Radiolabelled Substrates for Angiotensin Converting Enzyme

Abstract

Six [³H]benzoyl-tripeptides were prepared and tested as substrates for angiotensin converting enzyme. Each was prepared first as its [4-iodo]- benzoyl-analog, and an atom of ³H per molecule was introduced by catalytic dehalogenation in ³H₂-gas. Kinetic parameters were measured at 37°C using as buffer 0.05 M Hepes, pH 8.0, containing 0.1 M NaCl and 0.6 M Na₂SO₄. When the substrates were used at concentrations far below their respective Km values, fractional rates of substrate utilization per unit time for constant enzyme concentration were direct functions of respective second order rate constants (Kc/Km). Although absolute values of Kc/Km differed for human enzyme as opposed to rabbit enzyme, relative values of Kc/Km were virtually identical. Similarly, relative rates of substrate utilization during passage through lungs of anesthetized rats were similar to relative values of Kc/Km measured in vitro. Thus, there is now a range of ACE substrates usable, in vitro and in vivo, under conditions of first order enzyme kinetics, conditions under which values of V/Km and Ki can be measured directly.