Carbodiimide-mediated O-sulfation of hydroxy-amino acids and peptides: A reaction suitable for radiolabeling

Carbodiimide-mediated sulfation of hydroxy-amino acids, peptides, and proteins can be accomplished in dry dimethylformamide by incubation in a 20-50 molar excess of sulfuric acid and various concentrations of dicyclohexyl carbodiimide [(1-ethyl-(3-dimethylaminopropyl)carbodiimide or 1-cyclohexyl-3-(2-morpholoethyl)carbodiimide p-toluene sulfonate)] at 4 °C for 2-4 h. Under these conditions, hydroxy-amino acids are quantitatively converted into O-sulfates, while cysteine yields the S-sulfonate. Other amino acids, including tryptophan, do not react and are recovered quantitatively. Porcine sodium insulin yields a product that can be separated into six bands by nondenaturing polyacrylamide gel electrophoresis. Radioloabeling of peptides by this method can be carried out with a high degree of efficiency if the added [³⁵S]sulfuric acid is used carrier free with an acid excess provided by trifluoromethyl sulfonic acid. Under these conditions, over 60% of [³⁵S]sulfuric acid was incorporated into insulin and bovine serum albumin. This method may prove useful in the radiolabeling of other peptides and proteins.