Evaluation of taurine biosynthesis in the livers of the spear squid Heterololigo bleekeri and the swordtip squid Uroteuthis edulis

Taurine is an abundant amino acid essential to various physiological functions in many organisms; however, invertebrate taurine biosynthesis pathways are largely unknown. Here, we investigated the activities of taurine biosynthesis enzymes, cysteinesulfinic acid decarboxylase (CSD), cysteic acid decarboxylase (CAD), and cysteamine dioxygenase (CAO) in the livers of spear squid Heterololigo bleekeri and swordtip squid Uroteuthis edulis. When the enzyme reactions were performed at 35 °C for 60 min, CSD and CAD activities of spear squid were 1.49 ± 0.25 and 0.79 ± 0.11 nmol/(min mg protein), respectively; no CAO activity was observed. The CSD and CAD activities of swordtip squid were lower than those of spear squid, but CAO activity was also observed. These results suggest that taurine biosynthesis pathways in squids vary between species, as in fish. Under the conditions of our experiment, kinetic analysis revealed that in spear squid, the Michaelis–Menten constant and maximum reaction rate were 0.20 ± 0.02 mM and 1.78 ± 0.22 nmol/(min mg protein), respectively, for CSD, and for CAD 0.57 ± 0.02 mM and 1.29 ± 0.17 nmol/(min mg protein), respectively. Cysteic acid competitively inhibited spear squid CSD activity, indicating that a single enzyme catalyzes the decarboxylation of cysteinesulfinic and cysteic acids in this squid.