FIG. 5.

Cis-bixin inhibits thioredoxin and thioredoxin reductase activities, but not glutathione reductase activity. (A, B) Effects of cis-bixin on thioredoxin reductase activity. DTNB was used as the substrate, and the rate of change in absorbance at 405 nm, as shown in the inset in (A), reflects activity of thioredoxin reductase. (B) Effects of cis-bixin compared with those of related carotenoids cis-norbixin, acitretin, and crocin on thioredoxin reductase activity. (C) Effects of cis-bixin on thioredoxin activity as assessed by the insulin precipitation assay, with rate of change in absorbance at 620 nm shown as a percentage of diluent control activity. (D) Effects of cis-bixin on glutathione reductase activity. DTNB was used as the substrate; absorbance was monitored at 405 nm and plotted as percentage of control of diluent activity. Error bars, +1 standard deviation; statistical significance, *p < 0.02 and **p < 0.001; each panel contains representative results of one of three distinct experiments conducted in triplicate, except for reference drug experiments in (B), performed in duplicate.