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To understand the action and mechanism of hypotaurine, an immediate precursor of taurine, on orofacial nociceptive processing, we examined the direct effects and receptor types involved in hypotaurine-induced responses using the whole-cell patch clamp technique in the substantia gelatinosa (SG) neurons of the trigeminal subnucleus caudalis (Vc) of immature mice. Under the condition of high-chloride pipette solution, hypotaurine elicited inward currents or upward deflections of membrane potential, which increased in a concentration-dependent manner (30–3000 μM) with the EC50 of 663.8 and 337.6 μM, respectively. The responses to 300 µM hypotaurine were reproducible and recovered upon washout. The 300 µM hypotaurine-induced currents were maintained in the presence of TTX, CNQX, and AP5, indicating direct postsynaptic action of hypotaurine on SG neurons. Responses to both low (300 µM) and high (1 or 3 mM) concentrations of hypotaurine were completely and reversibly blocked by the glycine receptor antagonist strychnine (2 µM), but unaffected by the GABAA receptor antagonist gabazine (3 µM) which blocks synaptic GABAA receptors at low concentration. Furthermore, responses to 300 µM hypotaurine and a maximal concentration of glycine (3 mM) were not additive, indicating that hypotaurine and glycine act on the same receptor. Hypotaurine-induced currents were partially antagonized by picrotoxin (50 µM) which blocks homomeric glycine receptors and by bicuculline (10 µM) which is an antagonist of α2 subunit-containing glycine receptors. These results suggest that hypotaurine-induced responses were mediated by glycine receptor activation in the SG neurons and hypotaurine might be used as an effective therapeutics for orofacial pain.

Hypotaurine is the precursor of taurine production from L-cysteinesulfinate. It is recognized that hypotaurine production in the liver occurs in cytosol. In the present study, hypotaurine production from L-cysteinesulfinate in rat liver mitochondria was investigated. The mitochondrial preparation prepared according to the method of Hogeboom and washed repeatedly with 0.25 M sucrose solution was incubated with L-cysteinesulfinate. Products were derivatized with dabsyl chloride and dabsylated amino acids were analyzed by RP-HPLC. Presence of a peak corresponding to dabsyl-hypotaurine was confirmed. The peak of dabsyl-hypotaurine was converted quantitatively to dabsyl-taurine by the treatment with H(2)O(2). Optimum pH of the reaction was shown to be broad between 6.0 and 7.8 and Km for L-cysteinesulfinate was 0.11 mM. Results indicate the presence of L-cysteinesulfinate decarboxylase activity in liver mitochondria. Mitochondrial cysteine metabolism was summarized and possible antioxidant roles of cysteine metabolites including hypotaurine in mitochondria are discussed.

We studied in vivo production of taurine, hypotaurine and sulfate following subcutaneous administration of L-cysteinesulfinate (CSA) to rats and mice. When 5.0 mmol/kg of body weight of CSA was injected to rats, increased urinary excretions of taurine, hypotaurine and sulfate in 24 h urine were 617, 52 and 1,767 micromol/kg, respectively. From these results together with our previous data, sulfate production was calculated to be 1.6 times greater than taurine production. Increased contents (micromol/g of wet tissue) over the control of taurine and hypotaurine in mouse tissues at 60 min after the injection of 5.0 mmol/kg body weight of CSA were: liver, 3.5 and 9.9; kidney, 0.3 and 5.2; heart, 3.7 and 0.2; blood plasma, 0.4 and 0.2, respectively. Upon loading of hypotaurine or taurine, tissue contents of these amino acids in liver and kidney increased greatly. Our results indicate that liver is the most active tissue for taurine production, followed by kidney, and that external CSA, hypotaurine and taurine are easily taken up by these tissues.

We determined the alterations in metabolic conversion of cysteine into glutathione and taurine in liver of rats treated with ethanol acutely. Ethanol treatment reduced cysteine as well as glutathione levels in liver for 24 h. However, cysteine dioxygenase was up-regulated rapidly, and hypotaurine/taurine levels were significantly higher than those found in the saline-treated rats. It is therefore suggested that enhancement of cysteine catabolism into taurine contributes to the depletion of hepatic glutathione, which could exacerbate the ethanol-induced oxidative liver injury.

