ISOTHIAZOLONES INTERFERE WITH NORMAL MATRIX METALLOPROTEINASE ACTIVATION AND INHIBIT CARTILAGE PROTEOGLYCAN DEGRADATION

A series of isothiazolones that inhibit pro-(matrix metalloproteinase) (proMMP) activation but do not inhibit the active enzyme are effectiv e as cartilage protectants in bovine nasal cartilage organ culture, pr eventing interleukin-1 (IL-1)-induced proteoglycan (aggrecan) degradat ion without affecting its synthesis. These compounds were found to bin d to prostromelysin (proMMP-3) in a non-dialysable and stoichiometric manner. Preincubation with cartilage-protectant isothiazolones prevent ed the binding of [C-14]iodoacetamide to Cys(75) Of the MMP-3 propepti de, suggesting that the activity of these compounds involves their bin ding to the Cys(75) of the MMP zymogen. Studies following chymotrypsin activation of proMMP-3 by SDS/PAGE indicated that altered processing of the 57 kDa zymogen to the active form occurred in the presence of c ompound. The 53 kDa intermediate seen on normal activation was not for med; instead a different intermediate appeared with a molecular mass o f approx. 46 kDa. N-terminal sequence analysis indicated that this int ermediate was formed by cleavage at the putative 4-aminophenylmercuric acid cleavage site. Importantly the 45 kDa active MMP-3 species forme d in the presence of compound was one amino acid residue shorter than the native MMP-3. These results suggest that the inhibition of cartila ge proteoglycan degradation by isothiazolones might be due to their ab ility to bind to the Cys(75) in the propeptide region of the MMP zymog en and interfere with its normal activation process.