Sodium-dependent and sodium-independent binding of taurine to rat brain synaptosomes

Rat brain synaptosomes (P(2)B fraction) release ?M concentrations of taurine into the incubation medium when incubated at protein concentrations of 0.5-5.0 mg/ml. Repeated incubations, with or without sonication, also induce release of ?M concentration of taurine. When this release is taken into account, two classes of binding sites for taurine can be demonstrated over the intracellular-extracellular concentration range (3 ?M-30 mM) in the presence of 140 mM sodium. The higher affinity class has a K(d) of 25.1 +/- 7.7 ?M (mean +/- SE) and a B(max) of 15.08 +/- 4.27 pmol mg(?1) protein. This site is sodium-dependent, no binding being observed in the absence of sodium. Binding is also suppressed by the taurine transport inhibitor, guanidinoethane sulfonate. We equate this site with the transport site for taurine. The lower affinity site has a K(d) of 33.4 +/- 7.5 mM and a B(max) of 10.3 +/- 1.9 nmol mg(?1) protein. Binding to this site is unaffected by guanidinoethane sulfonate. In the absence of sodium, K(d) and B(max) are decreased to 5.8 +/- 0.56 mM and 2.55 +/- 0.21 nmol mg(?1) protein, respectively. We equate this site with the calcium-modulatory binding of taurine to phospholipid recently demonstrated in cardiac sarcolemma [Huxtable and Sebring, TIPS 7, 481 (1986)].