Primary structure of a pyrroloquinoline quinone (PQQ) containing peptide isolated from porcine kidney diamine oxidase.

After treating porcine kidney diamine oxidase (PKDAO, EC 1.4.3.6) with the inhibitor 2,4-dinitrophenylhydrazine (DNPH), the enzyme was subjected to proteolysis with trypsin. The hydrolysate contained a peptide to which the C(5) hydrazone of PQQ and DNPH (PQQ-DNPH) was bound. The peptide was purified to homogeneity after which the amino acid sequence was determined. It appeared to consist of 11 amino acids, with PQQ bound to number eight. Further proteolysis of the peptide with aminopeptidase and carboxypeptidase gave a compound which was identical to a product prepared from coupling of PQQ-DNPH to lysine. Therefore, the cofactor in PKDAO has most probably an amide bond between one of its carboxylic acid groups with the epsilon-NH2 group of a lysine residue. Possibilities for attachment of the cofactor to the protein chain are discussed.