Degradation of human epidermal keratin by fish pepsin

Stomach extract of Atlantic herring Clupea harengus, Atlantic salmon Salmo salar, cod Gadus morhua, redfish Sebastes marinus, and plaice Pleuronectes platessa, degraded human epidermal keratin effectively in vitro. The keratin-degrading activity of all extracts showed a pH optimum around 3.3–3.4, and sheets of plantar callus were degraded with about the same efficacy as keratin. Pepstatin sensitivity, heat lability, and the acidic pH optimum demonstrated that the keratin-degrading activity was pepsin. The keratin-degrading activity of cod stomach extract had a temperature optimum of around 42°C at optimal pH, and showed a similar pH dependency with collagen as with keratin as substrate. The keratin-degrading activity of pepsin I and pepsin II purified from cod showed a pH optimum of 3.7 and 3.1, respectively, similar to that obtained with hemoglobin as substrate. Pig pepsin showed a pH optimum of about 2 with keratin, hemoglobin, and collagen as substrates. The present investigation demonstrates that fish pepsin is effective in degrading human epidermal keratin in vitro, and in a contemporary study the same was shown with fish trypsin. This may suggest a possible mechanism for the development of irritative hand eczema caused by exposure to fish and acidified fish material.