Near Infrared Laser Exposure Enhancement of Cytochrome C Oxidase Enzyme Activity does not Exhibit Irradiance Reciprocity

Photobiomodulation (PBM) is the use of visible or near infrared (NIR) light to induce positive biological/therapeutic effects. It is generally believed that the chromophore for these effects is cytochrome c oxidase, a.k.a. complex IV (C-IV) of the mitochondrial electron transport chain. However, the mechanisms by which these pathways are initiated and how they establish observed beneficial outcomes are still unclear. We have found that C-IV enzyme kinetics can be enhanced by exposure to 808 nm NIR laser light. We used a spectrophotometric method to monitor C-IV activity, by observing changes in the redox state of the small electron carrier protein cytochrome c. By examining a range of irradiances and exposure times, we collected data from samples exposed to overlapping radiant exposures, but at different irradiances. If the mechanism by which NIR exposure enhances C-IV activity is a traditional photochemical reaction, it is expected to exhibit irradiance reciprocity, meaning the biological outcome of a photoreaction is dependent upon the radiant exposure to which the sample is exposed (e.g. the same bioeffect is observed if irradiance is halved and exposure duration is doubled). Surprisingly, this was not the case, and instead irradiance was found to be the exposure parameter best correlated with the degree of C-IV activity enhancement. This violation of irradiance reciprocity is strong evidence that NIR exposure does not rely on delivery of a particular number of photons. This implies either light acts on C-IV indirectly or there are other secondary processes at work, before an eventual enhancement of C-IV activity.