"It could be folly to assume that peripheral neuropathy and pigmentation ought to converge towards the same causal factor. These two symptoms could very well be etiologically unrelated despite their temporal association. I would even go so far as to say that they are quite distinct, and that all cases of peripheral neuropathy reduces down to one molecule. I have inferred this before, yet I have now found confirmation of this. Consider the following:
Stavniichuk, Roman. "
Peroxynitrite and protein nitration in the pathogenesis of diabetic peripheral neuropathy."
Diabetes/metabolism research and reviews (2014)
'Accumulation of nitrotyrosine, a product of peroxynitrite-induced protein nitration, has been documented in peripheral nerve [31], vasa nervorum [7–9] and dorsal root ganglion [32] in streptozotocin diabetic rats, and peripheral nerve, spinal cord and dorsal root ganglion of streptozotocin diabetic and ob/ob mice [31,33–35] indicating that diabetes creates not just oxidative but oxidative/nitrosative stress in the peripheral nervous system.' ―Stavniichuk
'Our findings implicate both peroxynitrite injury in toto and its component, protein nitration, in the development of advanced diabetic peripheral neuropathy; they also suggest that protein nitration does not account for all detrimental effects of peroxynitrite, and that peroxynitrite, the most potent oxidant in biological systems, contributes to the development of diabetes-associated neuropathic changes through both protein nitration and oxidative stress.' ―Stavniichuk
'Our findings indicate that both peroxynitrite injury
in toto and its component, protein nitration, play an important role in the development of chronic diabetic peripheral neuropathy. [...] Treatment with a peroxynitrite decomposition catalyst alleviated peripheral nerve dysfunction and increased intraepidermal nerve fibre density, whereas a protein nitration inhibitor resulted in a significant improvement of functional indices only.' ―Stavniichuk
Janes, Kali. "
Bioenergetic deficits in peripheral nerve sensory axons during chemotherapy-induced neuropathic pain resulting from peroxynitrite-mediated post-translational nitration of mitochondrial superoxide dismutase."
PAIN (2013)
'This chronic neuropathy is characterized by bilaterally symmetrical sensory symptoms (eg numbness, tingling, and pain) appearing in the feet, or in both the feet and hands [49], and occurs with chemotherapeutics across drug classes with distinctly different anti-tumor mechanisms, such as taxanes (eg paclitaxel), platinum-complexes (eg oxaliplatin), and proteasome-inhibitors (eg bortezomib). Recent work in rats indicates a common pathophysiology for CIPN from these agents due to a long-lasting dysfunction in mitochondria of peripheral nerve sensory axons (PNSAs) [51,53,55,56] that can be blocked by mitoprotective agents, such as acetyl-L-carnitine [53,56] and olesoxime [52], and exacerbated by mitochondrial poisons [51].' ―Janes
'The triggering mechanisms of this mitotoxicity are unknown, but the potent nitroxidative species peroxynitrite (PN) may play a critical role. We have recently reported that PN contributes to the development of paclitaxel-induced neuropathic pain and that administration of a peroxynitrite decomposition catalyst (PNDC) can prevent this mechano-hypersensitivity [12]. Protein nitration by PN, the product of superoxide (SO) and nitric oxide (NO) [4], in pathological settings can lead to gain or loss of protein function [17].' ―Janes
'Here we confirm our recent findings that PN is a critical determinant of paclitaxel-induced neuropathic pain [12] and now extend to CIPN observed with two other distinct chemotherapeutic agents: oxaliplatin, which is used for metastatic colon cancer and other gastrointestinal tumors, and bortezomib, which is used for multiple myelomas [13].' ―Janes
Peroxynitrite (ONOO⁻) is formed by the spontaneous union of nitric oxide (ṄO) and superoxide (Ȯ₂⁻). Although not a free radical like its' substrates, peroxynitrite is actually more dangerous and characteristically adducts with tyrosine side-chains on proteins. Although Kali Janes assumes above that peroxynitrite-induced neuropathy occurs upon tyrosyl nitration of mitochondrial proteins, this assumption fails when noting the persistence of neuropathic symptoms greatly outlasts the mitochondrial turnover rate. I think it is more reasonable to assume the nerve proteins themselves are the targets, and specifically α- and β-tubulin: The subunits of long microtubule fibers invariably found at the centre of every nerve. Mitochondrial theories also fail to account for the reductions in nerve velocity noted.
