Differential sensitivity of major types of opioid binding sites to U.V. irradiation

The three major types of opioid binding sites exhibited differential sensitivity to inactivation by ultraviolet irradiation. The order of sensitivity was delta greater than mu greater than kappa. Two separate rates of inactivation by U.V. light were observed for each type, an initial fast rate and a secondary slow rate. Addition of EDTA or EGTA to the buffer medium decreased only the initial fast rate of inactivation. The photosensitive tryptophan and tyrosine moieties have been shown to be photolyzed in a two step manner in a number of protein systems. It is suggested that the differential sensitivity of the multiple opioid binding sites may reflect either differing amino acid composition or differences in tertiary structure.