Quantification of pulmonary capillary endothelium-bound angiotensin converting enzyme inhibition in man

Angiotensin converting enzyme (ACE, kininase II) is an endothelial luminal ectoenzyme expressed abundantly on the pulmonary capillary endothelium and recognized as the site for the conversion of circulating angiotensin I to II. In the present study, we have applied recently developed methodologies for assaying pulmonary capillary endothelium-bound (PCEB) ACE activity in man, to estimate the interaction of an ACE inhibitor (enalaprilat) with PCEB ACE in human subjects. Trace amounts of the specific ACE substrate, ³H-benzoyl-Phe-Ala-Pro (³H-BPAP; 40 Ci or 2 nmol), was injected as a bolus into the subclavian vein and immediately blood was withdrawn from a radial arterial catheter. Plasma concentrations of surviving substrate and product (³H-benzoyl-Phe) were estimated and BPAP utilization was calculated during a single transpulmonary passage, at baseline (T₀) and at 15 min (T₁₅) and 2 h (T₁₂₀) after intravenous administration of 1.5 g/kg enalaprilat in 12 normotensive subjects. This treatment had no significant effect on mean arterial pressure (91 ± 6 vs. 84 ± 7 vs. 88 ± 6 mm Hg for T₀, T₁₅ and T₁₂₀, respectively), but significantly decreased serum and PCEB ACE activities. When normalized to predrug (T₀) activity levels, enalaprilat inhibited PCEB and serum ACE activities at T₁₅ 74 ± 6% and 68 ± 6%, respectively. However, 2 h after enalaprilat (T₁₂₀), PCEB ACE inhibition was maintained at 66 ± 7%, whereas serum ACE inhibition was reduced to 46 ± 8% (P < .01 from PCEB ACE), suggesting a preferential PCEB ACE inhibitory effect of enalaprilat.