Proline in vasoactive peptides: consequences for peptide hydrolysis in thelung

To examine the hypothesis that trans isomers of bradykinin and [Gly(6)]brad ykinin are preferentially hydrolyzed by lung peptidases, we studied the fra ctional inactivation of these peptides in the perfused rat lung using a bio assay after a single-pass bolus injection and high-performance liquid chrom atography after lung recirculation. In the bioassay studies, when the pepti des passed through the lung, 25.6-fold more bradykinin or 7-fold more [Gly( 6)]bradykinin was required to elicit a contraction equivalent to that produ ced when the peptides did not pass through the lung. In the recirculation s tudies, hydrolysis progress curves with rapid and slow phases were observed , with a higher fraction of bradykinin than [Gly(6)]bradykinin hydrolyzed i n the rapid phase. Cyclophilin increased the hydrolysis rate during the slo w phase for both peptides. Kinetic analysis indicated that the slowly hydro lyzed peptide fraction, presumably the cis fraction, was 0.13 for bradykini n and 0.43 for [Gly(6)]bradykinin with cis-trans isomerization rate constan ts of 0.074 and 0.049 s(-l), respectively, consistent with published nuclea r magnetic resonance studies.