In vitro enzymatic processing of radiolabelled big ET-1 in human kidney

We have investigated enzymatic processing of big ET-1 in sections of human renal cortex by examining selected binding characteristics of the radiolabelled precursor and cleaved peptide. Sections of histologically normal human kidney obtained from patients undergoing nephrectomy for hypernephroma (50-74 years, N = 10, male or female) were incubated with 0.1 nM [125I]-ET-1, [125I]-Tyr13 big ET-1 or [125I]-Tyr31 big ET-1 in culture media at 37 degrees to facilitate enzymatic activity. Specific binding measured from sections incubated with [125I]-Try13 big ET-1 (which would yield [125I]-ET-1 on enzymatic cleavage) was 39.7 +/- 2.5%. This was significantly reduced to 19.0 +/- 2.0% following co-incubation with 10 microM thiorphan, an inhibitor of neutral endopeptidase (NEP) but not the putative endothelin converting enzymes (ECE). No further reduction in specific binding was obtained with 100 microM thiorphan, indicating that this is a maximal effect. However phosphoramidon (100 microM), an inhibitor of ECE and NEP, almost abolished specific binding, indicating that both NEP and ECE cleave big ET-1 in the kidney. No specific binding was detected when sections were labelled with [125I]-Tyr31 big ET-1 (which would be expected to yield [125I] labelled C-terminal fragment). Binding of the product of processed [125I]-Tyr13 big ET-1 was inhibited mainly by the ET(B) selective antagonist (BQ788 = 75.1 +/- 2.1% inhibition; FR139317 = 9.7 +/- 7.3% inhibition), consistent with the predominance of this subtype in human kidney. We conclude that big ET-1 is processed by NEP and ECE in human kidney and that the cleaved product binds predominantly to the ET(B) receptor subtype. ECE may be a therapeutic target in the attenuation of renal diseases in which ET-1 has been implicated.