Prolyl oligopeptidase is involved in release of the antifibrotic peptide Ac-SDKP

N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is a ubiquitous tetrapeptide hydrolyzed almost exclusively by angiotensin-converting enzyme ( ACE). Chronic treatment with Ac-SDKP decreases cardiac and renal fibrosis and inflammatory cell infiltration in hypertensive rats. However, very little is known about endogenous synthesis of Ac-SDKP, except that thymosin-beta(4) may be the most likely precursor. Two enzymes are potentially able to release Ac-SDKP from thymosin-beta(4): prolyl oligopeptidase ( POP) and endoproteinase asp-N. POP is widely present and active in several tissues and biological fluids, whereas endoproteinase asp-N appears to be lacking in mammals. Therefore, we hypothesized that POP is the main enzyme involved in synthesizing the antifibrotic peptide Ac-SDKP. We investigated in vitro and in vivo production of Ac-SDKP. Using kidney cortex homogenates, we observed that Ac-SDKP was generated in a time-dependent manner in the presence of exogenous thymosin-beta(4), and this generation was significantly inhibited by several POP inhibitors ( POPi), Z-prolyl-prolinal, Fmoc-prolyl-pyrrolidine-2-nitrile, and S17092. Long-term administration of S17092 in rats significantly decreased endogenous levels of Ac-SDKP in the plasma ( from 1.76 +/- 0.2 to 1.01 +/- 0.1 nM), heart ( from 2.31 +/- 0.21 to 0.83 +/- 0.09 pmol/mg protein), and kidneys ( from 5.62 +/- 0.34 to 2.86 +/- 0.76 pmol/mg protein). As expected, ACE inhibitors significantly increased endogenous levels of Ac-SDKP in the plasma, heart, and kidney, whereas coadministration of POPi prevented this increase. We concluded that POP is the main enzyme responsible for synthesis of the antifibrotic peptide Ac-SDKP.