Characteristics of taurine transport in rat liver lysosomes

Taurine (2‐aminoethanesulfonic acid) is a unique sulfur amino acid derivative that has putative nutritional, osmoregulatory, and neuroregulatory roles and is highly concentrated within a variety of cells. The permeability of Percoll density gradient purified rat liver lysosomes to taurine was examined. Intralysosomal amino acid analysis showed trace levels of taurine compared to most other amino acids. Taurine uptake was Na+‐independent, with an overshoot between 5–10 minutes. Trichloroacetic acid extraction studies and detergent lysis confirmed that free taurine accumulated in the lysosomal space. Kinetic studies revealed heterogeneous uptake with values for Km1 = 31 ± 1.82 and Km2 > 198 ± 10.2 mM. The uptake had a pH optimal of 6.5 and was stimulated by the potassium specific ionophore valinomycin. The exodus rate was fairly rapid, with a t1/2 of 5 minutes at 37°C. Analog inhibition studies indicated substrate specificity similar to the plasma membrane β‐alanine carrier system, with inhibition by β‐alanine, hypotaurine, and taurine. α‐Alanine, 2‐methylaminoisobutyric acid (MeAIB), and threonine were poor inhibitors. No effects were observed with sucrose and the photoaffinity derivative of taurine NAP‐taurine [N‐(4‐azido‐2‐nitrophenyl)‐2‐aminoethanesulfonate]. In summary, rat liver lysosomes possess a high Km system for taurine transport that is sensitive to changes in K+ gradient and perhaps valinomycin induced diffusional membrane potential. These features may enable lysosomes to adapt to changing intracellular concentrations of this osmotic regulatory substance.