Photodynamic Action

injury to biological structures and disruption of their function upon the absorption of light by a pigment or dye in the presence of oxygen.

In photodynamic action, the unpigmented molecules, which do not directly absorb visible radiation, are damaged. A pigmented substance, which absorbs photons, serves as the photosensitizer, that is, the mediator in the photoreaction; it mediates the oxidation of the substrate and the formation of the product of photodynamic action. Molecules of the dye apparently participate in the photoprocess in the triplet excited state. Among the reactive dyes involved in photodynamic action are acridines, anthraquinones, a number of porphyrins, and riboflavin.

Various organic substances may serve as substrates of the reaction; many structures and functions on the organismic, cellular, and molecular levels are sensitive to photodynamic action. Thus, photodynamic action may produce erythema and photodermatitis when reactive dyes are applied to the skin; it may also produce intoxication when light is absorbed by the free porphyrins of the blood if there is disruption of porphyrin exchange. Poisonings have been known to occur in animals after they have eaten plants containing a photodynamically active pigment, for example, hypericin in Saint-John’s-wort. Photodynamic action may initiate carcinogenic processes in pigmented structures.

On the cellular level photodynamic action may stimulate or inhibit cell division, cause mutations, produce bactericidal action, and damage biomembranes. Photodynamic action is known to affect physiological and biochemical processes, such as respiration, oxidative phosphorylation, and photosynthesis. The basis of many of the effects of photodynamic action is the damage to the molecules of proteins, such as enzymes, caused by the oxidation of the amino acids in the molecules. The effect on the genetic apparatus, on bacteria, and on viruses is due to inactivation of the nucleic acids, which occurs as a result of the destruction of nitrogenous bases.