Before a cell is broken or lysed, mitochondria in an infected eukaryotic cell sense the presence of an intruding microbe by detecting the diversion of electrons (as NADH and NADPH) and carbon to viral biogenesis centers for polymer synthesis to make viral RNA, protein, and DNA from building blocks in the host cell. This “electron steal” is sensed as a voltage drop, or decrease in electron flow available within the cell for oxidative phosphorylation in mitochondria. The metabolic consequences are nearly instantaneous. Mitochondria rapidly decrease their oxygen consumption, which is coupled to electron flow. The dissolved oxygen concentration in the cell begins to rise because mitochondria are the oxygen sink in every eukaryotic cell. This makes the cellular redox chemistry more oxidizing (
Naviaux, 2012). Highly oxidizing environments strongly inhibit the assembly of monomeric building blocks into polymers, and rapidly decrease the efficiency of RNA, protein, and DNA synthesis by the infecting virus. Oxidizing conditions also result in the oxidation of sulfur in methionine, and thiols like cysteine, homocysteine, and glutathione, and the disassembly of iron–sulfur clusters in many enzyme systems, and decrease the availability of the thiol of coenzyme A that is essential for intermediary metabolism.
The ability of mitochondria to monitor electron flow and sulfur oxidation makes them ideally suited as generalized cell “danger alarms”. Their rapid metabolism makes mitochondria the “canaries in the coal mine” for the cell. Any trace or heavy metal that acts as an electrophile or sulfurophile in the cell will trigger a mitochondrial response that is similar to that of a viral infection, because metal electrophiles and replicating pathogens both divert and consume electrons. Likewise, a large number of molecules have been synthesized since the 1850s as dyes, pesticides, drugs, and industrial chemicals. Many are polyaromatic and halogenated. These modern chemicals with conjugated ring systems, multiple double bonds, and delocalized π orbital electron clouds are highly electrophilic and will produce an electron steal within the cell that can also activate the CDR. The CDR is a generic, but highly evolved response that often complicates more specific molecular effects that occur when a synthetic molecule binds to a receptor, or competes with and disrupts normal metabolic or hormone signaling. Mixtures of chemical and biological threats can have synergistic effects, and the total load of danger triggers can influence the magnitude and form of the CDR. When danger is detected, mitochondria alter cellular metabolism to help shield the cell from further injury. This is accomplished by stiffening cell membranes, activating the production of reactive oxygen species (ROS), and producing changes in many different pathways in intermediary metabolism that have the effect of limiting pathogen replication and limiting the spread of danger (
Naviaux, 2012). These pathways are immature in newborns and growing children (
Wood et al., 2010), leading to effects that are not limited to inflammation and innate immunity in peripheral tissues, but can also alter neurodevelopment (
Landrigan et al., 2012) and increase the risk of other chronic childhood diseases.