Methylene blue predictably lowers the concentration of nitric oxide (ṄO), yet does so by increasing superoxide (Ȯ₂⁻). Although the resulting product—peroxynitrite (ONOO⁻)—is not a free radical like its' substrates, it is more dangerous on account of tyrosyl nitration. Although nitric oxide is most well-known: nitrogen dioxide (ṄO₂), nitrosonium (NO⁺), and peroxynitrite (ONOO⁻) are considered to be the most dangerous reactive nitrogen species. Methylene blue lowers nitric oxide by converting it into a more dangerous product.
Wolin, Michael. "
Methylene blue inhibits vasodilation of skeletal muscle arterioles to acetylcholine and nitric oxide via the extracellular generation of superoxide anion."
Journal of Pharmacology and Experimental Therapeutics (1990)
'Since a reduced form of methylene blue can generate superoxide anion (McCord and Fridovich, 1970) and tissues contain enzymes known to reduce methylene blue (Thunberg, 1930), the generation of superoxide anion could be involved in the inhibitory actions of methylene blue.' ―Wolin
'The inhibition of vasodilation to 0.01 μg acetylcholine by methylene blue of 66 ± 13% was completely prevented by suffusion of superoxide dismutase, whereas the inhibitory action of methylene blue was not altered in the presence of catalase.' ―Wolin
'Methylene blue is generally assumed to be an inhibitor of the activation of the soluble form of guanylate cyclase, however, recent evidence suggests that this assumption may not be completely accurate.' ―Wolin
'Since it is currently thought that all of these agents produce vasodilation through the activation of soluble guanylate cyclase via the formation of nitric oxide (Ignarro, 1989), methylene blue, at the concentration used, does not appear to function as an inhibitor of guanylate cyclase activation in the cremaster microcirculation. The reversal of the action of methylene blue by topical suffusion of superoxide dismutatase, but not by catalase, suggests that methylene blue inhibits the action of vasodilators through the extracellular generation of superoxide anion.' ―Wolin
'The effects of methylene blue on rat cremasteric arteriolar diameter are most consistent with an increased generation of extracellular superoxide anion, since the actions of this probe are antagonized by suffusion or extracellular application of superoxide dismutase.' ―Wolin
'A reduced form of methylene blue can generate superoxide anion (McCord and Fridovich, 1970), however, the mechanism of generation of this oxygen species has not been extensively examined.' ―Wolin
'It has been known since the early 1900s that many dehydrogenase enzymes present in mammalian tissues can reduce methylene blue and that the reduced form is readily oxidized by oxygen (Thunberg, 1930). Thus, methylene blue should be readily reduced by enzyme activities present in the cremaster microcirculation and the levels of superoxide anion generated by its auto-oxidation could conceivably be dependent on the tissue oxygen tension and superoxide dismutase activity. Since superoxide dismutase completely reverses the actions of methylene blue, an intracellular effect of methylene blue-elicited generation of superoxide anion is not detectable under the present conditions.' ―Wolin
The enzymes most influential on peroxynitrite concentrations are nitric oxide synthase, superoxide dismutase, and cyclooxygenase. As can be gathered from the study above, superoxide dismutase converts superoxide (Ȯ₂⁻) into the less-harmful hydrogen peroxide (H₂O₂) or water (H₂O). Nitric oxide synthase comes in three related forms (iNOS, eNOS, nNOS), and is responsible for transforming arginine into nitric oxide (ṄO) and citrulline. Cyclooxygenase lowers peroxynitrite concentrations by using it in prostaglandin H formation, creating its' characteristic endoperoxide bridge through peroxynitrite's (ONOO⁻) peroxy moiety (–OO⁻). Peroxynitrite is actually the other substrate besides the 3 known lipids—
i.e. dihomo-γ-linolenic, arachidonic, eicosapentaenoic—for prostaglandin H, and thus for all subsequent prostaglandins (
Landino, 1996).