It has been suggested that invertebrates inhabiting deep-sea hydrothermal vent areas use the sulfinic acid hypotaurine, a precursor of taurine, to protect against the toxicity of hydrogen sulfide contained in the seawater from the vent. In this protective system, hypotaurine is accumulated in the gill, the primary site of sulfide exposure. However, the pathway for hypotaurine synthesis in mollusks has not been identified. In this study, we screened for the mRNAs of enzymes involved in hypotaurine synthesis in the deep-sea mussel Bathymodiolus septemdierum and cloned cDNAs encoding cysteine dioxygenase and cysteine sulfinate decarboxylase. As mRNAs encoding cysteamine dioxygenase and cysteine lyase were not detected, the cysteine sulfinate pathway is suggested to be the major pathway of hypotaurine and taurine synthesis. The two genes were found to be expressed in all the tissues examined, but the gill exhibited the highest expression. The mRNA level in the gill was not significantly changed by exposure to sulfides or thiosulfate. These results suggests that the gill of B. septemdierum maintains high levels of expression of the two genes regardless of ambient sulfide level and accumulates hypotaurine continuously to protect against sudden exposure to high level of sulfide.

Taurine activates and modulates GABA receptors in vivo as well as those expressed in heterologous systems. This study aimed to determine whether the structural analogs of taurine: homotaurine and hypotaurine, have the ability to activate GABA-A receptors that include GABAρ subunits. The expression of GABA-A receptors containing GABAρ has been reported in the STC-1 cells and astrocytes. In both cell types, taurine, homo-, and hypotaurine gated with low efficiency a picrotoxin-sensitive GABA-A receptor. The known bimodal modulatory effect of taurine on GABAρ receptors was not observed; however, differences between the activation and deactivation rates were detected when they were perfused together with GABA. In silico docking simulations suggested that taurine, hypo-, and homotaurine do not form a cation-π interaction such as that generated by GABA in the agonist-binding site of GABAρ. This observation complements the electrophysiological data suggesting that taurine and its analogs act as partial agonists of GABA-A receptors. All the observations above suggest that the structural analogs of taurine are partial agonists of GABA-A receptors that occupy the agonist-binding site, but their structures do not allow the proper interaction with the receptor to fully gate its Cl(-) channel.

Many studies have suggested an antioxidant role for taurine, but few studies have directly measured its free radical scavenging activity. The aim of the present study was to directly determine the action of taurine and taurine analogs to inhibit peroxynitrite-mediated oxidation of dihydrorhodamine 123 (DHR) to rhodamine. Taurine was also tested to determine if it could attenuate the toxicity of sodium nitroprusside (SNP) to neuronal cultures. Taurine at concentrations above 30 mM had a modest ability to inhibit peroxynitrite formation derived from SIN-1. Hypotaurine could inhibit peroxynitrite formation from both SIN-1 (decrease 75%) and SNP (decrease 50%) at 10 mM. Other taurine analogs (homotaurine, beta-alanine & isethionic acid) slightly potentiated DHR oxidation by SIN-1. Short-term (1-hour) treatment of PC12 cultures with either SNP (1-2mM) or taurine (20-40 mM) appeared to induce cellular proliferation. In contrast, 24-hour treatment with SNP (1 mM) induced cell death. Combination treatments with taurine and SNP appeared to interact in an additive fashion for both cell proliferation and neurotoxic actions. It appears unlikely that taurine is a major endogenous scavenger of peroxynitrite.

