Photoreactions of 3,3',4',5-tetrachlorosalicylanilide with proteins.

The photoreactions of 3,3',4',5-tetrachlorosalicylanilide anion (TCSA-) with two serum proteins were studied. TCSA- and anions of two similar compounds, N-ethyl-3,5-dichlorosalicylamide and salicylanilide, bind noncovalently to human serum albumin (HSA) without irradiation in aqueous pH 7.4 buffered solutions. TCSA- noncovalently bound to HSA yields three types of photoproducts when irradiated with ultraviolet light (lambda greater than 360 nm). A covalently bonded photoadduct between TCSA- and HSA is formed and histidines in HSA are chemically modified. In addition to these two types of photoproducts which involve HSA, two of the four TCSA- photoproducts which form when HSA is absent are also formed when the TCSA-/HSA complex is irradiated. The results presented indicate that not all proteins in the skin are capable of being the carrier protein in photoallergy of TCSA- and that cross reactivity to other halogenated salicylanilides can be explained by further photochemical reactions of TCSA- photoproducts.