Transport of d-[1-14C]-amino acids into Chinese hamster ovary (CHO-K1) cells: implications for use of labeled d-amino acids as molecular imaging agents

IntroductionThe fact that d-amino acids have been found in various tissues and are involved in various functions is a clue to how to develop new imaging agents. We examined d-amino acid transport mechanisms in Chinese hamster ovary (CHO-K1) cells because CHO-K1 cells are widely used in biomedical studies and are thought to be useful for expression of genes involved in metabolism of d-amino acids.MethodsUptake experiments were performed. CHO-K1 cells cultured in 60-mm plastic culture dishes under ordinary culture conditions were incubated with 18.5 kBq of radiolabeled amino acid in 2 ml of phosphate-buffered-saline-based uptake solution at 37°C. The following radiolabeled amino acid tracers were used: d-[1-14C]-alanine, l-[1-14C]-alanine, d-[1-14C]-serine, l-[1-14C]-serine, d-[1-14C]-methionine, l-[1-14C]-methionine, d-[1-14C]-phenylalanine, l-[1-14C]-phenylalanine, d-[1-14C]-leucine, l-[1-14C]-leucine, d-[1-14C]-valine, l-[1-14C]-valine, d-[1-14C]-tyrosine, l-[1-14C]-tyrosine, d-[1-14C]-glutamic acid, l-[1-14C]-glutamic acid, d-[1-14C]-lysine, l-[1-14C]-lysine, d-[1-14C]-arginine and l-[1-14C]-arginine. We tested the inhibitory effects of the following compounds (1.0 mM) on transport: 2-(methylamino)isobutyric acid (a specific inhibitor of system A, in Na+-containing uptake solution) and 2-amino-bicyclo[2,2,1]heptane-2-carboxylic acid (a specific inhibitor of system L, in Na+-free uptake solution).Resultsd-[1-14C]-methionine, d-[1-14C]-phenylalanine and d-[1-14C]-tyrosine accumulated mainly via system L. d-[1-14C]-alanine and d-[1-14C]-serine accumulated primarily via system ASC. High uptake of d-[1-14C]-alanine, d-[1-14C]-methionine, d-[1-14C]-phenylalanine and d-[1-14C]-leucine was observed. The uptake of radiolabeled serine, valine, tyrosine, glutamic acid and arginine into CHO-K1 was highly stereoselective for l-isomers.ConclusionsWe observed high uptake of d-[1-14C]-alanine via system ASC (most likely alanine–serine–cysteine-selective amino acid transporter-1) and high uptake of d-[1-14C]-methionine and d-[1-14C]-phenylalanine via system L (most likely l-type amino acid transporter-1).