Rat epidermal cathepsin L-like proteinase: purification and some hydrolytic properties toward filaggrin and synthetic substrates.

We have purified cathepsin L-like proteinase from rat epidermis, determined its NH2-terminal amino acid sequence, and investigated its proteolytic activities on an intermediate filament-associated protein filaggrin and several synthetic substrates. The amino acid sequence of its NH2-terminus was determined to be Val-Pro-Asn-Ser-Leu-Asp-Trp-Arg-Glu-Lys-Gly-Tyr-Val-Thr-Pro-, which differed from that of rat cathepsin L and was not found in the amino acid sequence data bank. The enzyme consisted of a single-chain form with M(r) 30,000. Its hydrolytic properties toward synthetic substrates were similar to those of cathepsin L in other tissues. The enzyme effectively proteolyzed rat epidermal filaggrin into small fragments at pH 4.0-6.0 and was inhibited by a specific cysteine proteinase inhibitor, N-[N-(L-3-trans-carboxyoxirane-2-carbonyl)L-leucyl]-agmatin. However, cathepsins D and E from rat epidermis did not hydrolyze filaggrin. This study demonstrated that filaggrin was susceptible to degradation by rat epidermal cathepsin L-like proteinase, suggesting that this proteolytic activity may have relevance to skin differentiation, in which acid proteases are thought to participate.