat liver microsomal systems. Reduced lipoic acid is only about one-third as active as DTT in both systems. Thioredoxin, however, is significantly more active than either DTT or reduced lipoic acid in both systems; thioredoxin binds 188 times better than does DTT. The thioredoxin must be in the reduced form since omission of either thioredoxin reductase or NADPH results in complete loss of enzyme activity. The concentration of DTT required to obtain maximal enzyme activity may be as much as 485 times greater than the corresponding concentration of reduced thioredoxin that gives the same enzyme activity.