Phospholipase C (Bacillus cereus) contains two apparently essential and very reactive lysine residues that may be labelled selectively by pyridoxal 5′-phosphate [Aurebekk & Little (1977) Biochem, J. 161, 159–165]. One of these lysine residues was found in the 25-amino acid N-terminal fragment liberated by CNBr digestion of the pyridoxal-labelled enzyme and identified as lysine-6. Two of the labelled peptides isolated from the chymotryptic digest of pyridoxal-labelled enzyme contained proline, suggesting that the other labelled lysine residue is situated in the same region of the primary structure as the single proline residue of the enzyme.
