CYCLOPHILIN-FACILITATED BRADYKININ INACTIVATION IN THE PERFUSED RAT LUNG

Cis and trans isomers of X-proline (X-Pro) bonds can influence some as pects of the kinetics of peptide metabolism. We previously used the pe ptidyl-prolyl cis-trans isomerase, cyclophilin, to show that angiotens in converting enzyme (ACE) preferentially hydrolyzes the trans isomer of a synthetic tripeptide that contains a C-terminal proline (Dawson e t at, Am J Physiol 257: H853-H865, 1989; Merker et at, J Appl Physiol 75: 1519-1524 1993). Bradykinin (Arg-Pro-Pro-Gly-Phe Ser-Pro-Phe-Arg) exists as both cis and trans isomers at all three X-Pro bonds, and alt hough its inactivation in the lung by pulmonary endothelial peptidases is extensive, commonly a small fraction of the peptide survives passa ge through the lung. To determine whether the presence of cis X-Pro bo nds might limit the extent of bradykinin metabolism in the lung, we st udied inactivation of bradykinin by the isolated perfused rat lung usi ng the rabbit jugular vein superfused with the pulmonary venous efflue nt as a bioassay for bradykinin. A large fraction (>90%) of the bradyk inin in a bolus injection was inactivated in a single transit through the pulmonary circulation, but a detectable fraction emerged in the ve nous effluent. The addition of cyclophilin to the bradykinin in the bo lus reduced the bradykinin emerging from the lungs to virtually undete ctable levels. When the isomerase inhibitor cyclosporin A was included with bradykinin and cyclophilin in the injectate, this effect of cycl ophilin was reversed. These observations suggest that the fraction of bradykinin that normally survives passage through the lungs contains i somers that have at least one X-Pro bond that is refractory to enzymat ic inactivation and whose isomerization time constant is significantly longer than the pulmonary capillary transit time.