Automated C-Terminal Protein Sequence Analysis Using the Alkylated-Thiohydantoin Method

Abstract

The alkylated-thiohydantoin (ATH) method for C-terminal sequence analysis, which exploits some unique properties of the thiohydantoin (TH) ring system, was first described in 1992 (1). The ATH method incorporates alkylation of the C-terminal TH with an aralkylbromide reagent. The resulting ATH is a better leaving group than the unmodified TH, eliminating the need for strongly acidic or strongly nucleophilic cleavage conditions and minimizing ring opening of the TH and hydrolysis of the protein. The ATH is readily cleaved under mildly acidic conditions by nucleophilic displacement with thiocyanate ion, [NCS]“, conditions that simultaneously form the penultimate (C-1) TH. This is a key advantage of the ATH method over other methods for C-terminal sequence analysis. Combination of ATH-cleavage and TH-derivatization into one step eliminates the need for activation of the C-terminal amino acid before each cycle of sequencing. Therefore, any new C-termini arising from fragmentation of peptide bonds in the protein can not form TH. This prevents detection of amino acid background resulting from internal cleavage of the protein. In other methods for C-terminal sequence analysis, internal cleavage followed by repeated C-terminal activation can produce a rising TH-background, making accurate sequence calling difficult. In the ATH method, each alkylation of the proteinyl-TH, followed by cleavage and derivatization with [NCS]^-, comprises one cycle of sequencing.