Abstract

Sodium-dependent transport of taurine into rat brain synaptosomes was studied using [³H]taurine of high specific activity (2.8 Ci/mmole). At 2.8 µM [³H]taurine, 57% of the total radioactive accumulation was directly proportional to the sodium ion concentration. The sodium-dependent, high-affinity transport was a linear function of added protein and incubation length, and was maximal between pH 6.6 and 8.6. Kinetic analyses indicated a high-affinity apparent K_m value of 4.76 µM and an apparent V_max value of 5.35 nmoles/g of protein per minute. After correction of the high-affinity transport for that portion contributed by the low-affinity system, the true kinetic constants of the high-affinity system were calculated to be 3.20 µM and 2.96 nmoles/g of protein per minute. Similarly, the true kinetic constants of the low-affinity system were calculated to be 3340 µM and 699 nmoles/g of protein per minute. The Hill plot for both the high- and low-affinity transport systems had a slope of about 1, which suggested a 1:l interaction between taurine and its transport molecule. The sodium-dependent, high-affinity transport of [³H]taurine was decreased by the removal of potassium or chloride ions, and was absent from lysed synaptosomes or when the assay was performed at 2°. The omission of glucose or the addition of dinitrophenol slightly reduced transport. Ouabain inhibited transport in a time- and dose-dependent manner. The Arrhenius plot of [³H]taurine transport revealed an energy of activation (E_a) of 15.6 kcal/mole and an energy quotient (Q₁₀) of 2.34, each of which indicated an active process. The regional distribution of uptake showed that the midbrain, thalamus, and olfactory bulbs had the highest velocity of transport, while the cerebral cortex, spinal cord, and cerebellum had the lowest velocity of transport. Several structural analogues were inhibitors of taurine transport, and analyses of structure-activity relationships revealed that the uptake site was very specific. Only molecules with free anionic and cationic groups were potent inhibitors. These data are consistent with the hypothesis that taurine is a neurotransmitter or neuromodulator in the brain, and we have investigated some of the molecular characteristics of this transport.