Characterization of a new amino acid transport system in human epidermal Langerhans cells: the L-dopa influx/efflux cycle.

Various aspects concerning the L-dopa uptake mechanism into human epidermal Langerhans cells (LC) and the energy metabolism of epidermis have been examined. The study is based on the discovery of a new amino acid transport system in human epidermal LC. The amino acid transport system has been characterised. This system is stereospecific and is unique for L-dopa and LC as it does not share any characteristics with other described transport systems of other amino acids in other cell types. The uptake assay is based on the Falck/Hillarp histofluorescence method, which enables studying uptake in individual LC in situ in their microenvironment. For the first time, it is shown that the human epidermis is capable of an aerobic lactate production, which is a strange phenomenon, as this means that epidermal cells, despite the formal presence of mitochondria, nevertheless produce lactate in excess. This probably depends on a disturbed Pasteur effect. The same phenomenon is found under normal conditions in smooth muscle cells, but has not been described in other human cells. Also, for the first time, it is shown that the human epidermis has a lower energy charge value than other body cells except for, again, smooth muscle cells. The increased lactate production in epidermis, which can be inhibited by iodoacetate, implies an increased intracellular proton production. The energetics of the L-dopa transport into the LC is probably a counter-transport (antiport) of protons due to the aerobic production of lactate. It is proved that L-dopa can be dislocated out of the LC by certain amino acids on the outside of the cell, some acting in a trans-stimulatory fashion and others probably by exchanging with intracellular L-dopa, hereby getting "useful" amino acids into the cell instead. This, now extracellular L-dopa, can then begin a new in- and out-transport cycle. For further characterisation of the L-dopa transport system, experiments have been carried out with an H+,K+-ATPase-inhibitor (omeprazole and a number of its analogues). Most of these are inhibitory of the L-dopa transport into LC. This is most probably not caused by an effect on a thiol group essential for this transport, but the actual mechanism is not known.