Characteristics of cysteine uptake in intestinal basolateral membrane vesicles

Uptake of cysteine in intestinal basolateral membrane vesicles was independent of Na+, was sensitive to medium osmolarity, exhibited saturation kinetics, and was selectively inhibited by other amino acids in a concentration-dependent manner, indicating transport by a carrier-mediated process. The kinetics indicated the existence of two transport systems: a high- and a low-Km system with Km and Vmax values that differed by greater than one order of magnitude. The substrate specificity pattern indicated two principal transport systems for cysteine: one corresponding to the high-Km system and one to the low-Km system. The high-Km system was inhibited primarily by neutral amino acids with small or polar side chains (alanine, serine, threonine, and glycine), resembling the ASC system in its specificity. The low-Km system was inhibited primarily by neutral amino acids with large, nonpolar side chains (leucine, phenylalanine, and methionine) and by the leucine analogue beta-2-aminobicyclo(2,2,1)heptane-2-carboxylic acid, identifying it as the L system. The two systems also exhibited trans stimulation, but only with those amino acids that caused cis inhibition.