Photochemical Labeling: Can Photoaffinity Labeling be Differentiated from Site‐Directed Photochemical Coupling?

Abstract

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 receptormediated mechanism.

Site-Directed Photochemical Coupling of Cytochrome b6 f-Associated Chlorophyll,

“…Following the exchange of the b 6 f-bound Chla molecule for [ 3 H]Chla and irradiation at 430 nm, covalently bound radioactivity was found to be associated predominantly with Cytf. The low yield of labeling (0.1-2.7% of the amount of [ 3 H]Chla introduced into the site) is typical for site-directed photocoupling (39). Some radioactivity, the specificity of which is more difficult to ascertain, also comigrated with the small, ∼4-kDa subunits.…”

Dynamic Structural Investigations on the Torpedo Nicotinic Acetylcholine Receptor by Time‐Resolved Photoaffinity Labeling

“…tagonists ([ 3 H]DDF, [43,44] [ 3 H]Bz 2 Ch, [45] [ 3 H]TDBzCh [46] ) and agonists ([ 3 H]DCTA [47] and [ 3 H]APFBzCh [48] ), or by site-directed photochemical cross linking [49] with unmodified modulators (the agonist [ 3 H]nicotine [50,51] [52,53] ), for reviews see ref. [32][33][34].…”

Substituted Isoquinolines and Quinazolines as Potential Antiinflammatory Agents. Synthesis and Biological Evaluation of Inhibitors of Tumor Necrosis Factor α

“…Photoaffinity Radiolabeled Derivative. The use of photoaffinity labels to elucidate the target of small molecule binding has been a valuable technique 18 for gaining information of biological interest. A compound having some activity in the TNFR in vitro assay and containing both a photoactivatable group as well as a radioisotope seemed most suitable for our purposes.…”