The Hidden History of Redox Signaling
Peter H. Proctor, PhD, MD
Forman (1) provides a concise history of the role of reactive species in cell signaling, e.g., noting Szent-Gyorgyi's conjectures concerning a role for electronic mobility in biological materials as a mechanism for (say) hormonal action. Interestingly, the first empiric evidence for electronically-activated processes in cell signaling was clinical observation (2).
Examples include the long-known association of radical-generating agents such as copper, iron, or manganese with specific symptomology. This includes dyskineas ( movement disorders ) and other neuropsychiatric symptoms, diabetes, fibrosis, and deafness, as well as associated-changes in the skin pigment melanin (reviews, 2-5).
For example, the skin pigment melanin is the only overtly-visible biological free radical. Melanin was also the first well-defined free radical in biological systems (38). In addition to skin, visible melanin is present in extradermal structures such as the inner ear and in catecholaminergic nerve cell bodies in the midbrain substantia nigra, and locus caeruleus. In such structures it may have a role in cellular function, in dopamine, serotonin, and norepinephrine-mediated cell signaling, and in organ development.
Thus, Charles Darwin ( 6 ) proposes that “..a slight arrest of development in the nervous system..” may explain deafness in white, blue-eyed cats. CNS development is per se a messenger-mediated process. Likewise, light exposure induces dermal melanin synthesis, another complex and closely-regulated cellular process.
Similarly, hyperbaric oxygen causes seizures, a reactive oxygen species (ROS ) -mediated process difficult to explain by simple tissue damage. Interocular copper or iron produce vitreous fibrosis disproportionate to actual damage. Further, in hemochromatosis and Wilson's disease iron or copper deposition accompany liver fibrosis or “cirrhosis”, skin pigmentation abnormalities, psychosis, and dyskinesia (2-5). The neuropsychiatric symptoms of Wilson's disease appear before any overt evidence of tissue damage, while another name for hemochromatosis is bronze diabetes.
Likewise, Hoffer and Osmond's Adrenochrome Hypothesis ( 7 ) holds that psychoactive oxidation products of catecholamines (adrenochromes ) figure in the etiology of schizophrenia and other neuropsychiatric diseases. Now long-supplanted, but the basic concept is correct. Interestingly, this work resulted in the use of nicotinic acid in the treatment of hypercholesterolemia and dyslipidemia. Six-decades later, this is still a primary treatment for dyslipidemias when statins are contra-indicated.
Another key discovery was the isolation of orgotein (11). This anti-inflammatory blue copper-zinc protein from bovine liver was eventually approved for veterinary as well as human use in some countries as an antiinflammatory and radioprotective agent. Orgotein proved to be ZnCu superoxide dismutase (SOD1). This provided a major tie-in between inflammation and other pathogenic processes such as cancer cell growth and redox cell signaling. Stated-simply-- an effect on inflammation is mechanistically too complicated to be simply-assigned to preventing non-specific tissue damage, particularly when we already knew that (e.g.) dopaminergic neurotransmission likely involves reactive species.
Over four decades ago, Cotzias and coworkers noted (8,9) the likely role of mid-brain melanin and charge-transfer processes ( including oxidative stress ) in (e.g.) chronic manganism and other extrapyramidal syndromes, particularly Parkinsonism. Patton (10) quotes Cotzias: “I don't believe God put the melanin granule in the central nervous system for nothing. It must be doing something. Something big, something that will win me the Noble prize." Eventually this led to their development of Levodopa therapy for Parkinson's disease( 9 ) -- again, still a primary treatment.
Similarly, in the late 1960's, we encountered a patient with Lesch-Nyhan Syndrome. Lesch-Nyhan's is characterized by massive overproduction of purines and their metabolite uric acid, along with bizarre behavior (including self-mutilation ) and choreoathetoid dyskinesia. Significantly, this patient had originally been diagnosed as “schizophrenic”. Lesch-Nyhans Syndrome is now thought a primary model for bipolar mania. This disorder is often acutely-indistinguishable from schizophrenia, except by clinical history.
Thus oxidative and nitrosative stress likely plays a role in bipolar and maigraine comorbidity. See, e.g.,The Comorbidity of Bipolar Disorder and Migraine: The Role of Inflammation and Oxidative and Nitrosative Stress.. The same authors report that the xanthine oxidase-inhibitor allopurinol works in bipolar mania.
