1. Two-site enzyme-linked immunosorbent assays have been developed for the rapid, sensitive and non-isotopic measurement of endothelin-1 and big endothelin-1. The sensitivities of detection were 0.5 and 0.3 fmol/well, with ED50 values of 13 and 12 fmol/well for the endothelin-1 and big endothelin-1 assays, respectively. Each assay is highly selective for its corresponding antigen. The ET-1 assay showed no detectable cross-reactivity with ET-1-(1-20), indicating that the assay only recognizes the 21-amino acid biologically active peptide.

2. The two assays were used to measure the effects of two classes of protease inhibitor on the basal release of enothelin-1 and big endothelin-1 from cultured first-passage human umbilical vein endothelial cells.

3. The secretion of both peptides was time-dependent over 12 h. The metalloprotease inhibitor phosphoramidon (1 × 10−4 mol/l) significantly reduced the amount of endothelin-1 secreted into the medium (P < 0.05), with a concomitant increase in the secreted levels of big endothelin-1 (P < 0.01). The aspartyl protease inhibitor, pepstatin A, also caused a significant decrease in the secretion of endothelin (P < 0.05). However, unlike phosphoramidon, there was no increase in the levels of big ET-1 compared with the controls. At these concentrations, neither inhibitor affected the viability of the cells as indicated by Trypan Blue exclusion.

4. The two assays permit the direct measurement of endothelin-1 and its precursor, and will be of use in the elucidation of the putative human endothelin-converting enzyme(s). In addition to a phosphoramidon-sensitive enzyme, the results also suggest the existence of a pepstatin A-sensitive enzyme in human endothelial cells. Furthermore, pepstatin A also appears to limit the generation of big endothelin-1. Thus pepstatin A-sensitive enzymes may be better targets for inhibitors to reduce the harmful vasoconstrictive effects of endothelin-1 in cardiovascular diseases.

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