SYMMETRICAL DISULFIDE BONDS AS S‐PROTECTING GROUPS AND THEIR CLEAVAGE BY DITHIOTHREITOL: SYNTHESIS OF OXYTOCIN WITH HIGH BIOLOGICAL ACTIVITY

Abstract

Oxytocin, possessing high rat uterotonic activity ( 560 ± 25 U/mg ) and avian vasodepressor activity ( 510±23 U/mg ) was obtained according to the following scheme: acylation of bis (L‐ tyrosyl ‐L‐ isoleucyl ‐L‐ glutaminyl ‐L‐ asparaginyl )‐L‐ cystinyl‐bis (L‐ prolyl ‐L‐ leucylglycinamide ) with bis ( tett‐butyloxycarbonyl )‐L‐ cystinyl‐bis (p‐ nitro‐phenyl ester ); liberation of the N‐terminal amino groups of the coupling products with TFA and reduction of all disulfide bonds with dithiothreitol in aqueous medium at pH 8; separation of oxytoceine from low molecular weight compounds; followed by air oxidation of the disulfhydryl monomer to the cyclic disulfide. The protected intermediates of oxytocin were prepared according to the stepwise method of peptide synthesis in which the peptide chain was elongated by the use of N‐protected activated esters as acylating agents; the cysteine residues were introduced into the peptide as symmetrically substituted cystines.