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

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Abstract
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 Km 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.