Synthesis of uric acid by xanthine oxidase produces reactive oxygen species. On the other hand, oxypurines are powerful-reducing agents. Thus, the classic clinical assay for urate depends upon its singular ability to directly reduce phosphotungstic acid, a property it shares with another antioxidant reducing substance, ascorbate (2). Also like ascorbate, uric acid is conditionally pro-oxidant (2), as are other purines (12).
Thus, drawing upon Cotzias et al (8) we proposed that electron-transfer processes produce the neuropsychiatric symptoms of Lesch-Nyhan's (2-6, 13-16), as they do those of chronic manganism. Most-importantly, oxidative-stress-mediated tissue damage alone seemed an unlikely cause of such bizarre and specific messenger-mediated neuropsychiatric symptoms as self-mutilation and choreoathetoid dyskinesia. The same seemed to hold for analogous symptoms in alcaptonuria, etc. (2-5,12-16).
Even four decades ago, the clear implication was that electron-transfer processes likely specifically mediate CNS cellular function-- say, by “doping” some organic semiconductor such as neuromelanin (2) and/or by (e.g.) modulating dopaminergic processes (13). This phenomenon is but one manifestation of what is now dubbed “redox signaling”.
Subsequent workers ( e.g.,17-20) refined the role of both dopaminergic processes and oxidative stress in Lesch-Nyhan's syndrome. Similarly, urate and/or xanthine oxidase-induced oxidative-stress have been implicated in (e.g.) atherosclerosis (6, 21), stroke (18,22,23), and diabetes (7, 23, 24 ). Moreover, SOD ameliorates hyperuricemic syndrome in Dalmatian dogs ( 25,26 ). As further proof of concept, such animals also tend to be both deaf and “bronzed” (25), a visible sign of underlying electronically-activated processes.
Likewise, higher primates have uniquely lost both the ability to make ascorbate and to break-down uric acid. So, we suggested that urate may substitute for ascorbate in human evolution (27), say, as an antioxidant or enzyme cofactor. Most importantly, “this does not exclude other physiological roles for uric acid” (27) . Other proposed functions (2-5) of uric acid and similar charge-transfer agents such as homocysteine included (e.g.) acting as a pro- or anti-oxidant, a “dopant” for some semiconducting biopolymer such as neuromelanin, or some more direct action as a cellular messenger. Accordingly, urate elevation is now in clinical trials for both stroke and Parkinson's disease.
Such multifunctionality ( including cell signaling ) has proven the case. However, researchers can still only speculate whether uric acid is causative, ameliorative, or (most likely) both in, e.g., atherosclerosis and stroke (22-24). We likewise extended this to more human diseases and more characteristic symptoms ( 2-5 ). This includes additional putative oxidative-stress-related symptoms such as psychosis, pigmentary abnormalities, and deafness, as well as atherosclerosis and diabetes ( e.g., in hemochromatosis ). E.g., ref (2) seems to be the first suggestion that homocysteine pathogenesis involves oxidative stress.
Investigating another aspect of redox signaling and as proof of concept for McGinness' mobility gap conduction model (28), we also constructed (29) an “active” organic polymer electronic device. An "active" device is one in which a current or voltage controls resistivity, as in a transistor. This was a voltage-controlled bistable switch using melanin. The “ON” state of this switch exhibits almost metallic conductivity. Significantly, doping with charge-transfer agents associated with (e.g.) neuropsychiatric symptomology modulated the electrical properties of the device, a la Szent-Gyorgyi. Cytochrome-C also switched, but at a much higher voltage gradient, though one achievable in vivo.
Concerning this device, Hush's history (36) of organic electronics states: “Also in 1974 came the first experimental demonstration of an operating molecular electronic device that functions along the lines of the biopolymer conduction ideas of Szent-Gyorgi.”. Hush also notes that the electronically-active material exhibits negative differential resistance, a hall-mark of present-day electronically-active organic polymers. Similar materials are now used in actual (literally) printed circuits. Organic electronics is also part of “nanotechnology”. So this is arguably the first “nanotech” device, well before the term was coined. Paradoxically, although virtually unknown in redox cell signalling, this organic semiconductor electronic device is now on the short Smithsonian chips list (37) of historic milestones in a rather different area, semiconductor physics and technology. It is also in the Smithsonian National Museum of American History's collection of historic electrical devices.** This device may also be an early example of protonic conduction in biological materials.
