Characterization of angiotensin converting enzyme by [³H]captopril binding

We demonstrate that [³H]captopril selectively labels angiotensin converting enzyme (EC 3.14.15.1) (ACE) and employ this technique to probe enzyme-inhibitor interactions. [³H]Captopril binding sites copurify with ACE activity from rat lung or rat brain. At each stage of the purification the V(max)/B(max) ratio, or k(cat) is 17,000 min^-1 with hippuryl-L-histidyl-L-leucine as substrate. The specificity of [³H]captopril binding is apparent in the similar pharmacologic profile of inhibition in crude and pure enzyme preparations. Furthermore, binding sites and enzyme activity comigrate in gel filtration and sucrose gradient sedimentation experiments. Equilibrium analysis of [³H]captopril binding to purified ACE reveals a B(max) of 6 nmol/mg of protein (K(D) = 2 nM), demonstrating the presence of one inhibitor binding site per polypeptide chain. The kinetics of [³H]captopril binding are characterized by monophasic association and dissociation rate constants of 0.026 nM^-1 min^-1 and 0.034 min^-1, respectively. The affinity of ACE for both [³H]captopril and enalaprilat is greater at 37° than at 0°, demonstrating that these interactions are entropically driven, perhaps by an isomerization of the enzyme molecule. The ionic requirements for [³H]captopril binding and substrate catalysis differ. Chloride and bromide ion, but not fluoride, are about 100-fold more potent stimulators of binding than catalysis. When the active site Zn²⁺ ion is replaced by Co²⁺, catalysis was stimulated 2-fold, whereas binding activity was decreased by 70%.