Regulation of taurine biosynthesis and its physiological significance in the brain

Cysteine sulfinic acid decarboxylase (CSAD), the rate-limiting enzyme in taurine biosynthesis, was found to be activated under conditions that favor protein phosphorylation and inactivated under conditions favoring protein dephosphorylation. Direct incorporation of (32)P into purified CSAD has been demonstrated with [gamma(32)P]ATP and PKC, but not PKA. In addition, the (32)P labeling of CSAD was inhibited by PKC inhibitors suggesting that PKC is responsible for phosphorylation of CSAD in the brain. Okadaic acid had no effect on CSAD activity at 10 mu M suggesting that protein phosphatase-2C (PrP-2C) might be involved in the dephosphorylation of CSAD. Furthermore, it was found that either glutamate- or high K(+)-induced depolarization increased CSAD activity as well as (32)P-incorporation into CSAD in neuronal cultures, supporting the notion that the CSAD activity is endogenously regulated by protein phosphorylation in the brain. A model to link neuronal excitation, phosphorylation of CSAD and increase in taurine biosynthesis is proposed.