Characterization of immunoreactive endothelin in human urine

Structure-activity analysis of C-terminal endothelin analogues

Several synthetic endothelin (ET) analogues of the C-terminal ET hexapeptide (ET16-21) were analyzed by radio-receptor competition binding assays and biologic activity using both ETA and ETB receptor subtypes. In addition, we produced a hybridoma monoclonal antibody, anti-ET15-21, that appeared to crossreact with the entire ET molecule and was able to neutralize its biologic activity. Antibody binding was measured with competition enzyme-linked immunosorbent assays and a surface plasmon resonance-based biosensor (BIA technology). The ET16-21 moiety was modified with systematic replacement of each residue by alanine (Ala-scan). Whereas the C-terminal residues (Asp18, Ile20, and particularly Trp21) were very important for both receptor binding and immunologic activity, Ala substitution in positions 16, 17, and 19 hardly affected such activities. Analysis of another series of synthetic ET16-21 analogues with the His16 residue replaced by a non-amino-acidic block confirmed that the last two C-terminal residues are essential for receptor and antibody binding, whereas the central region of this hexapeptide is much more tolerant to modification. However, a critical steric conformation of the active hexapeptide is necessary.

Degradation of endothelin-1 by extracts of rat lung, kidney, and liver

Degradation of 125I-labeled endothelin-1 (125I-ET-1) when incubated 120 min at 37 degrees C with rat lung, kidney and liver plasma membrane extracts was examined using HPLC. Lung and kidney extracts showed degrading enzyme activity, but none was found in liver extract. EDTA almost abolished degradation of 125I-ET-1 in lung and kidney extracts. Phosphoramidon and SCH 39370, both inhibitors of neutral endopeptidase 24.11 (NEP), markedly inhibited degradation of 125I-ET-I in lung extract and clearly less in kidney extract. Soybean trypsin inhibitor (STI) and elastase inhibitor partly inhibited degradation in lungs and in kidney extract. Leupeptin had no inhibitory effect neither in lung nor in kidney extract. Our results suggest: (1) at least two types of enzymes degrade ET-1 in lung and kidney extracts, namely metallo-proteinases and serine proteinases. (2) The ET-1 degrading effect appears to be different in lungs and kidneys, metallo-proteinases being more important in pulmonary than in renal degradation of ET-1.

Enzyme immunometric assay for endothelin using tandem monoclonal antibodies

Seven distinct mouse monoclonal antibodies (mAbs) directed against human endothelin-1 (ET-1) have been obtained. On the basis of specificity studies performed with competitive immunoassays and of complementary binding studies, these mAbs were classified in two groups. mAbs of group A (Endo-4, -5, -6 and -10) were shown to be directed against the N terminal loop while those of group B (Endo-2, -8 and -18) recognized the C terminal part of the peptide. A pair of monoclonal antibodies with optimal properties for a two-site immunometric assay were selected and the test was performed in 96-well microtiter plates coated with one mAb (Endo-18), while another mAb (Endo-4) covalently labeled with enzyme acetylcholinesterase was used as tracer. Under optimal conditions, the assay appeared to be very sensitive since concentrations as low as 1 pg/ml could be significantly detected. The precision was also very good with a coefficient of variation below 10% from 3 to 250 pg/ml. The assay was specific for mature endothelin presenting no cross-reactivity with the precursor Big ET-1. On the other hand, strong cross-reactivity was observed with other ET-1-related peptides, including ET-2, ET-3, VIC peptide and sarafotoxin 6-b. The assay permitted specific determination of ET-1 in supernatants of cultured endothelial cells and the validity of the test was demonstrated by HPLC fractionation experiments. In addition, the assay also appeared to be suitable for direct determination of ET-1 in plasma. Studies performed with plasma from healthy subjects revealed that circulating levels of ET-1 are below or close to the detection limit of the method (< 8 pg/ml).