Thioredoxin reductase. Role in free radical reduction in different hypopigmentation disorders.

Membrane-associated thioredoxin reductase (TR) has been discovered to reduce free radicals at the surface of the skin. An accurate bioassay for this enzyme has been developed by using a spin-labeled quaternary ammonium salt as a free radical substrate. Enzyme activity has been correlated with the surface area, and units of specific activity have been determined as the sequential decrease in nitroxide radical reduction per 3-mm punch biopsy per ten minutes. The TR activity in a random population of 30 healthy volunteers with different skin types (Fitzpatrick classification I through VI) could be correlated to the skin type. Ten patients with untreated vitiligo, two with piebaldism, three with albinism, and two with postinflammatory leukoderma were examined and the findings were compared with the expected rates for the individuals' skin types. The results from this survey on the human population support our previous molecular experiments on the control of melanin biosynthesis by TR in the epidermis.