Stability of tyrosine sulfate in acidic solutions.

Tyrosine O-sulfation is a posttranslational modification of secretory and membrane proteins transported through the Golgi apparatus, which is widespread among higher eukaryotes. O-Sulfated tyrosines are not immediately identified during sequencing of peptides and proteins, because the sulfate ester is acid labile and rapidly hydrolyses to tyrosine in strong acidic solutions. Little is known about the hydrolysis at mildly acidic solutions, which are used during several protein purification and analysis procedures. We have examined the stability of tyrosine sulfate using sulfated gastrin-17, caerulein, and drosulfokinin as models for tyrosine O-sulfated peptides. The peptides were incubated in acidic solutions in a pH range of 1 to 3 at different temperatures and time spans. Only marginal hydrolysis of gastrin-17 was observed in triflouroacetic acid at room temperature or below. Comparison of the acid hydrolysis of the three peptides showed that hydrolysis rate depends mainly on the primary amino acid composition of the peptide. The activation energy (E(a)) for the hydrolysis of sulfated gastrin-17 was found to be E(a)=98.7+/-5 kJ mol(-1). This study serves as a general reference for handling tyrosine sulfated peptides in aqueous acidic solutions. We conclude that tyrosine sulfate is more stable under normal protein purification conditions than previously assumed.