A Novel, Easy Assay Method for Human Cysteine Sulfinic Acid Decarboxylase

Cysteine sulfinic acid decarboxylase catalyzes the last step of taurine biosynthesis in mammals, and belongs to the fold type I superfamily of pyridoxal-5′-phosphate (PLP)-dependent enzymes. Taurine (2-aminoethanesulfonic acid) is the most abundant free amino acid in animal tissues; it is highly present in liver, kidney, muscle, and brain, and plays numerous biological and physiological roles. Despite the importance of taurine in human health, human cysteine sulfinic acid decarboxylase has been poorly characterized at the biochemical level, although its three-dimensional structure has been solved. In the present work, we have recombinantly expressed and purified human cysteine sulfinic acid decarboxylase, and applied a simple spectroscopic direct method based on circular dichroism to measure its enzymatic activity. This method gives a significant advantage in terms of simplicity and reduction of execution time with respect to previously used assays, and will facilitate future studies on the catalytic mechanism of the enzyme. We determined the kinetic constants using L-cysteine sulfinic acid as substrate, and also showed that human cysteine sulfinic acid decarboxylase is capable to catalyze the decarboxylation—besides its natural substrates L-cysteine sulfinic acid and L-cysteic acid—of L-aspartate and L-glutamate, although with much lower efficiency.

Estradiol decreases taurine level by reducing cysteine sulfinic acid decarboxylase via the estrogen receptor-α in female mice liver

Cysteine sulfinic acid decarboxylase (CSAD) and cysteine dioxygenase (CDO) are two rate-limiting enzymes in taurine de novo synthesis, and their expressions are associated with estrogen concentration. The present study was designed to determine the relationship between 17β-estradiol (E2) and taurine in female mice liver. We initially observed the mice had lower levels of CSAD, CDO, and taurine during estrus than diestrus. We then, respectively, treated the ovariectomized mice, the cultured hepatocytes, and Hep G2 cells with different doses of E2, and the CSAD and CDO expressions and taurine levels were analyzed. The results showed that E2 decreased taurine level in the serum and the cultured cells by inhibiting CSAD and CDO expressions. Furthermore, we identified the molecular receptor types through which E2 plays its role in regulating taurine synthesis, and our results showed that estrogen receptor-α (ERα) expression was much higher than estrogen receptor-β (ERβ) in the liver and hepatocytes, and the inhibiting effects of E2 on CSAD, CDO, and taurine level were partially abrogated in the ICI-182,780-pretreated liver and hepatocytes, and in ERα knockout mice. These results indicate that estradiol decreases taurine content by reducing taurine biosynthetic enzyme expression in mice liver.

Development of a Novel Cysteine Sulfinic Acid Decarboxylase Knockout Mouse: Dietary Taurine Reduces Neonatal Mortality

We engineered a CSAD KO mouse to investigate the physiological roles of taurine. The disruption of the CSAD gene was verified by Southern, Northern, and Western blotting. HPLC indicated an 83% decrease of taurine concentration in the plasma of CSAD-/-. Although CSAD-/- generation (G)1 and G2 survived, offspring from G2 CSAD-/- had low brain and liver taurine concentrations and most died within 24 hrs of birth. Taurine concentrations in G3 CSAD-/- born from G2 CSAD-/- treated with taurine in the drinking water were restored and survival rates of G3 CSAD-/- increased from 15% to 92%. The mRNA expression of CDO, ADO, and TauT was not different in CSAD-/- compared to WT and CSAD mRNA was not expressed in CSAD-/-. Expression of Gpx 1 and 3 was increased significantly in CSAD-/- and restored to normal levels with taurine supplementation. Lactoferrin and the prolactin receptor were significantly decreased in CSAD-/-. The prolactin receptor was restored with taurine supplementation. These data indicated that CSAD KO is a good model for studying the effects of taurine deficiency and its treatment with taurine supplementation.