Although there are a great number of studies concerning the uptake of taurine in several tissues, the regulation of taurine transport has not been studied in the retina after lesioning the optic nerve. In the present study, isolated retinal cells of the goldfish retina were used either immediatly after cell suspension or in culture. The high-affinity transport system of [3H]taurine in these cells was sodium-, temperature- and energy-dependent, and was inhibited by hypotaurine and beta-alanine, but not by gamma-aminobutyric acid. There was a decrease in the maximal velocity (Vmax) without modifications in the substrate affinity (Km) after optic axotomy. These changes were mantained for up to 15 days after the lesion. The results might be the summation of mechanisms for providing extracellular taurine to be taken up by other retinal cells or eye structures, or regulation by the substrate taurine, which increases after lesioning the optic nerve. The in vivo accumulation of [3H]taurine in the retina after intraocular injection of [3H]taurine was affected by crushing the optic nerve or by axotomy. A progressive retinal decrease in taurine transport was observed after crushing the optic nerve, starting at 7 hours after surgery on the nerve. The uptake of [3H]taurine by the tectum was compensated in the animals that were subjected to crushing of the optic nerve, since the concentration of [3H]taurine was only different from the control value 24 hours after the lesion, indicating an efficient transport by the remaining axons. On the contrary, the low levels of [3H]taurine in the tectum after axotomy might be an index of the non-axonal origin of taurine in the tectum. Axonal transport was illustrated by the differential presence of [3H]taurine in the intact or crushed optic nerve. The uptake of [3H]taurine into retinal cells in culture in the absence or in the presence of taurine might indicate the existence of an adaptive regulation of taurine transport in this tissue, however taurine transport probably differentially occurs in specific populations of retinal cells. The use of a purified preparation of cells might be useful for future studies on the modulation of taurine transport by taurine in the retina and its role during regeneration.

Hypotaurine, concentrated under reduced pressure in HCl solution, partially (30-40%) degrades into taurine (about 30%), 2-aminoethyl-2-aminoethanethiolsulfonate (about 10%) and ethanolamine. The degradation products were identified using LC/APCI-MS, NMR, amino acid analysis and various chromatographies. The identities were confirmed by comparing the HPLS, MS and NMR characteristics of authentic compounds. One of the degradation processes during concentration of HCl solution of hypotaurine is therefore a disproportionation reaction which can interfere with the experimental results, when studying hypotaurine in biological systems.

This study followed the time course of urinary taurine and hypotaurine excretion after two-thirds hepatectomy in rats. The excretion of both taurine and hypotaurine was elevated during 18 h following the hepatectomy, with maximal excretion during the first 6 h. Twelve and 24 h after partial hepatectomy, the hepatic hypotaurine concentration was increased but liver taurine did not differ significantly from controls. No changes were observed in hypotaurine and taurine concentrations of heart, kidney, lung, muscle tissue and spleen. We postulate that partial hepatectomy induces a rapid increase of hepatic (hypo)taurine synthesis from precursor amino acids. The increased (hypo)taurine concentrations spill over into urine.

Two precursors of taurine have been studied: cysteamine and hypotaurine. Cysteamine has been quantified in genital secretions and found in follicular fluids of all species tested. On the contrary cysteamine was not detected (or traces) in tubal fluids of the same species. Addition of 50, 100 or 250 microM of cysteamine to the maturation medium used in the culturing of bovine oocytes did not improve the cleavage rate nor the embryo's developmental potential in vitro. Furthermore, at 250 microM, cysteamine seems to be toxic to the embryo. Addition of 0.5-1 mM hypotaurine to the bovine embryo culture medium improved significantly blastocyst production and quality. The respective roles of these 2 taurine precursors on maturation and embryo development are discussed.

L-Cysteine is mainly metabolized to sulfate and taurine through cysteinesulfinate pathway. Alternatively, sulfate is formed in rat liver mitochondria via 3-mercaptopyruvate pathway. Intraperitoneal administration of 5 mmol ofL-cysteine per kg of body weight resulted in the increase in sulfate and taurine (plus hypotaurine) excretion in the 24-h urine, which corresponded to 45.3 and 29.3%, respectively, ofL-cysteine administered. Subcutaneous injection of (aminooxy)acetate, a potent inhibitor of transaminases, together withL-cysteine halved the sulfate excretion and doubled the taurine excretion. In vitro sulfate formation fromL-cysteine and fromL-cysteinesulfinate in rat liver mitochondria was inhibited by (aminooxy)-acetate. The sulfate-forming activity of liver mitochondria obtained from rats injected with (aminooxy) acetate was also inhibited. These results indicate that the transamination reaction is crucial in sulfate formation and in the regulation of sulfur metabolism. Sulfur equilibrium in mammals was discussed.

