APCE cleavage of polymorphic forms of Met-α₂AP. Equal amounts (40 μg) of purified Met-α₂AP(Arg6) and Met-α₂AP(Trp6) were digested by APCE. After stopping the reaction at selected times, samples were assessed by electrospray mass spectrometry for the quantity of the amino-terminal 12-amino acid peptide produced from each Met-α₂AP form.