For more modern examples of the possible role of organic semiconductor processes in redox signaling, see references 39 and 40.
By the late 1970's, convincing evidence had accumulated indicating redox signaling is a general phenomenon, mediating much more than neuropsychiatric symptoms and dermal melanization. We also had experimental evidence (28,29) supporting Szent-Gyorgyi's conjectures about conductive biological polymers and their modulation by charge-transfer processes. So, at a 1979 “International Congress of Free Radical Researchers” we proposed (21) that "One explanation for this data is that various active oxygen species ( or such products as hydroperoxides ) may act as specific transmitter substances" and “...We suggest that active oxygen metabolites act as specific intermediary transmitter substances for a variety of biological processes including inflammation, fibrosis, and possibly, neurotransmission ..".
Simply-stated, electron-transfer processes mediate, e.g.. dyskinetic symptoms in various diseases, self-mutilation in Lesch-Nyhan's, and melanocyte function. So why not also equally complex processes such as (say) inflammation and fibrosis ?
This initial effort to generalize the concept of redox signaling failed to gain traction. In fact, due to strong reviewer objection, this manuscript was omitted from the published proceedings of the conference, although another paper from our group was included (30). Among other things, the latter paper reports (as did others at the meeting) that superoxide dismutase suppresses tumor growth in experimental animals. This was one more indirect confirmation of the cellular messenger properties of reactive species. ROS are now known to be important mediators of cell growth and replication in cancer.
Now a given, for a time opposition to a messenger role for electronically-active species was so strong that it took years to publish. Tellingly, this was only by taking advantage of the freedom allowed reviews and invited book chapters (4,5).
Eventually, other researchers such as Bochner et al ( 31 ) began to relate oxidative stress to modulation of specific biochemical pathways, making the concept more generally-palatable. By the mid-1990's, the concept was respectable enough that major review articles started to appear (39). Meanwhile, the early work had been forgotten.
*Szent-Gyorgyi, A., 1941b. The study of energy-levels in biochemistry. Nature 148 (3745), 157–159. Szent-Gyorgyi, A., 1957. Bioenergetics. Academic Press, New York. Szent-Gyorgyi, A., 1960. Introduction to a Submolecular Biology. Academic Press, New York. Szent-Gyorgyi, A., 1968. Bioelectronics. Academic Press, New York. Szent-Gyorgyi, A., 1976. Electronic Biology and Cancer. Marcel Dekker, Inc., New York. Szent-Gyorgyi, A., 1978. The Living State and Cancer. Marcel Dekker, Inc., New York.
** However, as Nicolaus and Parisi note (34), all melanins are polyacetylenes and vice-versa. Well before us, other researchers reported passive "resister-like" high conductivity in polyacetylene derivatives. E.g., highly-conductive iodine-doped polyaniline "melanins" were first reported in 1963 (32,33). By the mid 1960's, researchers achieved conductivities less than 1 ohm/cm, comparable to present-day efforts.
Remarkably, the 1977 rediscovery of passive high conductivity in almost-identical iodine-"doped" oxidized polyacetylenes (35) eventually won the 2000 Nobel Prize in Chemistry for " the discovery and development of conductive polymers...''. In an unprecedented lapse, the Nobel committee completely missed the previous substantial body of research on highly-conductive polyacetylene derivatives, of which we were merely the last.
Thus, Inzelt's textbook "Conductive Polymers" ( 33 ) contests the Nobel “discovery” assignment, noting that such highly-conductive polymers were well-known and had even been applied well before the work of the Nobel laureates. The ”development” part also involved a restatement of McGinness' mobility-gap model for electronic conduction in organic polymers (28)-- e.g., adding the special case of soliton migration in pure polyacetylene.
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Keywords: uric acid urate ascorbic urate ascorbate pro-oxidant proxidant nanotechnology antioxidant atherosclerosis stroke purine melanin pigment cell nitric oxide NO uric acid pro oxidant schizophrenia synthetase NADPH oxidase NOX pbn mnp organic semiconductor pigmentation abnormalities excitotoxicity redox cell signaling fructose substantia nigra hydrogen peroxide organic metals conductive polymer antioxidant myocardial infarction vitamin-c. free radical scavenger tempol tempo spintrap hypertension human evolution molecules.