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Rat endothelin-converting enzyme-1 forms a dimer through Cys412 with a similar catalytic mechanism and a distinct substrate binding mechanism compared with neutral endopeptidase-24.11.

ABSTRACT: Endothelin-converting enzyme-1 (ECE-1) is involved in the conversion of big endothelins (big ETs) into endothelins (ETs) and shows sequence similarity with neutral endopeptidase-24.11 (NEP). Unlike NEP, ECE-1 exists as a disulphide-linked dimer. Here we reveal that Cys412 is solely responsible for the dimerization of rat ECE-1. The C412S mutant enzyme, which existed as a monomer, showed no difference in glycosylation level, subcellular localization of clustering structure formation, but showed a higher K(m) and lower kcat for big ET-1 compared with the wild-type enzyme. These results indicate that dimerization of ECE-1 is preferential for effective conversion of big ETs into ETs. In addition, complete loss of activity in the mutants E592Q, E651Q and H716Q confirmed that these residues are responsible for catalytic activity, zinc binding and stabilization of the intermediate during the transition state respectively. In contrast, the catalytic properties of mutant enzymes containing a substitution at Arg129 or Glu752 were not markedly different from those of the wild-type enzyme, suggesting that these residues play only a minor role, if any, in substrate binding, in contrast with their role in NEP.

SUBMITTER: Shimada K

PROVIDER: S-EPMC1217286 | biostudies-other | 1996 May

REPOSITORIES: biostudies-other

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Project description:The human colon cancer cell line Caco-2 undergoes spontaneous enterocytic differentiation during growth, and expresses a number of brush-border-membrane-associated hydrolases typical of a differentiated phenotype. Among these are alkaline phosphatase, dipeptidyl peptidase IV and sucrase-isomaltase (sucrase, EC 3.2.1.48). Neutral endopeptidase 24.11 [EC 3.4.24.11, neprilysin (NEP)] is another abundant protease of normal enterocytes but its presence in Caco-2 cells has not been fully documented yet. In this paper, we show that Caco-2 cell extracts hydrolyse tritiated [D-Ala2Leu5]enkephalin with a Km of 180 microM, very close to the value obtained for the NEP present in the rabbit kidney (118 microM). Western-blot analysis of brush-border membranes purified from post-confluent cells revealed a protein with an apparent molecular mass of 94000 Da similar to that of the rabbit kidney NEP. The amount of enzyme in cell extracts increased as a function of the age of the culture, indicating that NEP expression is correlated with the degree of cell differentiation as is also the case for sucrase and dipeptidylpeptidase IV (DPP-IV). Binding of a radiolabelled antibody to Caco-2 cell monolayers grown on semi-permeable filters indicated that 95% of NEP molecules present at the cell surface are on the apical side. Immunocytochemical and flow cytometric analysis of intact and permeabilized cells were also used to investigate the presence of NEP and DPP-IV at the surface of Caco-2 cells. Whereas DPP-IV staining appeared to be homogeneous throughout the entire cell population, NEP-related fluorescence exhibited a bimodal distribution which indicates an uneven expression of the protein at the cell surface. Permeabilization of monolayers with saponin before staining restored a labelling pattern for NEP similar to the one obtained for DPP-IV. This suggests that although DPP-IV and NEP follow similar patterns of expression when enzymic activities are measured on whole-cell extracts, targeting of these brush-border proteins to the cell surface appears to be regulated in different ways.

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Rat endothelin-converting enzyme-1 forms a dimer through Cys412 with a similar catalytic mechanism and a distinct substrate binding mechanism compared with neutral endopeptidase-24.11.

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