Photodynamic properties of pyridoxal phosphate bound to cystathionase-gamma-lyase.

The cofactor pyridoxal phosphate bound through an aldimine linkage to lysine residues of the enzyme cystathionase (L-Cystathione cysteine-lyase (deaminating), EC 4.4.1.1) is very stable to irradiation with light of 420 nm. The catalytic function of the enzyme remains unaffected indicating that the cofactor is not an efficient photosensitizer of essential amino acid residues. This unusual stability of the cofactor to irradiation can be ascribed to the presence of aldimine linkages as demonstrated by studies conducted on model compounds. The binding of a reversible inhibitor (L-allylglycine) to the catalytic site of the enzyme does not facilitate photooxidation of the cofactor. On the contrary, irradiation of the cofactor in the presence of the inhibitor results in photodestruction of the inhibitor.