Extraction of Peptidase Substrates by the Isolated Perfused Rat Lung

Peptidases in the lung are well placed to have an important role in regulating levels of circulating endogenous and therapeutic peptides. They also present a first-pass metabolic barrier for peptides delivered to the lung for systemic absorption. The activities of five peptidases were surveyed in the pulmonary circulation of the asanguinous isolated perfused rat lung (IPRL) using synthetic substrates and selective inhibitors. Extraction ratios (ER) were calculated for the substrates of: aminopeptidase N (AMN), ER 0·37 ± 0·04; dipeptidyl peptidase IV (DPP), ER 0·69 ± 0·05; and angiotensin-converting enzyme (ACE), ER 0·40 ± 0·02. These activities were inhibited (> 95%) by the selective inhibitors bestatin, diprotin A, and captopril, respectively. Substrates for neutral endopeptidase 24.11 (NEP) and carboxypeptidase M (CPM) were minimally degraded with ER of 0·02 and 0·00, respectively. The low activity of NEP, a major membrane bound endopeptidase, indicates that the lungs may not contribute greatly to degradation of either systemic NEP substrates, or exopeptidase-resistant peptides absorbed from the peripheral lung. In contrast peptides susceptible to the exopeptidases AMN, DPP, and ACE, will be substantially degraded during passage through the lung. The absence of CPM activity in the pulmonary circulation of the asanguinous IPRL implies that basic carboxypeptidase activity in blood perfused lungs reported in the literature is more likely to be the result of plasma carboxypeptidase N activity.