Heart taurine function

A quantitatively important pathway of cysteine catabolism in animals is oxidation to cysteine sulfinate (Fig. 24-25, reaction z),450 a two-step hydroxyl-ation requiring 02, NADPH or NADH, and Fe2+. Cysteine sulfinic acid can be further oxidized to cyste-ic acid (cysteine sulfonate),454 which can be decarbox-ylated to taurine. The latter is a component of bile salts (Fig. 22-16) and is one of the most abundant free amino acids in human tissues 455-457 Its concentration is high in excitable tissues, and it may be a neurotransmitter (Chapter 30). Taurine may have a special function in retinal photoreceptor cells. It is an essential dietary amino acid for cats, who may die of heart failure in its absence,458 and under some conditions for humans.459 In many marine invertebrates, teleosts, and amphibians taurine serves as a regulator of osmotic pressure, its concentration decreasing in fresh water and increasing in salt water. A similar role has been suggested for taurine in mammalian hearts. A chronically low concentration of Na+ leads to increased taurine.460 Taurine can be reduced to isethionic acid... 

R.J. Huxtable, The regulation and function of taurine in the heart relationship to the adrenergic system, iji "Low molecular weight sulfur-containing natural products". Plenum Press, New York (1979) ... 

THE REGULATION AND FUNCTION OF TAURINE IN THE HEART AND OTHER ORGANS... 

In this article, I shall review the evidence from my laboratory as to the source of taurine in the heart, and its possible function. 

We have, therefore, found a very unusual modulation of the influx of an amino acid. Stimulated uptake of taurine in the presence of adrenergic activation is probably the reason for the high levels found in congestive heart failure. Such a state is produced as the end result of prolonged stress on the heart, in which there has probably also been a prolonged or continuous activation of the 6-adrenergic system. A possible physiological function for this modulation is discussed below. 

Does the stimulation in taurine influx as a result of adrenergic excitation serve any function The editors have invited speculation, so I will take advantage of that to suggest two functions for the adrenergic modulation of taurine flux into the heart, one presynaptic and the other postsynaptic. 

Active transport of 3-alanine occurs in the heart apparently by the same system that transports taurine. However, as shown in figure 4, there is a nonsaturable component to transport. The pKa of 3-alanine is 3.6. Thus, at a pH of 7.2, the proportion of 3-alanine with a non-ionized acid function is 125 times higher than for an equivalent concentration of taurine. Guanidinoethyl sulfonate is transported in place of taurine by a saturable process (fig. 5). The Lineweaver-Burk plot for transport is shown in figure 5. Over the concentration range 0-400yM, transport occurs by a one component system with an affinity (Km) of 153 yM, and a Vmax of 65 nmole/g dry weight/min. 

A hypothesis is proposed for the function of taurine in the heart, and for the Importance of a transport process in these functions. It is proposed that taurine has both a pre- and postsynaptic action. 

R. Huxtable, Metabolism and function of taurine in the heart, in "Taurine, R. Huxtable and A. Barbeau, eds.. Raven Press,... 

The attempts to correlate the high levels of taurine which are present in cardiac tissue with a physiological function have generated much speculation. However, success has thus far been quite fleeting as unfortunately a mechanism(s) of action for taurine in the myocardium has eluded all investigations and the role of taurine remains an unknown entity in the heart. The only observation concerning taurine in cardiac tissue that is non-controver-sial is that it constitutes approximately 50% of the free amino acid pool of the heart (Jacobsen and Smith, 1968). 

Huxtable,R."Metabolism and function of taurine in the heart"in"Taurine"Huxtable,R.and Barbeau,A.ed.Raven Press,New York(1976). 

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