engleski

Human dipeptidyl peptidase III: broadened repertoire of potential substrates

Dipeptidyl peptidase III (DPP III ; EC 3.4.14.4) is a zinc-exopeptidase which was discovered in extracts of pituitary gland through hydrolysis of Arg-Arg-2-naphthylamide (Arg-Arg-2NA). Until now, DPP III was purified and biochemically characterized from several human and animal tissues, and lower eukaryotes. It is generally found to be a cytosolic protein, but a membrane-associated DPP III has been reported in brain as well. It cleaves dipeptides sequentially from the N-terminus of its substrates, optimally sized from tetra- to octapeptides. In vitro this peptidase displays a broad specificity, and high affinity for angiotensins and enkephalins. A preference for hydrophobic residues at position P1', and proline as a prohibited amino acid at P1 or P1' was postulated fo mammalian DPPs III. The physiological substrates of this metallopeptidase are mostly unknown. Recent data support the role for DPP III in endogenous pain-modulatory system: high concentrations of DPP III were found in the superficial laminae of rat spinal cord dorsal horn where this peptidase co-localizes with opioid peptides enkephalins and endomorphins. In our study of mammalian DPPs III we determined, by activity measurements and Western blotting, a level of this enzyme in the rat tissues, in the brain being among the highest. To investigate further a possible involvement of DPP III in the metabolism of neuropeptides, we examined its affinity (by competitive inhibition of Arg-Arg-2NA hydrolysis) and hydrolytic activity (by TLC) towards several novel opioid peptides. Surprisingly, human DPP III (purified from erythrocytes) cleaved endomorphins, in contrast to the accepted notion that proline imposes a restriction on the hydrolysis of the peptide bond catalysed by this type of enzyme. Furthermore, DPP III hydrolytic activity towards endomorphin-1 (EM-1) was compared to that of dipeptidyl peptidase IV (DPP IV ; EC 3.4.14.5), a well-known post-proline cleaving enzyme. DPP IV was isolated from rat kidney membranes. The enzymatic hydrolysis products of EM-1 were separated and quantified by capillary electrophoresis and kinetic parameters determined. Both DPP III and DPP IV cleaved this tetrapeptide with comparable rate, by liberating the N-terminal Tyr-Pro. In conclusion, our study revealed hydrolytic activity of human DPP III towards representatives of three new groups of opioid peptides: endomorphins, hemorphins and exorphins. These results show for the first time that human DPP III can act as a post-proline-cleaving enzyme.

aminopeptidase; capillary electrophoresis; hydrolase; opioid peptides

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