Sulfate transport in human neutrophils

The mechanism by which SO4(2-) is transported across the plasma membrane of isolated human neutrophils was investigated. Unlike the situation in erythrocytes, SO4(2-) and other divalent anions are not substrates for the principal Cl-/HCO3- exchange system in these cells. At an extracellular concentration of 2 mM, total one-way 35SO4(2-) influx and efflux in steady-state cells amounted to approximately 17 mumol/liter of cell water per min. The intracellular SO4(2-) content was approximately 1 mM, approximately 25-fold higher than the passive distribution level. Internal Cl- trans stimulated 35SO4(2-) influx. Conversely, 35SO4(2-) efflux was trans stimulated by external Cl- (Km approximately 25 mM) and by external SO4(2-) (Km approximately 14 mM), implying the presence of a SO4(2-)/Cl- countertransport mechanism. The exchange is noncompetitively inhibited by 4-acetamido-4'-isothiocyanostilbene-2,2' -disulfonate (SITS) (Ki approximately 50 microM) and competitively blocked by alpha-cyano-4-hydroxycinnamate (Ki approximately 230 microM) and by ethacrynate (Ki approximately 7 microM); furosemide and probenecid also suppressed activity. The carrier exhibits broad specificity for a variety of monovalent (NO3- approximately Cl- greater than Br- greater than formate- greater than I- approximately p-aminohippurate-) and divalent WO4(2-) greater than oxalate2- greater than SO4(2-) greater than MoO4(2-) greater than SeO4(2-) greater than AsO4(2-) anions. There was little, if any, affinity for HCO3-, phosphate, or glucuronate. The influx of SO4(2-) is accompanied by an equivalent cotransport of H+, the ion pair H+ + SO4(2-) being transported together in exchange for Cl-, thereby preserving electroneutrality. These findings indicate the existence of a separate SO4(2-)/Cl- exchange carrier that is distinct from the neutrophil's Cl-/HCO3- exchanger. The SO4(2-) carrier shares several properties in common with the classical inorganic anion exchange mechanism of erythrocytes and with other SO4(2-) transport systems in renal and intestinal epithelia, Ehrlich ascites tumor cells, and astroglia.