PHYSIOLOGICAL REVIEWS Vol. 72, No. 1, January 1992 Printed in U.S.A. Physiological Actions of Taurine R. J. HUXTABLE Department of Pharmacology, University of Arizona College of Medicine, Tucson, Arizona I. Introduction .......................................................................................... 101 II. Physicochemical Considerations ..................................................................... 102 III. Biochemical Considerations .......................................................................... 104 A. Distribution of taurine ........................................................................... 104 B. Metabolism of taurine ............................................................................ 107 IV. Nonmetabolic Actions of Taurine .................................................................... 108 A. Osmoregulation ................................................................................... 108 B. Calcium modulation .............................................................................. 114 C. Phospholipid interactions ........................................................................ 119 D. Protein interactions .............................................................................. 123 E. Interactions with zinc ............................................................................ 129 V. Metabolic Actions: Taurine as Product .............................................................. 130 A. Antioxidation: hypotaurine story ................................................................. 130 B. Radioprotection ................................................................................... 133 C. Cysteine detoxification ............................................................................ 134 VI. Metabolic Actions: Taurine as Precursor ............................................................ 135 A. Antioxidation: chloramine story .................................................................. 135 B. Radioprotection by taurine ....................................................................... 136 C. Energy storage (phosphagen) .................................................................... 136 D. Metabolism and energy production ............................................................... 138 E. Surfactant and detergent actions ................................................................ 139 F. Xenobiotic conjugation ........................................................................... 141 G. Isethionic acid and anion balance ................................................................ 141 H. Taurine-containing peptides ...................................................................... 142 I. Other taurine metabolites ........................................................................ 142 VII. Conclusions ........................................................................................... 142 I. INTRODUCTION carries a functional significance above and beyond the presence of GABA in bacteria. The osmoregulatory ac- Z-Aminoethane sulfonic acid, or taurine, is a phylo- tions of GABA in the latter species are superseded by its genetically ancient compound with a disjunct distribu- neurotransmitter function in the former. A moment’s tion in the biosphere. It is present in high concentration thought will multiply these examples. Thus it can be in algae (159, 649, 748) and in the animal kingdom, in- readily deduced that, in considering the physiological cluding insects and arthropods, but is generally absent significance of taurine, it, in all likelihood, will exhibit or present in traces in the bacterial and plant kingdoms. polyvalent functions. In many animals, including mammals, it is one of the Taurine was so named because it was first isolated most abundant of the low-molecular-weight organic con- from the bile of the ox, Bos taurus (134). The modern era stituents. A 70-kg human contains up to 70 g of taurine. of research on taurine may be considered to have been One is not tumbling into the abyss of teleology in think- introduced by the seminal and thorough review of Ja- ing that a compound conserved so strongly and present cobsen and Smith (338), which appeared in this journal in such high amounts is exhibiting functions that are in 1968. At that time, the functions suggested for tau- advantageous to the life forms containing it. rine were limited to bile salt synthesis, osmoregulation As the phylogenetic tree is ascended, substances in marine invertebrates, energy storage in marine tend to accrete functions. The adaptive advantages pro- worms, and neuroinhibition in the central nervous sys- vided by serotonin in bananas (Muss sapienturn), norepi- tem (CNS). Since then, the