Inhibition of cathepsin L and B by haptoglobin, the haptoglobin–hemoglobin complex, and asialohaptoglobin. "In vitro" studies in the rat

In broadening our research on the inhibition of cathepsin B (EC 3.4.22.1) by rat haptoglobin, we have used the haptoglobin–hemoglobin complex and asialohaptoglobin. The inhibition of cathepsin L (EC 3.4.22.15), another lysosomal thiol proteinase, by haptoglobin and its related molecules has also been investigated.With azocasein as substrate, both enzymes were inhibited by both haptoglobin and its related molecules. When azocasein was used as a substrate, the apparent Michaelis constant (Km, app.) for cathepsin L was 1 × 10−5 ± 0.4 × 10−5 M. When haptoglobin was added, the apparent inhibition constant (Ki, app.) was 3 × 10−8 ± 2.5 × 10−8 M.The results suggest that rat haptoglobin specifically inhibits lysosomal thiol proteinases and that it has a regulatory role in tissue proteolysis associated with the inflammatory reaction. On the other hand, these properties would seem to be peculiar to the systems rat haptoglobin – rat liver cathepsin B or L.