Inactivation of endothelin I by deamidase (lysosomal protective protein)

Deamidase cleaves ester and peptide bonds in various substrates and deamidates protected COOH-terminal amino acids. It preferentially hydrolyzes peptides which contain hydrophobic amino acids in the P1' and/or P1 position. Because the COOH-terminal end of endothelin I contains the hydrophobic sequence -Ile19-Ile20-Trp21-OH, we investigated whether human deamidase, purified from platelets, could inactivate this peptide. We found that deamidase readily cleaved off Trp21 with an acid pH optimum, a Km = 22 microM, a kcat of 1454 min-1, and a kcat/Km of 68 microM-1 min-1. We also found the enzyme to be present in target cells of endothelin, in vascular smooth muscle cells. Extracts of cultured vascular smooth muscle cells cleave both the synthetic fluorescent substrate 5-dimethylaminonaphthalene-1-sulfonyl(Dns)-Phe-Leu-Arg and endothelin I by releasing the COOH-terminal amino acid. The reaction was inhibited by diisopropyl fluorophosphate, benzyloxycarbonyl-Gly-Leu-Phe-CH2Cl, and p-chloromercuribenzenesulfonate, which inhibit the purified deamidase, but not by inhibitors of some other peptidases. The rate of hydrolysis of endothelin I in the soluble, 100,000 x g final supernatant of the homogenized smooth muscle cells was 2.1 mumol/h/mg and 3.1 mumol/h/mg for Dns-Phe-Leu-Arg. Thus, smooth muscles, platelets, and many other tissues which contain the deamidase can inactivate endothelin by cleaving the COOH-terminal tryptophan.