L-DOPA (L-3,4-dihydroxyphenylalanine) uptake by human red blood cells

The uptake of L-DOPA (L-3,4-dihydroxyphenylalanine) was studied in normal human red blood cells in vitro using L-[3-14C]DOPA. Uptake was slow, tending towards a distribution ratio close to unity with a half-time to equilibrium of one hour. Uptake was not Na+-dependent. Concentration dependence studies showed both saturable and non-saturable components of uptake, and inhibition studies using L-leucine and L-tryptophan suggest that the L and T systems of red cell amino acid uptake are involved. A powerful inhibitor of both systems, 3,4-dihydroxy-2-methylpropriophenone (U-0521), is described. It is concluded that uptake is by carrier-mediated facilitated diffusion via the L and T systems for which L-DOPA has low affinity.