A novel antioxidant mechanism of ebselen involving ebselen diselenide, a substrate of mammalian thioredoxin and thioredoxin reductase

The antioxidant mechanism of ebselen involves recently discovered reductions by mammalian thioredoxin reductase (TrxR) and thioredoxin (Trx) forming ebselen selenol. Here we describe a previously unknown reaction; ebselen reacts with its selenol forming an ebselen diselenide with a rate constant of 372 M(-1)s(-1). The diselenide also was a substrate of TrxR forming the selenol with K-m of 40 mum and k(cat) of 79 min(-1) (k(cat)/K-m of 3.3 x 10(4) m(-1)s(-1)). Trx increased the reduction because of its fast reaction with diselenide (rate constant 1.7 x 10(3) m(-1)s(-1)). Diselenide stimulated the H2O2 reductase activity of TrxR, even more efficiently with Trx present. Because the mechanism of ebselen as an antioxidant has been assumed to involve glutathione peroxidase-like activity, we compared the H2O2 reductase activity of ebselen with the GSH and Trx systems. TrxR at 50 nm, far below the estimated physiological level, gave 8-fold higher activity compared with 1 mm GSH; addition of 5 pm Trx increased this difference to 13-fold. The rate constant of ebselen selenol reacting with H2O2 was estimated to be faster than 350 m(-1)s(-1). We propose novel mechanisms for ebselen antioxidant action involving ebselen selenol and diselenide formation, with the thioredoxin system rather than glutathione as the predominant effector and target.