Metabolic Functions of Taurine

14.5.2.1 Taurine Conjugation of Bile Acids The bile acids are conjugated with either taurine or glycine to increase their polarity. Increased availability of taurine results in decreased glycine conjugation and an increase in biliary taurocholic acid. Conversely, glycine overload results in an increase in the plasma concentration of taurine, apparently as a result of increased glycocholic acid formation and reduced utilization of taurine for bile acid conjugation.

Although supplements of taurine alter the ratio of taurocholic:glycocholic acids, they have no effect on the total output of bile salts or on fat absorption in normal subjects. There is some evidence that patients with cystic fibrosis have improved fat absorption if given taurine supplements. This may be because taurine-conjugated bile acids are generally reabsorbed lower down the small intestine than glycine conjugates; in patients whose intestinal absorption is compromised, this may give a beneficial increase in the total length of intestinal tract available for fat absorption.

In addition to bile acid conjugation, a variety of other compounds may also be excreted as taurine conjugates, including retinoic acid (Section 2.2.1.3) and a number of xenobiotics.

14.5.2.2 Taurine in the Central Nervous System There is a relatively high concentration of taurine in the central nervous system - higher than would be expected for a neurotransmitter and without a specific anatomical localization. As in the retina, the main function of taurine in the central nervous system seems to be as an osmolyte (Hussy et al., 2000; Saransaari and Oja, 2000).

The concentration of taurine in the developing brain is three- to four-fold higher than in the adult brain, and falls rapidly between birth and weaning. Unlike other amino acids, taurine is transported within axons to a greater extent in young animals than in adults. The highest concentrations of taurine in the brain and the greatest rates of axonal transport occur before and during the process of synaptic development, suggesting that it may have a role in the development of the central nervous system and the postnatal development of synaptic connections (Lima et al., 2001).

14.5.2.3 Taurine and Heart Muscle Cardiomyopathy is a major problem in taurine-deficient cats, and after prolonged deficiency, there is a failure of contractility, leading to heart failure. Heart muscle concentrates taurine from the bloodstream, and the heart can synthesize taurine by oxidation of cysteamine, although not by the cysteine sulfinic acid decarboxylase pathway. Pharmacologically, taurine affects drug-induced cardiac arrhythmias by depressing the hyperirritability caused by loss of potassium - a digitalis-like action that suggests an effect on membrane permeability and ion flux, and perhaps especially on the maintenance of stable intracellular concentrations of calcium (Nittynen et al., 1999; Militante et al., 2000).

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