Figure 4.

Heterologous expression of SA1 in HeLa cells confers amino acid transport. (a) HeLa cells expressing the SA1 cDNA (closed circles) accumulate substantially more [¹⁴C]MeAIB than cells transfected with vector alone (open circles). (b) At an early time point during which uptake remains linear (1 min), the transport of [¹⁴C]MeAIB by cells expressing SA1 is saturable. Lineweaver-Burke analysis indicates a K_m of 140 μM. (c) Neutral amino acids (10 mM) inhibit the uptake of [¹⁴C]MeAIB by cells expressing SA1. Charged amino acids and the System L substrate 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH) do not significantly inhibit [¹⁴C]MeAIB uptake whereas histidine and D-serine inhibit partially. Uptake was measured at 2 min, and the uptake by cells transfected with vector alone subtracted as background. The letters indicate the standard amino acid code. (d) Alanine, proline, glutamine, and serine potently inhibit [¹⁴C]MeAIB uptake by SA1 at 1 min. IC₅₀ values are 0.27 ± 0.05 mM for alanine, 0.41 ± 0.08 mM for proline, 0.51 ± 0.19 mM for glutamine, and 0.36 ± 0.1 mM for serine.