HYDROLYSIS OF HUMAN AND PIG BRAIN NATRIURETIC PEPTIDES, URODILATION, C-TYPE NATRIURETIC PEPTIDE AND SOME C-RECEPTOR LIGANDS BY ENDOPEPTIDASE-24.11

Endopeptidase-24.11 (E-24.11, EC 3.4.24.11) is widely believed to play a physiological role in metabolizing atrial natriuretic peptide (ANP) . Since the discovery of ANP, new natriuretic peptides have been isola ted and other peptides synthesized as receptor ligands. The hydrolysis in vitro of six related peptides by the endopeptidase has been studie d, mainly by h.p.l.c. The initial attack on the 32-residue form of pig brain natriuretic peptide (pBNP-32) was shown to be at the Ser20-Leu2 1 bond, as had been previously shown for the 26-residue form. In contr ast, human brain natriuretic peptide-32 (hBNP-32), which differs in te n residues from pBNP-32, was attacked first at the Met4-Val5 bond, rel easing the N-terminal tetrapeptide, and only later at bonds within the ring: at Arg17-Ile18 and subsequently at four other sites. Urodilatin , which has a four-residue extension at the N-terminus compared with a lpha-human atrial natriuretic peptide-28 (alpha-hANP), was degraded at about half the rate of the latter, though the C-terminal Phe-Arg-Tyr was released at the same rate. The 22-residue C-type natriuretic pepti de was hydrolysed more rapidly than alpha-hANP, as were two C-receptor ligands (peptides with deletions within the ring): C-ANP4-23 (rANP4-2 3 des-Gln18,Ser19,Gly20,Leu21,Gly22) and SC 46542 (hANP5-28 des-Phe8,G ly9,Ala17,Gln18). Angiotensin-converting enzyme failed to hydrolyse pB NP-32, hBNP-32 or I-125-rat (r) ANP, even after prolonged incubation. K(m) and k(cat) values were determined for the hydrolysis of alpha-hAN P, porcine BNP-26, porcine BNP-32 and I-125-rANP by E-24.11. K(i) valu es were determined for six peptides, alpha-hANP, urodilatin, hBNP-32, C-type natriuretic peptide (CNP), SC 46542 and C-type natriuretic pept ide (C-ANP4-23), in radiometric assays of E-24.11 with either [I-125] insulin B chain or [I-125] rANP as substrate. The K(i) values (2.5-13 muM) for CNP were the lowest of any of the group, whereas those for hB NP-32 (151-172 muM) were the highest. The physiological significance o f these results is discussed, especially in regard to the relative res istance of hBNP-32 to attack and the ability of the C-receptor ligands to compete with natriuretic peptides for hydrolysis by E-24.11.