Photoinactivation of enzymes by linear and angular furocoumarins

Furocoumarins with linear (psoralen, 8-methylpsoralen, 8-methoxypsoralen and 3-carbethoxypsoralen) and angular molecular structures (angelicin and 4,5'-dimethylangelicin) were found to inactivate enzymes to different extents through UV-A irradiation. Moreover, enzymes with different structures (glutamate dehydrogenase, lysozyme, 6-phosphogluconate dehydrogenase, enolase, thermolysine and ribonuclease) are inactivated to different extents by the same furocoumarin. UV-A irradiation produces both covalent incorporation of the furocoumarins into the protein molecule and photodegradation of amino acid residues; the latter phenomenon seems to be mainly responsible for the photoinactivation process. A close correlation was found between the capacity of the furocoumarins to photoinactivate enzymes and their capacity to modify free amino acids. A study of the effects of quenchers of various forms of activated oxygen on the photoinactivation of glutamate dehydrogenase, used as a model enzyme, and psoralen and 8-methylpsoralen as a reference for furocoumarins, showed that singlet oxygen is the species most involved in the photoinactivation process. (author)