Animal aspartate decarboxylase (ADC), glutamate decarboxylase (GDC) and cysteine sulfinic acid decarboxylase (CSADC) catalyze the decarboxylation of aspartate, glutamate and cysteine sulfinic acid to β-alanine, γ-aminobutyric acid and hypotaurine, respectively. Each enzymatic product has been implicated in different physiological functions. These decarboxylases use pyridoxal 5-phosphate (PLP) as cofactor and share high sequence homology. Analysis of the activity of ADC in the presence of different amino determined that beta-alanine production from aspartate was diminished in the presence of cysteine. Comparative analysis established that cysteine also inhibited GDC and CSADC in a concentration-dependent manner. Spectral comparisons of free PLP and cysteine, together with ADC and cysteine, result in comparable spectral shifts. Such spectral shifts indicate that cysteine is able to enter the active site of the enzyme, interact with the PLP-lysine internal aldimine, form a cysteine-PLP aldimine and undergo intramolecular nucleophilic cyclization through its sulfhydryl group, leading to irreversible ADC inactivation. Cysteine is the building block for protein synthesis and a precursor of cysteine sulfinic acid that is the substrate of CSADC and therefore is present in many cells, but the presence of cysteine (at comparable concentrations to their natural substrates) apparently could severely inhibit ADC, CSADC and GDC activity. This raises an essential question as to how animal species prevent these enzymes from cysteine-mediated inactivation. Disorders of cysteine metabolism have been implicated in several neurodegenerative diseases. The results of our study should promote research in terms of mechanism by which animals maintain their cysteine homeostasis and possible relationship of cysteine-mediated GDC and CSADC inhibition in neurodegenerative disease development.

Hypotaurine is an intermediate in taurine biosynthesis from cysteine in astrocytes. Although hypotaurine functions as an antioxidant and organic osmolyte, its physiological role in the central nervous system remains unclear. This study used behavioral assessments to determine whether hypotaurine influenced nociceptive transmission in acute, inflammatory, and neuropathic pain. The tail flick, paw pressure, and formalin tests were performed in male Sprague–Dawley rats to examine the effects of the intrathecal administration of hypotaurine (100, 200, 400, 600 μg) on thermal, mechanical, and chemical nociception. Chronic constriction injury (CCI) to the sciatic nerve was induced in the rats, and the electronic von Frey test and plantar test were performed to assess the effects on neuropathic pain. To determine which neurotransmitter pathway(s) was involved in the action of hypotaurine, in this study, we examined how the antagonists of spinal pain processing receptors altered the effect of 600 μg hypotaurine. To explore whether hypotaurine affected motor performance, the Rotarod test was conducted. Hypotaurine had antinociceptive effects on thermal, mechanical, and chemical nociception in the spinal cord. In CCI rats, hypotaurine alleviated mechanical allodynia and thermal hyperalgesia. These effects were reversed completely by pretreatment with an intrathecal injection of strychnine, a glycine receptor antagonist. Conversely, hypotaurine did not affect motor performance. This study demonstrated that intrathecal hypotaurine suppressed acute, inflammatory, and neuropathic pain. Hypotaurine may regulate nociceptive transmission physiologically by activating glycinergic neurons in the spinal cord, and it is a promising candidate for treating various pain states.

