Effect of transit time on metabolism of a pulmonary endothelial enzyme substrate

Fractional hydrolysis (M) of the synthetic angiotensin-converting enzyme (ACE) substrate [³H]benzoyl-Phe-Ala-Pro (BPAP) on passage through the isolated dog lung lobe was found to be relatively independent of flow rate and transit time (t̄). The most commonly expressed explanation for this kind of observation is that recruitment of ACE-containing surface area occurs when flow is increased. To test this, as well as other hypotheses that might explain the behavior of this substrate, we compared M obtained after the first pass of a BPAP-containing bolus through isolated rabbit lungs with that obtained after two sequential passes through the lungs. In this way, we could double t̄ with no change in flow or vascular pressure. We found that, when the second pass occurred within a few seconds of the first, M after both the first and second pass was only slightly larger than that after the first pass alone. If the time between passes was increased to a few minutes, M after the second pass was substantially increased. These results are contrary to the recruitment hypothesis and suggest that this substrate may exist in alternative forms that are in slow equilibrium relative to the capillary t̄. When albumin was present in the perfusate, an albumin-bound fraction appeared to be one such alternative form. However, experiments carried out using protein-free perfusate suggest the possibility that conformational variants of the substrate may also exist.