Prolyl endopeptidase: inhibition in vivo by N-benzyloxycarbonyl-prolyl-prolinal

The activity of prolyl endopeptidase in homogenates of mouse tissues was determined 30 min after intraperitoneal injection of N-benzyloxycarbonyl-prolyl-prolinal (1.25 mg/kg), a potent transition state analog inhibitor (K1 = 14 nM) of prolyl endopeptidase (EC 3.4.21.26). A more than 85% decrease of enzyme activity was obtained in all tissues. The in vivo degradation of potential prolyl endopeptidase substrates was studied by following the release of sulfamethoxazole from N-benzyloxycarbonylglycyl-prolyl-sulfamethoxazole, a model synthetic substrate of the enzyme. When this substrate was given intraperitoneally, its enzymatic degradation was blocked after administration of the inhibitor in a dose- and time-dependent manner, indicating inhibition of the enzyme in vivo. Of interest is the long duration of the inhibition. After a relatively low inhibitor dose (5 mg/kg) significant inhibition was seen in most tissues even after 6 h. The brain was particularly sensitive to the effect of the inhibitor. Since prolyl endopeptidase readily degrades many proline-containing neuropeptides, the inhibitor should be of value in studies on the role of the enzyme in neuropeptide metabolism.