We studied the significance of alterations in the metabolomics of sulfur-containing substances in rapidly regenerating rat livers. Male rats were subjected to two-thirds partial hepatectomy (PHx), and the changes in hepatic levels of major sulfur-containing amino acids and related substances were monitored for 2 weeks. Liver weight began to increase from 24 h after the surgery, and appeared to recover fully in 2 weeks. Serum alanine aminotransferase and aspartate aminotransferase activities were elevated immediately after the surgery and returned slowly to normal levels in 2 weeks. Methionine, S-adenosylmethionine (SAM), cystathionine and cysteine were increased rapidly and remained elevated for longer than 1 week. Hepatic glutathione concentration was increased gradually for 24 h, and then decreased thereafter, whereas hypotaurine was elevated drastically right after the surgery. Hepatic concentrations of polyamines were altered significantly by PHx. In the hepatectomized livers putrescine concentration was elevated rapidly, reaching a level 40- to 50-fold greater than normal in 6–12 h. Ornithine, the metabolic substrate for putrescine synthesis, was also elevated markedly. Spermidine was increased significantly, whereas spermine was depressed below normal, which appeared to be due to the increased consumption of decarboxylated SAM for spermidine biosynthesis. The results show that the metabolomics of sulfur-containing amino acids and related substances is altered profoundly in regenerating rat livers until the original weight is recovered. Hepatic concentrations of polyamines after PHx are closely associated with the alteration in the metabolomics of sulfur-containing substances. The implication of these changes in the progression of liver regeneration is discussed.

Alterations in hepatic metabolism of S-amino acids were monitored over one week in male rats treated with a single dose of ethanol (3 g/kg, ip). Methionine and S-adenosylhomocysteine concentrations were increased rapidly, but S-adenosylmethionine, cysteine, and glutathione (GSH) decreased following ethanol administration. Activities of methionine adenosyltransferase, cystathionine gamma-lyase and cystathionine beta-synthase were all inhibited. gamma-Glutamylcysteine synthetase activity was increased from t = 8 hr, but GSH level did not return to control for 24 hr. Hepatic hypotaurine and taurine levels were elevated immediately, but reduced below control in 18 hr. Changes in serum and urinary taurine levels were consistent with results observed in liver. Cysteine dioxygenase activity was increased rapidly, but declined from t = 24 hr. The results show that a single dose of ethanol induces profound changes in hepatic S-amino acid metabolism, some of which persist for several days. Ethanol not only inhibits the cysteine synthesis but suppresses the cysteine availability further by enhancing its irreversible catabolism to taurine, which would play a significant role in the depletion of hepatic GSH.

The amino acid taurine plays an important trophic role during development and regeneration of the central nervous system. Other amino acid systems, such as those for glutamate and gamma-aminobutyric acid (GABA), are modified during the same physiological and pathological processes. After crushing the optic nerve, goldfish retinal explants were plated in the absence and in the presence of different amino acids and amino acid receptor agonists. The length and the density of the neurites were measured at 5 days in culture. Taurine increased the length and the density of neurites. Glutamate and glycine increased them at low concentration, but were inhibitors at higher concentration. The combination of N-methyl-D-aspartate (NMDA) and glycine produced a greater inhibitory effect than NMDA alone. NMDA or alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) added simultaneously with taurine impaired the stimulatory effect of the latter. GABA stimulated the emission of neurites in a concentration dependent manner. Hypotaurine also elevated the length of neurites, but cysteinesulfinic acid did not produce a significant effect. The concentrations of taurine, glutamate and GABA were determined by HPLC with fluorescent detection in the retina of goldfish at various days post-crushing the optic nerve. The levels of taurine were significantly increased at 48 h after the crush, and were elevated up to 20 days. Glutamate level decreased after the lesion of the optic nerve and was still low at 20 days. GABA concentration was not significantly different from the control. The interaction of these amino acids during the regenerative period, especially the balance between taurine and glutamate, may be a determinant in restoring vision after the crush.

The synthesis of taurine fromN-acetylcysteine has been examined in ratsin vivo and in rat hepatocyte suspensionsin vitro. In ratsin vivo, administration ofN-acetylcysteine significantly increased urinary taurine (3 fold) 24h after dosing and liver glutathione levels. Liver taurine was not increased significantly. In hepatocytes incubated in the presence ofN-acetylcysteine, glutathione concentration increased to a maximum after 1 hour but the increase was not dependent on the concentration ofN-acetylcysteine. In contrast, after an initial lag phase, taurine synthesis increased in relation to the concentration ofN-acetylcysteine and continued for 3 hours. Glutathione synthesis seems to be preferential to taurine synthesis. Taurine synthesis from cysteine sulphinate was greater and from hypotaurine was greatest and maximal after 1 hour. Implications for the mechanism of protection byN-acetylcysteine are discussed.