Uptake of α-aminoisobutyric acid (AIB) by MCF-8, a line of malignant mouse mammary epithelial cells derived from a D2 hyperplastic nodule outgrowth, was found to be mediated by two different transport systems. In actively dividing cells about 60% of total AIB uptake is mediated by an A-like system: transport was Na+ -dependent, was inhibited by N-methyl-AIB, was maximal at pH 7.5 and ceased at pH 6. Transport by this system was reduced by nearly 65% when cell division ceased in confluent cultures, but the addition of insulin and/or calf serum to the medium of quiescent cells restored activity. Transport by the A system had a Km of 1.0 mM AIB and Vmax of 5.5 pM AIB transported per μg cellular protein per minute. About 20% of total AIB uptake was mediated by an L-like transport system which was Na+ -independent, was inhibited by α-2-aminobicyclo (2,2, l)-heptane-2-carboxylic acid, and was active at both pH 7.5 and pH 6. The activity of the L system was not influenced by cell growth rate or by insulin or serum. The remaining 20% of total AIB uptake occurs via a non-saturable route. No ASC-like system was detected
