Regulatory steps involved in the cellular uptake of vitamin C. We identified three possible regulatory sites at which HL-60 neutrophils regulate their uptake and content of vitamin C. These steps are also likely active in cells such as human neutrophils and monocytes that transport dehydroascorbic acid, but lack the capacity to transport ascorbic acid. (1) The first regulatory site relates directly to the availability of the substrate that is transported. Activation of HL-60 neutrophils with the chemotactic peptide fMetLeuPhe triggers the respiratory burst, with the subsequent generation of reactive oxygen species that can oxidize locally available ascorbic acid to dehydroascorbic acid. As a result, the locally generated dehydroascorbic acid will be rapidly transported intracellularly. (2) The second point of regulation is related to the transport step, and its regulatory potential is determined by changes in the number, molecular identity, or functional status of the transporters of vitamin C. GM-CSF treatment increases the affinity of GLUT1 for the transport of dehydroascorbic acid. As a result, increased transport of dehydroascorbic acid is observed. Growth factors and cytokines that affect the level of expression, subcellular localization, or the intrinsic functional activity of the glucose transporters can potentially modulate the cellular transport of dehydroascorbic acid. (3) A third potential regulatory step is defined by the existence of intracellular mechanisms that determine the capacity of cells to accumulate characteristic intracellular levels of vitamin C. At least two general enzymatic systems with dehydroascorbic acid reductase activity, one GSH dependent and one GSH independent, exist in mammalian cells. No data regarding their regulatory potential are currently available.