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.