Biochemical demonstration and immunohistochemical localization of calpain in human skin.

The biochemical properties and immunohistochemical localization of calpain, a Ca++-dependent, intracellular, nonlysosomal cysteine proteinase was examined in human skin. Human epidermal calpain I was fractionated on a DEAE-cellulose column and was found to be half-maximally activated at 3.5 microM free Ca++ and fully activated at 10 microM Ca++ as measured by casein hydrolysis. Immunoelectrophoretic blotting of calpain revealed only a single band of Mr 83,000, when the blot was made with affinity-purified anti-calpain I heavy subunit IgG. Immunohistochemical staining of normal human epidermis showed that calpain I was localized in the cytoplasm of keratinocytes in the mid to upper epidermis but not in the basal cells. In untreated psoriatic epidermis, the deposition of this proteinase was visualized weakly just beneath the stratum corneum. However, remarkable staining was observed after photochemotherapy of topical psoralen plus long-wave UV irradiation. Whether the photochemotherapy induced a quantitative increase in the amount of calpain or merely made calpain more stainable by altering the membrane remains unknown.