Autoproteolytic activation and processing of human polymorphonuclear leucocyte (PMNL) type IV procollagenase (gelatinase) was initiated by HgCl2 and was investigated by kinetic analysis and N-terminal sequence determination of the reaction products. In the first instance the propeptide domain was lost by subsequent cleavage of the Asp15-Leu16, Glu40-Met41, Leu52-Leu53 and Ala74-Met75 peptide bonds. The PRCGVPD sequence motif (residues Pro78-Asp84), which is conserved in all metalloproteinases and expected to be relevant for latency, remained uncleaved at the activated enzyme. The generated intermediate was further processed by three C-terminal cleavages. The Glu666-Leu667, Ala506-Glu507 and Ala398-Leu399 bonds were hydrolysed sucessively. From the fragmentation products we were able to conclude that three released fragment peptides contained unpaired free cysteine with the residues Cys497, Cys653, Cys683. Cleavage of the first C-terminal peptide bond resulted in the loss of one of the conserved Cys residues of the hemopexin-like domain, whereas the Cys residue of the PRCGVPD motif was retained at the fully active enzyme. The possibility of an entirely different activation mechanism for PMNL type IV procollagenase is discussed.
