ZINC UPTAKE BY PROXIMAL CELLS ISOLATED FROM RABBIT KIDNEY - EFFECTS OF CYSTEINE AND HISTIDINE

The aim of this study was to characterize the mechanisms of zinc transport in proximal cells isolated from rabbit kidney cortex. Uptakes of Zn-65 were assessed under initial rate conditions, after 0.5 min of incubation. The kinetic parameters obtained at 20-degrees-C were a K(m) of 15.0 +/- 1.5-mu-M, a J(max) of 208.0 +/- 8.4 pmol min-1 (mg protein)-1, and an unsaturable constant of 0.259 +/- 0.104 (n = 8). Cadmium competitively inhibited the zinc uptake, with a K(i) value of 13.0 +/- 2.8-mu-M, while zinc competitively inhibited Cd-109 uptake by isolated cells. Cysteine and histidine stimulated zinc transport at an amino acid: zinc molar ratio ranging from 1:1 to 8:1. This stimulation was not observed in the absence of a sodium gradient. At a molar ratio greater than 16:1 (i.e., 400-mu-M cysteine or histidine and 25-mu-M Zn), there was evidence of inhibition. These data suggest that zinc enters renal proximal cells (a) as a free ion via a saturable carrier-mediated process or an unsaturable pathway and (b) complexed with cysteine or histidine, by means of a sodium/amino acid cotransport mechanism.