Noninvasive procedure for in situ determination of skin surface aspartic proteinase activity in animals; Implications for human skin

The present study demonstrated a noninvasive procedure for in situ determination of stratum corneum aspartic proteinase in the living animal. A non-leaky well, containing [¹²⁵I]S-carboxymethylated insulin B-chain (ICMI) as a substrate, was constructed on the shaved back of anesthetized guinea pigs and rats. The enzymatic activity was determined by measuring the radiolabeled trichloroacetic acid soluble material. We demonstrated pepstatin-sensitive proteinase activity bound to the skin surface indicating the involvement of aspartic proteinase(s) such as cathepsin D and/or E. Aged rats had about six fold lower activity than young animals. The proteinase activity was inhibited by the alkylating agent mechlorethamine and by the cosmetic propylene glycol. A similar procedure was carried out with intact human skin pieces obtained during plastic surgery. The activity was inhibited by antihuman cathepsin D antibodies. Cathepsin D was immunohistochemicaliy localized in the corneal and granular layers of the epidermis. Skin surface aspartic proteinase/cathepsin D activity may serve as a marker for skin aging or for certain skin disorders leading to a new approach to their medical treatments.