3. Taurine in aquaculture3.1. Biosy

3. Taurine in aquaculture
3.1. Biosynthesis pathways

The description and quantitative characterization of the taurine biosynthetic pathways has been the subject of several decades of research, in spite of which some parts remain poorly characterized. Jacobsen and Smith (1968) described and discussed five pathways for taurine biosynthesis (Fig. 1). However, there is no convincing evidence supporting pathway V, since cystine disulfoxide is quite unstable and decomposes into cysteinesulfinate and cystine (Lavine, 1936), and previous studies supporting this pathway were rejected when the investigator subsequently realized that the measured cystine disulfoxide was in fact hypotaurine (Cavallini, 1956). Hence pathway V remains unsubstantiated. Pathway IV sees taurine biosynthesis by incorporation of inorganic sulfate, via phosphoadenosine phosphosulfate (PAPS) and cysteic acid. Although evidence shows that this pathway occurs in insects such as cockroaches, it was also shown that the initial conversion of inorganic sulfur to cysteine is actually performed by bacterial endosymbionts, and not by the insect itself (Block and Henry, 1961). In vertebrates, this pathway’s shortcomings were compellingly demonstrated by radioisotope tracing (Huxtable, 1981, Jacobsen and Smith, 1968 and Knopf et al., 1978), in spite of which this pathway is sometimes referred to in more recent literature. Consequently it is now generally accepted that the non-bacterial conversion of inorganic sulfate to organic sulfur compounds (i.e., pathway IV) is insignificant. Likewise, it was hypothesized that taurine could be synthesized in vivo from its deaminated form, isethionic acid. However, no evidence for such reaction was found; rather it is the opposite reaction that occurs, at quite a slow rate ( Huxtable and Bressler, 1972).