There is no more toxic and operatively acting pathogenic factor than oxygen deficiency in the body due to the presence of such a large number of enhancers and distributors of its pathogenic effect on cell metabolism.
The whole history of oxygen life takes place under the sign of the economical consumption of always scarce oxygen, at all levels of the organization.
An important mechanism for this saving is the creation of oxygen reserves, especially in intensively functioning tissues and organs. The central nervous system, which is the most powerful and most intensive consumer of oxygen (per gram of mass per unit of time) as the main energy carrier, uses glucose, a semi-oxidized product containing its own oxygen. Glial cells that perform auxiliary functions contain glycogen, which also allows them to conserve oxygen, which is necessary for the functioning of neurons. I will dwell on other mechanisms for saving oxygen later.
I will list some of the main primary consequences of hypoxia for cells and the body as a whole.
1. Activation of an energy-dependent, regulated process of programmed cell death – apoptosis, which is safe for the surrounding tissues and for the organism as a whole, as a result of external influences. Apoptosis is not self-destruction of a cell, but it killing by external factors, in the extreme case, apoptosis can be considered as forcing cells to commit suicide by external factors: – the main physiological – cortisol (circadian rhythm), which with age increasingly becomes pathological (age-dependent growth basal level of cortisol and distress), and the main pathological one – hypoxia.
There is not enough oxygen for the simultaneous work of all cells of the body, it is necessary to save the “most valuable” ones, getting rid of ineffective cells for the survival of the body under hypoxic conditions, and also get rid of cells that may be restored from stem cells. Cascade mechanisms of the sequential elimination of cell components in a certain order require the expenditure of free energy in the form of ATP hydrolysis (for example, ubiquitin).
1.1. Activation of the production of free oxygen radicals by the respiratory chain of dying mitochondria. Free radicals of oxygen (* OH) and nitrogen (* NO), possessing high values of the oxidative potential, as well as ATP and NAD(P)H are mass carriers of free energy and are involved in the normal energy metabolism of cells. Free oxygen radicals generated by dying mitochondria are products of cell apoptosis, but not vice versa, as is often found in the literature.
It is the oxygen deficiency that leads to a network of events ending in apoptosis: – slowing down of the transport of electrons along the respiratory chain; – a decrease in the electrochemical potential difference of hydrogen ions on the inner mitochondrial membrane; – swelling of mitochondria with disruption of the integrity of the outer mitochondrial membrane; – exit from the intermembrane space into the cytoplasm of cytochrome C, which leads to disconnection from the respiratory chain of cytochrome oxidase and to the termination of direct transfer of electrons to oxygen (disconnection of cytochrome oxidase from the respiratory chain is an elegant evolutionary device that excludes the possibility of senseless and therefore harmful “eating” oxygen that is already deficient under conditions of hypoxia); – activation of the reverse transfer of electrons (against the redox potential of the electron carriers of the respiratory chain) entering the respiratory chain from dehydrogenases of the second conjugation point; – increasing the concentration of the reaction product of one-electron reduction of Coenzyme Q; – chemical reaction of oxygen with the Coenzyme Q radical, leading to an increase in the concentration of free oxygen radicals.
1.2. The main results of the impact of free oxygen radicals generated by dying mitochondria. The most important result of the action of free oxygen radicals is the chemical modification of mitochondrial DNA, which is surrounded on all sides by outgrowths of the inner membrane (cristae), in which the enzymes of the respiratory chain are localized. The number of DNA copies in mitochondria reaches 10, and the number of mitochondrial DNA copies per cell is several tens of thousands due to the large number of mitochondria in it.
The main function of free oxygen radicals generated by the respiratory chain of mitochondria of cells that have entered apoptosis, which is positive for the body, is the covalent modification of mitochondrial DNA and mitochondrial enzymes of its duplication. The meaning of these processes is the inactivation or neutralization of mitochondrial DNA, which is in origin and structure (without introns and without histones) bacterial DNA, capable of integrating into cellular DNA and thereby facilitating cell transformation [20].
This does not mean that the appearance of free oxygen radicals (like many other, especially chemically active metabolites) in the wrong place and/or in unusually high concentrations exceeding the capabilities of antioxidant protection does not harm the cell and the body as a whole. This situation, apparently, is realized under conditions of intense radiation exposure.
The function of free oxygen radicals generated by NADPH oxidase of the plasma membrane of immunocompetent cells is also similar, the activity of which increases when they interact with bacteria and viruses. The meaning of the generation of free oxygen radicals, and in this case, lies in the covalent modification of foreign DNA. To destroy a bacterium or cell means, first of all, to damage its DNA.
The pathogenic function of an excess of antioxidants consumed by humans is to reduce the rate of mitochondrial DNA detoxification by free oxygen radicals, which, apparently, leads to an increase in the likelihood of oncological diseases [10].
1.3. Safety of free oxygen radicals generated by the mitochondria of a dying cell for neighboring cells. Due to the high chemical reactivity of free oxygen radicals and due to the small distances of their free path, neighboring cells with intact mitochondria are probably not susceptible to the pathogenic effects of these radicals.
First, in order to leave the mitochondria of a dying cell and get into a neighboring healthy cell, free radicals need to overcome many membranes with built-in densely packed proteins that contain a large number of potential targets for free radicals (unsaturated bonds in lipids and proteins; strong and numerous reducing agents in the form of natural antioxidants – vitamins, glutathione and thiol groups of proteins; as well as enzymes – catalase, peroxidase and superoxide dismutase, which neutralize radicals.
Secondly, even single free radicals that have reached the mitochondria of a neighboring healthy cell are able to engage in the normal functioning of their respiratory chains due to a chemical reaction with Coenzyme Q, a 50-fold excess of which in relation to other electron carriers (cytochromes, ferredoxins and dehydrogenases) is present in the inner membrane of mitochondria and diffuses freely in the membrane.
2. Activation of the disordered process of cell death – necrosis under conditions of deep or prolonged hypoxia, harmful to the surrounding tissues and to the organism as a whole. Disruption of apoptosis into necrosis is caused by a deficiency of oxygen and, consequently, a deficiency of free energy in the form of ATP and NAD(P)H, which are necessary to bring the energy-dependent process – apoptosis to the logical end.
3. Inflammation and autoimmune diseases. One of the last substrates inaccessible to proteases involved in apoptosis are transmembrane proteins of the plasma membrane. These proteins are present in apoptotic bodies, the end products of apoptosis, which are successfully captured by cells and digested by lysosomal enzymes of cells of the immune system. Interruption of this sequence of events under hypoxic conditions leads to the appearance of transmembrane proteins in the blood and to inflammation. The production of antibodies simultaneously against the external and intracellular epitopes of such proteins is likely to lead to autoimmune diseases accompanied by inflammation.
Some of these proteins may play the role of anchoring, that is, devices for mechanically fixing the contacts of a neuron and its extended processes with neighboring cells that have similar proteins in their membranes, the external water-soluble fragments of which form strong isological dimers with similar fragments of proteins of neighboring cells. After the death of a neuron and the triggering of a specific protease that cleaves off the outer fragments of these proteins, the latter form a densely packed and poorly metabolized conglomerate – beta-amyloid, which