DEGRADATION AND INACTIVATION OF HUMAN ATRIAL-NATRIURETIC-PEPTIDE BY HUMAN PULMONARY PLASMA-MEMBRANES

Atrial natriuretic peptide (ANP) is extracted from plasma during its p assage through the lungs. ANP is metabolized in rat lung membrane prep arations by the enzyme neutral endopeptidase-24.11 (EC 3.4.24.11), but the hydrolysis of ANP in human lung has not been characterized. In th e present study synthetic human atrial natriuretic peptide 1-28 (alpha -hANP) was separately incubated with human pulmonary plasma membranes from two non-smoking patients, and the major degradation products were separated from alpha-hANP by reverse-phase high pressure liquid chrom atography. The degradation products were identified by sequence analys is and by mass-spectrometry, and biological activity was studied in vi tro by exposing precontracted rabbit pulmonary arteries to alpha-hANP and the degradation products. The initial cleavage appeared, with memb rane preparations from both patients, in the central ring structure be tween Arg14 and Ile15, followed by a cleavage of the bond Arg3-Arg4 at the N-terminal region of the peptide. The biological activity of this ring-opened product was about 1/500 of the activity of uncleaved alph a-hANP. Cleavage of the Arg3-Arg4 or Arg14-Ile15 bonds could not be in hibited by EDTA, iodoacetamide, benzamidine hydrochloride or pepstatin A. Neither did phosphoramidon (1 muM) or thiorphan (1 muM) inhibit th e hydrolysis, indicating the presence in human lung of an ANP-degradin g enzyme different from endopeptidase-24.11.