Over the years it became apparent t

Over the years it became apparent that pathway I – the cysteinesulfinate-dependent pathway – was the main pathway for taurine biosynthesis in mammals. Consequently, it has been extensively studied and is reviewed in greater details (Huxtable, 1989). Nevertheless, two important points should be made about this pathway. First, the flux of cysteine sulfur to taurine is mainly determined by the highly regulated cysteine dioxygenase (CDO), although the oxidation of hypotaurine into taurine may also be enzymatically regulated (Vitvitsky et al., 2011). CDO activity is highly responsive to changes in substrate availability (e.g. through dietary changes), thus playing an important role in maintaining the steady-state intracellular cysteine levels (Stipanuk and Ueki, 2011). Second, cysteinesulfinate can be either transaminated into β-sulfinylpyruvate or decarboxylated into hypotaurine. The partitioning between the two is determined by the relative saturation constants Km of the two enzymes responsible for these reactions, aspartate aminotransferase (AAT) and cysteinesulfinate decarboxylase (CSD), respectively (de la Rosa and Stipanuk, 1985, Huxtable, 2000 and Knopf et al., 1978). The Km of AAT is about 50 times higher than that of CSD, hence cysteinesulfinate is predominantly converted to β-sulfinylpyruvate rather than to hypotaurine (Huxtable, 2000). In mammals this paradigm is tissue specific however: CSD activity is comparatively higher in hepatocytes compared to muscle, kidney or enterocytes (Stipanuk et al., 2006). This explains the importance of the liver in taurine biosynthesis. In non-mammalian species, taurine biosynthesis pathways remain poorly described. There is indication that birds rely on pathway II: chickens convert cysteine to cysteic acid to taurine in the absence of hypotaurine formation (Chapeville, 1960 and Simmonet et al., 1960). Insects also synthesize taurine using pathway II, although hypotaurine was found in some tissues, indicating that pathway I is also used (Whitton et al., 1995). In pathway III hypotaurine is produced from cysteamine, which can be seen as a decarboxylated cysteine. However, there is no evidence of cysteine decarboxylase activity (Huxtable, 1981). More likely, cysteamine is produced via coenzyme A turnover and pantetheine oxidation by cysteamine dioxygenase (Coloso et al., 2006, Stipanuk and Ueki, 2011 and Ueki and Stipanuk, 2009). Hepatic hypotaurine in cysteamine-fed rats was about 40% of that of cysteine-fed rats, while levels were about 7% in an unsupplemented group (Stipanuk et al., 2006). Although the quantitative partitioning between pathway I and III has not been precisely quantified, such results suggest that pathway III could be biologically significant.