To the Editor.—
I would like to comment on the recent article by Schallreuter and Pittelkow1 on thioredoxin-reductase activity in psoriasis and its inhibition by anthralin. Schallreuter and Pittelkow present experimental data that thioredoxin-reductase activity in patients with psoriasis is elevated in involved, but not in uninvolved, skin. The measurements of thioredoxin-reductase activity in skin biopsy specimens are based on a bioassay, developed by Schallreuter et al.2 Using electron resonance spectroscopy (ESR), the signal loss of a membraneimpermeable, cationic nitroxide (free radical) is monitored in skin biopsy specimens and keratinocytes. The nitroxide is reduced in the epidermis to an ESR silent product. It is presumed that reduction of this particular nitroxide is specific for thioredoxin reductase in the epidermis. Indeed, under test tube conditions, nitroxides are reduced by mammalian and Escherichia coli thioredoxin reductase in the presence of nicotinamide-adenine dinucleotide phosphate (NADPH). There is, however, absolutely no rationale
Fuchs J. Validity of the `Bioassay' for Thioredoxin-Reductase Activity. Arch Dermatol. 1988;124(6):849–850. doi:10.1001/archderm.1988.01670060007002
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