Pulmonary metabolism of exogenous enkephalins in isolated perfused rat lungs

Metabolism of enkephalins during transit through the pulmonary circulation may be of significance in regulating systemic levels of these opioids. To determine whether Leu- and Met-enkephalin are metabolized by the pulmonary circulation, [3H]Tyr-Leu-enkephalin (10 microM) or [3H]Tyr-Met-enkephalin (10 microM) were each administered to isolated rat lungs perfused in a recirculating manner with a physiologic salt solution and a recently developed high-performance liquid radiochromatographic analytical method was used to identify and quantitate metabolites in the perfusion medium. Both Leu- and Met-enkephalin were metabolized in a curvilinear, time-dependent manner. The principal metabolites were identified as tyrosine and Tyr-Gly-Gly. Neither Tyr-Gly nor Tyr-Gly-Gly-Phe were detected in significant amounts. After a 20-min perfusion, residual Leu- or Met-enkephalin accounted for 28.4 and 21.5%, respectively, of the radioactivity present in the perfusate. In addition, 97% of the initial radioactivity for both Leu- and Met-enkephalin were found in the perfusion medium, indicating that neither the parent compounds nor metabolites were avidly sequestered in pulmonary tissue. The angiotensin converting enzyme inhibitor, captopril (18 microM) blocked the formation of Tyr-Gly-Gly and attenuated slightly the production of tyrosine. Inhibition of aminopeptidase with bestatin (116 microM) blocked the formation of tyrosine and enhanced production of Tyr-Gly-Gly. Inhibition of enkephalinase with thiorphan (0.3 microM) did not appear to affect Met-enkephalin metabolism. These observations indicate that in isolated, buffer perfused rat lungs Leu- and Met-enkephalin are metabolized during pulmonary transit by at least two enzymes, angiotensin converting enzyme and aminopeptidase.