The cathepsin L propeptide (phcl-2) was expressed in Saccharomyces cerevisiae using a human procathepsin L/alpha-factor fusion construct containing a stop codon at position -1 (the C-terminal amino acid of the proregion). Since the yield after purification was very low, the cathepsin L propeptide was also obtained by an alternate procedure through controlled processing of an inactive mutant of procathepsin L (Cys25Ser/Thrl10Ala) expressed in Pichia pastoris, by small amounts of cathepsin L. The peptide resulting from the cleavage of the proenzyme (phcl-1) was then purified by HPLC. The purified propeptides were characterized by N-terminal sequencing and mass spectrometry and correspond to incomplete forms of the proregion (87 and 81 aa for phcl-1 and phcl-2 respectively, compared to 96 aa for the complete cathepsin L propeptide). The two peptides were found to be potent and selective inhibitors of cathepsin L at pH 5.5, with Ki values of 0.088 nM for phcl-1 and 0.66 nM for phcl-2. The Ki for inhibition of cathepsin S was much higher (44.6 nM with phcl-1), and no inhibition of cathepsin B or papain could be detected at up to 1 microM of the propeptide. The inhibitory activity was also found to be strongly pH-dependent. Two synthetic peptides of 75 and 44 aa corresponding to N-terminal truncated versions of the propeptide were also prepared by solid phase synthesis and displayed Ki values of 11 nM and 2900 nM, respectively, against cathepsin L. The data obtained for the 4 propeptide derivatives of various lengths indicate that the first 20 residues in the N-terminal region of the propeptide are more important for inhibition than the C-terminal region which contributes little to the overall inhibitory activity.

The autocatalytic processing of procathepsin L was investigated in vitro using purified recombinant proenzyme expressed in Pichia pastoris. Pure intermolecular processing was studied by incubating the mutant procathepsin L (C25S), which cannot autoactivate with a small amount of mature active cathepsin L. The results clearly establish that, contrary to recent reports, intermolecular processing of procathepsin L is possible. The main cleavage sites are located at or near the N terminus of the mature enzyme, in an accessible portion of the proregion, which contains sequences corresponding to the known substrate specificity of cathepsin L. Contrary to procathepsins B, K, and S, autocatalytic processing of procathepsin L can generate the natural mature form of the enzyme. A continuous assay using the substrate benzyloxycarbonyl-Phe-Arg 4-methylcoumarinyl-7-amide hydrochloride has also been used to obtain information on the nature of the steps involved in the autocatalytic processing of wild-type procathepsin L. Processing is initiated by decreasing the pH from 8.0 to 5.3. The influence of proenzyme concentration on the rate of processing indicates the existence of both unimolecular and bimolecular steps in the mechanism of processing. The nature of the unimolecular event that triggers processing remains elusive. Circular dichroism and fluorescence measurements indicate the absence of large scale conformational change in the structure of procathepsin L on reduction of pH. However, the bimolecular reaction can be attributed to intermolecular processing of the zymogen.Cathepsin L is a lysosomal cysteine protease that plays a major role in intracellular protein degradation (1, 2). Like many mammalian proteases, cathepsin L is synthesized as an inactive preproenzyme, which is subsequently processed to the mature form (3, 4). Cleavage of the 96-residue proregion, which is located between the signal sequence and the N terminus of the mature enzyme, is necessary to generate the fully active 221-residue mature enzyme (5); therefore, the proregion serves as a regulator of catalytic activity. Accordingly, the propeptide of cathepsin L has been shown to be a potent inhibitor of the mature protease (6). The proregion is also required for the proper folding of the protein (7). Procathepsin L is stable at high pH, and the proregion protects the protein from the denaturing effect of neutral to alkaline pH (8). The proregion also mediates the pH-dependent membrane association of procathepsin L, which may play a role in transport to the lysosome or activation of the proenzyme (9). In addition to its role in protein degradation, evidence has accumulated for the participation of cathepsin L in various physiological and pathological processes, e.g. tumor invasion and metastasis (10, 11), bone resorption (12), spermatogenesis (13), and arthritis (14, 15). In some cases, extracellular events involve the zymogen form (e.g. Refs. 16 and 17), and the role for the proregion in regulation of the catalytic activity and stabilization of procathe...

Exposure of human synovial cells and fibroblasts in monolayer culture to interleukin 1 results in prominent secretion of proteins with Mr values of 6000 and 7000. By N-terminal sequence analysis, the Mr-6000 protein is identified as the protein encoded by a recently described gro mRNA. The Mr-7000 protein is identical to a neutrophil chemotactic factor released from monocytes. Stimulation of normal human fibroblasts with tumour necrosis factor alpha also results in expression and secretion of these two proteins. In addition to these cytokine-induced proteins, we have identified beta 2-microglobulin as an Mr-8000 protein constitutively secreted by synovial cells.