Photochemical labeling: Can photoaffinity labeling be differentiated from site-directed photochemical coupling?

Site-directed photochemical labeling is a methodology designed to irreversibly and specifically label, through the action of light, a ligand binding site of a biological macromolecule. Photoaffinity labeling, a widely used site-directed labeling methodology, uses photosensitive ligand analogs generally obtained after chemical modification of the ligand by introducing an appropriate photoactivatable moiety, This methodology can be applied to natural ligands showing inherent photosensitivity, without any additional modification, and which can be linked efficiently to their receptor binding site by direct photoactivation. The emergence of an alternative methodology that links nonphotosensitive ligands to their receptors has raised the question of their potential use and their mechanisms of photocoupling, This article presents a series of examples that are meant to compare the general characteristics of the different site-directed labeling reactions and proposes distinct photochemical activation processes between photoaffinity labeling and site-directed photochemical coupling reactions. We suggest in particular that the former is necessarily a ligand-mediated activation process while the latter might involve a receptor-mediated mechanism.