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New mechanism for amino acid influx into human epidermal Langerhans cells: L‐dopa/proton counter‐transport system

Abstract:  We have characterized a stereospecific transport mechanism for L‐dopa into human epidermal Langerhans cells (LCs). It is different from any other amino acid transport system. It is highly concentrative, largely pH‐independent, and independent of exogenous Na+, glucose and oxygen, and fuelled by a renewable intracellular energy source inhibited by iodoacetate but not by arsenate. We propose that the mechanism is a unidirectional L‐dopa/proton counter‐transport system. We have recently demonstrated anaerobic glycolysis in human epidermis, which substantiates the need of proton pumps for resident LCs. The findings prompt a re‐evaluation of the profound changes LCs undergo when exposed to oxygen in aerobic culture. L‐dopa is not metabolized by LCs but can rapidly be dislocated to the intercellular space by certain extracellular amino acids, i.e. LCs can profit by L‐dopa in a dualistic way, altogether a remarkable biological phenomenon.