Tryptic peptide analysis of nanogram quantities of proteins: radioiodination of proteins detected by silver staining in polyacrylamide gels.

The silver-staining procedure for detecting proteins in polyacrylamide gels has been modified so that the polypeptides remain suitable for subsequent radioiodination and tryptic peptide analysis. The procedure, which involves a silver-staining/destaining protocol that minimizes crosslinking, is more rapid than many other methods, and can detect as little as 1 ng of protein. Following elimination of silver, the proteins can be radioiodinated and digested with trypsin by a modification of the method described by J. H. Elder, R. A. Pickett, J. Hampton, and R. A. Lerner (1977, J. Biol. Chem. 252, 6510-6515). Together, these improvements have increased the sensitivity of the tryptic peptide mapping technique by three orders of magnitude and thereby enabled us to perform reproducible structural analysis of femtomolar quantities of proteins.