Ultraviolet and N-formyl-kynurenine-sensitized photoinactivation of bovine carbonic anhydrase

Abstract —Direct photoinactivation by UV light of bovine carbonic anhydrase, as well as its photosensitization by N ‐formyl‐kynurenine, a tryptophan photooxidation product, have been investigated. In the presence of oxygen both methods lead to similar results: the enzyme loses its activity, the tryptophanyl, histidyl and, to a lesser extent, tyrosyl residues being destroyed. In nitrogen‐saturated solutions, a dramatic drop is observed in the photoinacitivation yield under the direct action of ultraviolet light, whereas histidyl residues remain intact. Evidence indicates an internal photodynamic action of N ‐formyl‐kynurenine in the protein core produced by the UV photooxidation of tryptophanyl residues. Photoinactivation of oxygenated enzyme solutions by external and internal photodynamic action correlates with histidyl residue destruction via singlet oxygen. The possible importance of the photodynamic ability of N ‐formyl‐kynurenine in the photochemistry of proteins, DNA, and cells is discussed.