The Hidden History of Redox Signaling
Peter H. Proctor, PhD, MD
Forman (1) gives a concise history of the role of reactive species in cell signaling, e.g., noting Szent-Gyorgyi's hypotheses 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 actually clinical observation (2). Examples include the long-known association of radical-generating agents such as copper, iron, or manganese with specific symptomology. This includes dyskinesias ( 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 and was the first well-defined
free radical in biological systems (38). Melanin is also found
in extradermal structures such as the inner ear and in
catecholaminergic nerve cell bodies in the midbrain substantia nigra,
and locus caeruleus, where it may have a mediator role.
Thus, Charles Darwin ( 6 ) proposed 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. There are also early reports of skin pigmentary abnormalities in schizophrenics.
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 ( “chalchosis” ) 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”.
However, the first direct suggestion that redox processes are related to cellular communication is apparently Hoffer and Osmond's controversial “Adrenochrome Hypothesis” ( 7 ). This holds that psychoactive oxidation products of catecholamines ( adrenochromes ) figure in the etiology of schizophrenia and other neuropsychiatric diseases. Whatever the quibbles over mechanism, their underlying concept that redox processes mediate psychiatric disease is sound. Free radical research has had direct clinical application almost from the start-- This theory also indirectly resulted in the clinical use of nicotinic acid in hypercholesterolemia, still a primary treatment. Similarly, Cotzias and coworkers noted (8,9) the likely role of mid-brain melanin and charge-transfer processes ( including what is now dubbed “oxidative stress” ) in the etiology of (e.g.) chronic manganism and other extrapyramidal syndromes, particularly Parkinson's disease. 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.... ".
Eventually this led to their development of Levodopa therapy for Parkinson's disease ( 9 ) -- again, still a primary treatment. 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, providing a major tie-in between inflammation and other pathogenic processes such as cancer cell growth and redox cell signaling.
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 misdiagnosed as “schizophrenic”. 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 can specifically mediate CNS cellular function-- say, by “doping” some organic semiconductor such as neuromelanin (2) and/or by (e.g.) modulating dopaminergic processes (13). Globally, this phenomenon is now called “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, 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.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.
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 tightly-controlled processes such as (say) inflammation and fibrosis ?
Due to reviewer skepticism about redox signaling, 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--- one more indirect confirmation of the cellular messenger properties of reactive species. Now a given, for a time opposition to a messenger role for electronically-active species was so strong that we finally published this concept only by taking advantage of the freedom allowed reviews and invited book chapters (4,5), where the global concept was "lost" until resurrected a decade or so later. 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.
Similarly, as proof of concept for McGinness' mobility-gap organic polymer conduction model (28), we constructed (29) an “active” conductive 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 (such as iodide) associated with neuropsychiatric symptomology and pigmentary abnormalities 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.
Passive "resister-like" high conductivity in analogous “doped” polyacetylene derivatives was described well before us ( e.g., 32,33).* As Nicolaus and Parisi note (34), all melanins are polyacetylenes and vice-versa. Some early “melanins” even had resistivities below 1 ohm/cm, comparable to present efforts. However, we do seem to have reported the first "active" organic-polymer electronic device.
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 electronically-active organic polymers. Similar materials are now used in actual “printed” circuits. Organic electronics is also part of “nanotechnology”. So this is arguably the first “nanotech” device, well before the term was coined. Unheralded in redox cell signaling, this conductive polymer device is now on the short Smithsonian chips list (37) of milestones in another area, semiconductor technology. It is also in the Smithsonian National Museum of American History's collection of historical electrical devices.
Footnotes:
* While first reported in 1963 (32), the 1977 rediscovery of passive high conductivity in an iodine-"doped" oxidized polyacetylene (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 substantial previous body of research in highly-conductive and semiconductive polymers, of which we were merely the penultimate. This did not go un-noticed. E.g., conceding that the Nobelists deserve credit for popularizing and publicising the field, Inzelt ( 33 ) contests the Nobel “discovery” assignment. noting that highly-conductive polymers were clearly already well-known and had been synthesized, studied and " even 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.
A personal note: This event particularly illustrates why I now question the ability of the present highly-competitive and politicised scientific community to properly assign discovery credit to smaller, less-connected and less-publicised players. Thus, my in-your-face exercise here. Personally, I went over to the dark side-- medical practice and patenting drugs-- years ago. So did Abram Hoffer of adrenochrome fame, who until his recent death ( in his 90's ) maintained that it would take a generation or two before people understood his work. So my stake here is small-- just bragging rights and a little "I told you so" vindication. Admittedly, organic electronics may be uniquely troubled. E.g., the Jan Hendric Schon science fraud case falsely-asserted nano-scale versions of our organic electronic device.
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