Taurine synthesis and cysteine metabolism in cultured rat astrocytes: effects of hyperosmotic exposure.

We investigated mechanisms controlling taurine synthesis in cultured rat cerebral astrocytes. The mean +/- SE rate of taurine synthesis from extracellular cysteine was 21.2 +/- 2.0 pmol.mg protein-1.min-1, whereas taurine degradation was < 1.3% of this rate. Eliminating cellular glutathione and inhibiting glutathione biosynthesis increased taurine synthesis from extracellular cysteine by 39%. In cell homogenates, cysteine dioxygenase (CDO) and cysteine-sulfinate decarboxylase activities were 2.4 +/- 0.2 and 8.3 +/- 2.8 nmol.mg protein-1.min-1, respectively. CDO activity was strongly dependent on cysteine concentration over physiological and pathophysiological ranges of intracellular cysteine concentration. Growth in hyperosmotic medium caused a greater increase in culture medium taurine content than that measured from cells in isosmotic growth medium. Hyperosmotic treatment transiently increased the rate of cysteine accumulation and cellular cysteine and glutathione contents but had no effect on the synthesis rate of taurine from extracellular cysteine. Thus cysteine is accumulated and then metabolized to taurine through CDO, whose activity depends on the intracellular cysteine concentration and appears to be rate limiting for taurine synthesis. Hyperosmotic exposure increases net taurine production yet has no effect on taurine synthesis from exogenously applied cysteine. Availability of substrate from intracellular pools must contribute to maintenance of high intracellular taurine during hyperosmotic exposure.