Superoxide dismutase activity has consistently been shown to lower both superoxide and peroxynitrite, an enzyme also been shown to attenuate peripheral neuropathy; this is fully-consistent with peroxynitrite being the causal factor. Since peroxynitrite has another precursor besides superoxide contributing approximately half its molecular weight, a person could infer that the inhibition of nitric oxide synthase might also lower peroxynitrite synthesis and peripheral neuropathy. Indeed, this has been tested and confirmed in mice genetically-engineered to lack inducible nitric oxide synthase (iNOS⁻):
Vareniuk, I. "
Inducible nitric oxide synthase gene deficiency counteracts multiple manifestations of peripheral neuropathy in a streptozotocin-induced mouse model of diabetes."
Diabetologia(2008)
'Diabetic distal symmetric sensorimotor polyneuropathy affects ~50% of patients with diabetes mellitus, and is a leading cause of foot amputation [1]. Evidence for the important role of the highly reactive oxidant peroxynitrite [2,3] in peripheral diabetic neuropathy is emerging from both experimental [4-7] and clinical [8-10] studies. Accumulation of nitrotyrosine, a footprint of peroxynitrite injury, has been found in peripheral nerve, vasa nervorum, spinal cord and dorsal root ganglion neurons in animal models of both type 1 and type 2 diabetes [3-7,11-13] and high glucose-exposed cultured human Schwann cells [14].' ―Vareniuk
'Clinical studies revealed increased plasma nitrotyrosine levels and their correlation with endothelial dysfunction and redistribution of sudomotor responses, an early sign of sympathetic nerve dysfunction, in type 1 diabetic patients [8-10]. Furthermore, plasma peroxynitrite generation assessed by the pholasin chemiluminescence test correlated with the diabetic neuropathy impairment score of the lower limbs [10].' ―Vareniuk
'Nitrated tyrosine immunofluorescence was increased by 69% in the sciatic nerves of diabetic wild-type mice compared with non-diabetic controls. In contrast, no diabetes-induced nitrotyrosine accumulation was detected in the sciatic nerves of iNos⁻ mice.' ―Vareniuk
'Diabetic wild-type mice displayed peroxynitrite injury in peripheral nerve and dorsal root ganglion neurons. They also developed motor and sensory nerve conduction velocity deficits, thermal and mechanical hypoalgesia, tactile allodynia and ~36% loss of intraepidermal nerve fibres. Diabetic iNos⁻ mice did not display nitrotyrosine and poly(ADP-ribose) accumulation in peripheral nerve, but were not protected from nitrosative stress in dorsal root ganglia. Despite this latter circumstance, diabetic iNos⁻ mice preserved normal nerve conduction velocities.' ―Vareniuk
'Inducible NOS plays a key role in peroxynitrite injury to peripheral nerve, and functional and structural changes of diabetic neuropathy. Nitrosative stress in axons and Schwann cells, rather than dorsal root ganglion neurons, underlies peripheral nerve dysfunction and degeneration.' ―Vareniuk
By lowering the concentration of either of the two peroxynitrite precursors, peripheral neuropathy is predictably attenuated. This is presumed to be caused by peroxynitrite and correlates with nitrotyrosine concentration, nerve conduction velocity, and the sensitivity of mice feet towards heat.
Microtubules are tubular structures residing within the center of myelinated and unmyelinated nerves, long polymers of alternating α- and β-tubulin monomers of indefinite length. Due to their locality and structure, it's hard to imaging that any structure besides could be responsible for long-distance nerve conduction. In the lumen of microtubules are repeating arrays of the aromatic side-chains of tryptophan, histidine, phenylalanine, and tyrosine—the canonical target of peroxynitrite. Peripheral nerves are less-myelinated and would be the most vulnerable to tyrosyl nitration, a process that could even occur to nascent tubulin in the cytosol before it's addition to microtubule ends. Kali Janes' mitochondrial theory of peroxynitrite-induced neuropathy also fails to account for its' the peripheral nature of the neuropathy because mitochondria are found everywhere.
Under this paradigm, methods to induce or inhibit peripheral neuropathy suggest themselves. Progesterone and γ-tocopherol would likely inhibit protein nitration by myelinating nerves and adducting-with peroxynitrite (
Christen, 1997). Both free iron and methylene blue have been shown to increase both superoxide & peroxynitrite concentrations, and cyclooxgenase inhibitors might increase peroxynitrite affinity for tyrosine by inhibiting it's conjugation to lipids."
Posting this here because people may find it helpful, very detailed comment by a user on another forum
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