Relaxed specificity of endoproteinase Asp-N: this enzyme cleaves at peptide bonds N-terminal to glutamate as well as aspartate and cysteic acid residues.

Asp-N, an endoproteinase specific for cleavage of protein or polypeptide bonds N-terminal to aspartate or cysteic acid residues, has been shown to possess a similar affinity for certain glutamate residues. Of 18 glutamate residues present in 2 cyanogen bromide fragments of apolipoprotein A-I, 5 residues were cleaved at rates comparable to that of cleavage at the 12 internal aspartate residues present in these polypeptides (all of which were cleaved). Cleavage of these 5 glutamate residues was obtained under standard enzyme digestion conditions, and the identities of all peptides obtained by Asp-N digestion were determined by amino acid sequencing of peaks obtained from reversed-phase high performance liquid chromatography.