# Healing Effects of Photobiomodulation on Diabetic Wounds
## 3 Photobiomodulation (PBM)
### Mechanisms of PBM  
The precise mechanisms of action of PBM are not fully known and understood, however a broad range of effects at the molecular, cellular, and tissular levels have been observed. What is known is that PBM appears to have a profound effect on the cellular mitochondria where the photons are absorbed by various components of the respiratory chain, primarily complex IV or cytochrome c oxidase (Cox), which transfers electrons from cytochrome c to molecular oxygen [17; 18; 19]. The photoreactivity of Cox is due to its four metal centers; two heme moieties (heme a and heme a3) and two redox-active copper sites (CuA and CuB) [20]. [PERSON] and colleagues [21] revealed that the redox state of Cox is influenced by red light, and is dependent on the initial redox state at the time of irradiation. It was also shown that irradiation intensified the transfer of electrons in Cox, resulting in accelerated oxidative phosphorylation [20]. An increase in other electron transfer complexes has also been observed. [PERSON] and co-workers [22] found an increase in enzyme activity of complex I, III, and IV in irradiated isolated mitochondria (660 nm; 10 mW/cm\({}^{2}\); 0.6, 1.2, 2.4, and 4.8 J/cm\({}^{2}\)). [PERSON] et al. [23] found up-regulation of genes coding for complex I, IV, and V following irradiation at 660 nm (11 mW/cm\({}^{2}\), 5 J/cm\({}^{2}\)). The effect on mitochondria results in increased adenosine triphosphate (ATP) synthesis [19; 24], proton electrochemical potential [24; 25], and oxygen consumption, as well Nicotinamide adenine dinucleotide (NADH) synthesis [26]. The increase in ATP has been found to peak immediately in irradiated mouse embryonic fibroblasts (810 nm; 0.3, 3, and 30 J/cm\({}^{2}\)) and decline to baseline levels over 6 h [27]. Increased levels of ATP following PBM have also been found under hypoxic conditions [28; 29; 30], which normally results in decreased ATP synthesis [29]. There is also modulation of intracellular reactive oxygen species (ROS), which are involved in cell signaling pathways and gene transcription. Under normal physiological conditions, ROS are produced during the synthesis of ATP, and since PBM boosts oxygen metabolism, it also acts to increase ROS production [16]. PBM also displaces nitric oxide (NO) from Cox. NO inhibits mitochondrial respiration by binding to the heme iron:copper binuclear center (a3/CuB) of Cox, thus displacing oxygen [31; 32]. This photodissociation allows for the influx of oxygen and resumption of respiration [31; 33]. It has also been hypothesized that PBM may also dissociate NO from intracellular stores, such as nitrosylated hemoglobin and myoglobin, leading to vasodilation [34]. Laser irradiation (830 nm, 4.4 mW/cm\({}^{2}\), 5 J/cm\({}^{2}\)) of human skin fibroblast cells resulted in an increase in ROS and NO 15 min post-irradiation in what would appear to be a direct photochemical effect [35]. [PERSON] et al. [27] demonstrated an increase in mitochondrial ROS fluorescence in irradiated (810 nm; 1-30 mW/cm\({}^{2}\); 0.3, 3, and 30 J/cm\({}^{2}\)) murine embryonic fibroblasts. [PERSON] et al. [36] found that the generation of ROS in irradiated (632.8 nm, 0.5 to 16 J/cm\({}^{2}\), 0.64 to 1.16 mW/cm\({}^{2}\) for whole cell culture irradiation; and 330 mW/cm\({}^{2}\) to 20 W/cm\({}^{2}\) for single cell irradiation) human skin fibroblasts was dependent on laser fluence and not on laser intensity. [PERSON] et al. [30] showed that PBM increased intracellular NO in irradiated cardiac cells (670 nm, 25 mW/cm\({}^{2}\), 7.5 J/cm\({}^{2}\)), an effect which was no longer evident when NO scavengers were added and partially impeded by nitric oxide synthase (NOS) inhibitors. [PERSON] et al. [30] came to the conclusion that the increase in NO was due to NOS and a second unidentified source, possibly photodissociation from Cox. When it comes to irradiation and destruction of bacteria with blue/violet light, the mechanism of action centers around the production of high amounts of ROS.