Rapid removal of acetimidoyl groups from proteins and peptides. Applications to primary structure determination

Methylamine buffers can be used for the rapid quantitative removal of acetimidoyl groups from proteins and peptides modified by treatment with ethyl or methyl acetimidate. The half-life for displacement of acetimidoyl groups from fully amidinated proteins incubated in 3.44 M-methylamine/HCl buffer at pH 11.5 and 25 degrees C was approx. 26 min; this half life is 29 times less than that observed in ammonia/HCl buffer under the same conditions of pH and amine concentration. Incubation of acetimidated proteins with methylamine for 4 h resulted in greater than 95% removal of acetimidoyl groups. No deleterious effects on primary structure were detected by amino acid analysis or by automated Edman degradation. Reversible amidination of lysine residues, in conjunction with tryptic digestion, has been successfully applied to the determination of the amino acid sequence of an acetimidated mouse immunoglobulin heavy chain peptide. The regeneration of amino groups in amidinated proteins and peptides by methylaminolysis makes amidination a valuable alternative to citraconoylation and maleoylation in structural studies.