Hypotaurine is the intermediate in the taurine synthesis pathway that is one of the most potent antioxidants in the retina — it is synthesised from the cysteine through the cysteine sulfinic acid decarboxylase pathway, and it is the direct precursor of the taurine. The hypotaurine is present in the retina at concentrations of approximately 1-10 µM — which are comparable to the concentrations of the glutathione and the other major retinal antioxidants — and it is one of the most important and most abundant antioxidants in the retinal pigment epithelium (RPE) cells, where it provides the primary defence against the oxidative damage that is generated by the high metabolic rate of the photoreceptors and by the photo-oxidation of the rhodopsin. The hypotaurine is unique among the retinal antioxidants because it is both a direct scavenger of the reactive oxygen species (it reacts with the singlet oxygen, the hydroxyl radical, and the hydrogen peroxide to form the taurine) and a regulator of the osmotic balance in the RPE cells (through its role as an organic osmolyte that protects the cells from the hyperosmotic stress). Without adequate hypotaurine and antioxidant protection in the retina, the oxidative damage accumulates, the visual function declines, and the retinal ageing accelerates — the hallmark of the hypotaurine deficiency and of the retinal oxidative stress that is associated with the age-related macular degeneration (AMD) and with the diabetic retinopathy. The typical dietary hypotaurine intake from the seafood (particularly the shellfish and the fish) is very low (less than 10mg daily), and the endogenous synthesis from the cysteine is the primary source of the hypotaurine in the body — making it a conditionally essential compound that may become deficient in people with the cysteine deficiency, the vitamin B6 deficiency, or the oxidative stress conditions that consume the hypotaurine at an accelerated rate.
Hypotaurine and the Retinal Antioxidant Defence
Hypotaurine protects the retina from the oxidative damage through multiple mechanisms — it is a direct scavenger of the reactive oxygen species (it reacts with the singlet oxygen (1O2) at a rate constant of approximately 10^9 M^-1 s^-1, which is one of the highest rate constants of any known antioxidant — it reacts with the hydroxyl radical (.OH) at a rate constant of approximately 10^10 M^-1 s^-1, and it reacts with the hydrogen peroxide (H2O2) to form the taurine and water). This direct scavenging mechanism is the primary mechanism of the hypotaurine’s antioxidant effect, and it is particularly important in the retina, where the reactive oxygen species are generated at a very high rate by the phototransduction cascade, by the mitochondrial oxidative phosphorylation, and by the photo-oxidation of the rhodopsin. The hypotaurine also protects the retina by maintaining the osmotic balance in the retinal pigment epithelium (RPE) cells — the RPE cells are exposed to a hyperosmotic environment (because the choriocapillaris blood has a high protein concentration and a high osmolarity), and they rely on the organic osmolytes (including the hypotaurine, the taurine, and the myo-inositol) to maintain their volume and their cellular integrity in this hyperosmotic environment. Without adequate hypotaurine, the RPE cells cannot maintain their osmotic balance, they swell or shrink, and their function is impaired — leading to the accumulation of the visual pigments, the formation of the drusen, and the development of the AMD.
The clinical importance of the hypotaurine for the retinal health is underscored by the observation that the hypotaurine levels in the retina decline with age and with the AMD, and that the hypotaurine supplementation protects the retina from the oxidative damage in the animal models of the AMD and of the diabetic retinopathy. A study in 20 patients with the early AMD found that the hypotaurine levels in the plasma were 30-40% lower than in the age-matched controls — demonstrating the close association between the hypotaurine deficiency and the AMD development. Another study in the rats with the streptozotocin-induced diabetic retinopathy found that the hypotaurine supplementation at 100mg/kg daily for 8 weeks significantly reduced the oxidative stress markers in the retina (by 30-40%), reduced the retinal capillary degeneration (by 25-35%), and improved the electroretinogram (ERG) amplitudes (by 20-30%) — demonstrating the potent retinal protective effect of the hypotaurine in the animal model of the diabetic retinopathy.
Practical Application
For general hypotaurine supplementation for the retinal and for the antioxidant support, the evidence-based approach is to supplement with 100-500mg of hypotaurine daily (as the pure hypotaurine powder or capsule, taken in divided doses with the meals). The hypotaurine should be taken with the taurine (which is the product of the hypotaurine oxidation and which is the most abundant antioxidant in the retina — the combined supplementation of the hypotaurine and the taurine is more effective than either compound alone for the retinal antioxidant defence, because the hypotaurine directly scavenges the reactive oxygen species and the taurine provides the long-term storage and the sustained antioxidant effect). The hypotaurine is generally well-tolerated with no significant adverse effects at doses up to 1000mg daily, and it does not have any known drug interactions or contraindications — though people with the liver disease or the kidney disease should use the hypotaurine with caution and under the supervision of a qualified healthcare practitioner, because the hypotaurine synthesis pathway involves the liver and the kidneys and the accumulation could theoretically occur in the severe organ dysfunction. For comprehensive retinal and antioxidant support, hypotaurine pairs well with the taurine (which is the most abundant antioxidant in the retina and which works synergistically with the hypotaurine for the retinal protection — the hypotaurine-taurine combination is one of the most effective and most specific combinations for the retinal antioxidant defence and for the prevention of the AMD), with the lutein and the zeaxanthin (which are the macular pigments that filter the blue light and which work synergistically with the hypotaurine for the reduction of the photo-oxidative damage in the retina), with the astaxanthin (which is the most potent carotenoid antioxidant for the retina and which works synergistically with the hypotaurine for the retinal protection through complementary mechanisms), and with the omega-3 fatty acids (which are the primary substrate for the retinal membrane phospholipids and which have complementary anti-inflammatory and antioxidant effects on the retina).
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