Comparison of angiotensinogen and tetradecapeptide as substrates for human renin. Substrate dependence of the mode of inhibition of renin by a statine-containing hexapeptide

The kinetic properties of two different substrates for human renin, a synthetic tetradecapeptide and the natural substrate human angiotensinogen, have been compared. While the Vmax was similar for the two substrates, the Km values differed by a factor of 10, i.e., 11.7 +/- 0.7 microM (tetradecapeptide) and 1.0 +/- 0.1 microM (angiotensinogen). The mode of inhibition of renin by a statine (Sta)-containing hexapeptide, BW897C, that is a close structural analog of residues 8-13 of human angiotensinogen (Phe-His-Sta-Val-Ile-His-OMe), was determined for the two substrates. Competitive inhibition was observed when tetradecapeptide was the substrate (Ki = 2.0 +/- 0.2 microM), but a more complex mixed inhibition mode (Ki = 1.7 +/- 0.1 microM, Ki' = 3.0 +/- 0.23 microM) was found with angiotensinogen as substrate. This mixed inhibition probably results from the formation of an enzyme-inhibitor-substrate or enzyme-inhibitor-product complex and reflects the more extensive interactions that the protein angiotensinogen, as opposed to the small tetradecapeptide substrate, can make with renin. We conclude that the mixed inhibition observed when angiotensinogen is used as renin substrate could be important in the clinical application of renin inhibitors because it is less readily reversed by increased concentrations of substrate than is simple competitive inhibition.