The role of calcium in the regulation of free radical reduction by thioredoxin reductase at the surface of the skin.

Membrane associated thioredoxin reductase has been previously shown to reduce free radicals on the outer plasma membranes of human keratinocytes and melanocytes to provide a possible first line of defense against free radical damage at the surface of the skin. Preliminary experiments with cell cultures of human keratinocytes and melanocytes grown in serum-free medium showed that the enzyme activity depends on extracellular calcium concentration in the medium. Thioredoxin reductase activity at the surface of the skin, at the surface of human keratinocytes and melanocytes, and purified thioredoxin reductase from E. coli and adult human keratinocytes all exhibited calcium-dependent allosteric control. Since thioredoxin reductase contains two extremely reactive thiolate groups at the active site with pK values close to neutrality, both of these anions can form covalent complexes with N-ethylmaleimide by nucleophilic attack on the double bond. In our experiments we used spin-labeled maleimide [4-maleimido-tempo] to examine the local environment in the active site of thioredoxin reductase in the presence and absence of calcium. Both spin-labeled thioethers are distinguishable by EPR spectroscopy, with one site being significantly more immobilized than the other. Hence, it has been possible to observe direct evidence for active site closure by calcium. These results suggest that extracellular calcium may play an important role in regulation of thioredoxin reductase activity for the defense mechanism against UV-mediated free radical damage at the surface of human skin.