increase in the range of phe- nephrine in Solarium, and dopamine in the giant sa- nomena with which taurine has been associated has guaro cactus (Cereus giganteus) are extended to addi- been little short of astounding. Phenomena currently tional phenomena when these same compounds are associated with taurine are listed in Table 1. The pur- found in the mammalian brain. The presence of y-ami- pose of this review is not to examine these phenomena nobutyric acid (GABA) in the brains of higher animals per se but to elucidate the mechanisms by which taurine 0031-9333/92 $2.00 Copyright 0 1992 the American Physiological Society 101 102 R. J. HUXTABLE Volume 72 TABLE 1. Some biological actions of taurine Action References Action References Cardiovascular system Retina Antiarrhythmic 185,253, 652,747 Maintenance of structure and 590, 741 Positive inotropy at low calcium 183,186,187,362, 643 function of photoreceptors, outer Negative inotropy at high calcium 183,187 segments, and tapetum lucidum Potentiation of digitalis inotropy Summarized in 315 Antagonism of calcium paradox 145,402 Liver Hypotensive (central and 1, 3, 68, 198-201, 322, 323, 539, peripheral action) 581, 699, 829 Bile salt synthesis 243,244 , 293 Retardation of lesion development 33,510 in calcium overload Reproductive system cardiomyopathy Increased resistance of platelets to 248 Sperm motility factor 529,580 aggregation Muscle Brain Muscle membrane stabilizer 284,300 Anticonvulsant 63,151,217,286,288,552 Modulator of neuronal excitability 13,123-126,191,232,233,350, General 538 Maintenance of cerebellar function 55,'738-740 Modulation of neurotransmitter 8, 29, 115, 279, 321, 382, 419, Antinociceptive against chemical 697 and hormone release 577a, 633,676,678,762,814 stimuli Osmoregulation 261,329,349,357,433 Thermoregulation 12, 59, 105, 106, 215, 276, 361, Stimulation of glycolysis and 147,409,527,673 460,532,634,698,700 glycogenesis Antiaggressive actions 491,492 Attenuation of 617 Central regulation of 69,202,207,271,808 hypercholesterolemia cardiorespiratory responses Cell proliferation and viability 280,605 Alteration of sleeping duration 168,319,505,509 Antioxidation 592,688,756,758,823 Resistance to anoxia/hypoxia 185,438,483,689 Regulation of phosphorylation 453,466 Altered learning 10,483, 662 Xenobiotic conjugation 155,281,340-343,347,352 Altered motor behavior 46,205,353,459,614,615 Antitremor actions 213 Suppression of drinking 275,761 Suppression of eating 761 may be acting and how the same mechanism expressed Smith in publishing a complete and exhaustive review of in different settings may affect apparently unrelated the taurine area has vanished irretrievably, entombed physiological phenomena. I have, perhaps unwisely, within a voluminous literature. For those who want listed conclusions at the end of each section. The need more information, the literature on taurine can be pur- for brevity in language and the desire to avoid qualify- sued through the numerous reviews and symposia pro- ing nearly every statement inevitably gives these con- ceedings that have appeared since 1976 (45,87,247,286, clusions an assurance that may be lacking in the pri- 288,291,297,298,305,311,327,338,416,549,551,598,672, mary literature. In attempting to make the information 735, 823). digestible to the general reader, I may have trodden on a few toes of the taurine specialists. However, there are enough toes in the area that a few can be spared in a II. PHYSICOCHEMICAL CONSIDERATIONS good cause. If nothing else, I hope the conclusions stimu- late further investigations by those desirous of disprov- The biological actions of a compound are an inevita- ing them. ble consequence of its physicochemical properties. Tau- Despite the impressive progress of the past two de- rine is an amino acid that differs from the more familiar cades, an understanding of the mechanisms underlying substances of that class in being a sulfonic rather than a the effects of taurine has been slow to evolve. However, carboxylic amino acid and in being a P-amino acid recent investigations have narrowed the gap between rather than an a-amino acid. Compared with carboxyl- observation and understanding, and one can be confi- ate groups, the sulfonate group is a strong acid, having dent that a further review of this topic in this journal in an acidic dissociation constant (pK,) equivalent to that another 20 years will reveal an advance in mechanistic of a mineral acid, such as hydrochloric acid (Table 2). insight sufficient to systematize in a rational way the The high acidity makes taurine almost completely zwit- biological actions of taurine. terionic over the physiological pH range. In contrast, a It is a measure of the progress made over the last 22 significant fraction of carboxylic