Kinetic study of the interaction between rat haptoglobin and rat liver cathepsin B

One of the roles of the acute phase reactants (APR), according to Koj, is to regulate the action of tissue proteinases released during the inflammatory reaction. To study this phenomenon in vitro, rat liver cathepsin B (EC 3.4.22.1), a lysosomal proteinase, and a typical APR, rat serum haptoglobin, were purified. By kinetic study, haptoglobin was found to inhibit cathepsin B and the inhibitory activity of a competitive type was increased with the molecular weight of the substrate. When 125I-labelled denatured bovine serum albumin (BSA) was used as a substrate, the apparent Michaelis constant (Km app.) for cathepsin B was 5 +/- 0.4 X 10(-6) M. When haptoglobin was added, the apparent inhibition constant (Ki app.) was 3.5 +/- 1.5 X 10(-8) M. Native haptoglobin was not catabolized by cathepsin B while under the same conditions denatured haptoglobin was degraded by the enzyme.