Oxidation/reduction explains heterogeneity of pancreatic somatostatin

Somatostatin 14 (SS 14) has been isolated from pancreatic extracts, but open gel filtration immunoreactive SS often elutes in two peaks. We isolated both peaks, but upon sequence analysis only authentic SS 14 could be identified. By further gel filtration experiments it turned out that both synthetic and extractable SS appeared homogeneous at neutral pH 7.5, but showed an additional, earlier peak in acetic acid. After addition of mercaptoethanol, all of the SS eluted at this earlier position regardless of the pH. We conclude that partial reduction/oxidation of SS explains the heterogeneity.