High-affinity uptake system for cysteine in crude synaptosomal fractions of rat cerebral cortex

The in vitro uptake of [³⁵S] cysteine was studied in crude synaptosomal preparation of the cerebral cortex of rat. The accumulation of cysteine was found to be temperature- and time-dependent. It was linear at least for four minutes at 37 C with characteristics of saturable kinetics. Uptake of cysteine was Na⁺- and K⁺-dependent. Increasing the Na⁺ ion concentration increased the accumulation of cysteine in synaptosomal preparations; unlike the Na⁺ ion, an increase in the K⁺ ion, an increase in the K⁺ ion inhibited cysteine uptake. Cysteine was accumulated against concentration gradients by a saturable mechanism. Double reciprocal plot of the cysteine uptake suggests two types of affinity systems, with K_m values for the high-affinity uptake of about 12.2 μM and for the low-affinity uptake of about 4 mM. The high-affinity uptake was also significantly inhibited by ouabain, a potent inhibitor of the Na⁺-K⁺-dependent ATPase, and other metabolic inhibitors. The results of the effects of cysteine analogues on uptake also suggested that it is a substrate-specific high-affinity uptake system for cysteine.