The photoreactions of 8-methoxypsoralen with tryptophan and lens proteins.

Abstract

Abstract— We have previously demonstrated that 8‐methoxypsoralen (8‐MOP) can be found in the lenses of rats injected (i.p.) with this drug, and that its presence can lead to a photosensitized enhancement of lenticular fluorescence. The cutaneous photosensitizing properties of psoralens are thought to be mediated via their excited triplet states, resulting in photoaddition cyclobutane products between pyri‐midine bases and 8‐MOP. We have now investigated the possibility that similar types of photoadducts could be generated between 8‐MOP and the aromatic amino acid residues in lens proteins. Our experiments involved in vitro irradiation (at 360 nm) of aqueous solutions of 0.1 mM 8‐MOP plus purified alpha, beta, or gamma crystallins from calf or normal human (under 20 years of age) lenses. UV absorption and fluorescence emission spectra were measured before and after radiation, and aliquots from all experiments were frozen and kept in the dark for subsequent phosphorescence and EPR spectroscopy. Similar experiments were performed with irradiated aqueous solutions of tryptophan or thymine plus 8‐MOP. All controls consisted of solutions kept in the dark. NMR spectra demonstrated that the hydrogen atoms at the 3,4 and 4',5' positions of the 8‐MOP molecule were lost following irradiation, suggesting that these two sites were involved in the photoproduct formed between tryptophan and 8‐MOP. These studies strongly suggest that 8‐MOP is capable of forming photoaddition products with tryptophan and with lens proteins as well as DNA in vivo, resulting in its permanent retention within the ocular lens.