Determination of taurine pathway and its physiological importance for dogs and cats

Taurine (2-aminoethanesulfonic) is a unique beta-amino-sulfonic acid that has been discovered by Tiedemann and Gmelin (1827) after isolating taurine from ox bile (WRIGHT, TALLAN and LIN, 1986). The nutritional importance of taurine was first observed by HAYES, CAREY and SCHMIDT (1975). They observed that after three months using casein as a protein source in a cat diet, the cats developed retinal degeneration. The disease was associated to low serum levels of sulfur amino acids due their low concentration in casein. Assuming that the physiological effect was a response to the low levels of taurine in the metabolism, it was possible to determine that taurine is an essential amino acid. Since that, the importance of taurine to metabolic routes have been studied, which was essential to determine that cats can convert just small amounts of taurine due their low activity of cysteine dioxygenase and cysteine sulfinic acid decarboxylase – two essential enzymes for taurine synthesis. On the other hand, dogs do not require dietary sources of taurine as cats do. Dogs can synthesize taurine from methionine through the transmethylation and transsulfuration pathway. Also, cats can only conjugate bile salts with taurine, while other mammals also use glycine. During the bile salts syntheses, taurine is conjugated with chenodeoxycholic acid and cholic acid forming the primary bile salts: taurochenodeoxycholic acid and taurocholic acid. The primary bile acids that do not react in the liver with taurine go to the small intestine and are converted into secondary bile acids. After formed, secondary bile salts can also react with taurine forming tauro-deoxy-cholic acid and taurolitocholic acid. Not only important for bile salts syntheses, taurine has been related to metabolic disorders in cats and dogs. The most common disease caused by taurine depletion is dilated cardio myopathy, which is directly correlated to serum taurine low levels. Despite dogs have the ability to synthesize taurine through sulfur amino acids, the literature has been relating cardio myopathy disease in large breed dogs with low taurine serum levels. This effect in dogs still not completely understood, but has been related to dog’s maintenance energy requirement and certain fibers, which can bind with taurocholic acid in the small intestine and make it unavailable for enterohepatic reutilization (KO et al., 2007; CASE et al., 2011). Showing that dog’s vegetable-based diets may need a dietary taurine supplementation as well as cat’s diets.