Uptake of ascorbic acid by human granulocytes.

The uptake of ascorbic acid by isolated human granulocytes is investigated under different conditions. The rate of uptake depends on the concentration of ascorbic acid in the incubation medium as well as on temperature, with a maximum at 40 degrees C. At 0 degrees C no uptake can be observed. N-formylated peptides being known to stimulate human granulocytes, considerably increase ascorbic acid uptake, but the non-formylated methionyl-leucyl-phenylalanine is without effect. Glucose is a strong inhibitor of ascorbate uptake with a Ki of 3.7 mM and a stoichiometry of 1:1. Fructose does not inhibit at all and galactose only at elevated concentrations. Phlorizin, a known inhibitor of glucose transport, inhibits the uptake of ascorbic acid to the same extent as that of glucose. Elevated glucose levels do not induce a release of ascorbic acid out of the cells. It is concluded that ascorbic acid is actively accumulated in human granulocytes and that stimulated cells respond with a pronounced increase of ascorbate uptake. The site of ascorbic acid transport across the membrane is probably the same as of glucose.