Assignment of disulphide bonds in synthetic endothelin-1 isomers by fast atom bombardment mass spectrometry

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Abstract
Endothelin‐1 (ET‐1), a 21‐residue vasoconstrictor peptide possessing four cysteinyl residues at positions 1, 3, 11 and 15, was synthesized by random oxidation of a tetrahydro‐ET‐1. On reverse‐phase high‐performance liquid chromatography, crude product was shown to be a mixture of two disulphide isomers. A method was developed to determine the disulphide structure of the isomers. The method consisted of (a) limited digestion with chymotrypsin, (b) cleavage with cyanogen bromide and (c) manual Edman degradation. Through this procedure, each isomer afforded specific fragments containing a single disulphide bond, which were identified by fast atom bombardment mass spectrometry. Isomer 1, the minor component, afforded a fragment containing Cys 3 and Cys 15, and isomer 2, the major component, afforded fragments containing Cys 3 and Cys 11. Since little disulphide exchange was observed, it could be concluded clearly that the disulphide bond pairs in isomer 1 were Cys 1–Cys 11 and Cys 3–Cys 15, while those in isomer 2 were Cys 1–Cys 15 and Cys 3–Cys 11 (the same as natural ET‐1). The procedure was successfully applied to two synthetic analogues, [Gly18]‐ET‐1 and [Pro16]‐ET‐1.