Plasma protein binding and endothelial enzyme interactions in the lung

The influence of plasma albumin binding of the synthetic angiotensin-converting enzyme (ACE) substrate [³H]benzoyl-phenylalanyl-alanyl-proline (BPAP) on BPAP hydrolysis by pulmonary endothelial ACE was studied in isolated rabbit lungs perfused with a salt solution containing either 5% bovine serum albumin (BSA) or 5% dextran. The single-pass indicator-dilution method was used to measure the fraction (M) of [³H]BPAP hydrolyzed. Lung M was greater with albumin-free perfusate than when BSA was present. M decreased as the time (t(i)) that the BPAP was in contact with the BSA before reaching the lung was increased, suggesting that some BSA binding sites for BPAP were not in equilibrium during bolus transit through the lungs. The M vs. t(i) data were correlated using a model incorporating both rapid and slow binding kinetics of BPAP and BSA. For the slow BPAP-BSA interaction, the dissociation rate constant was ~0.015 s^-1, and the fraction of the BPAP bound to these slowly equilibrating sites at equilibrium was ~22%. The results indicate that transient plasma protein binding kinetics can affect lung BPAP hydrolysis.