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 ⁶⁵Zn were assessed under initial rate conditions, after 0.5 min of incubation. The kinetic parameters obtained at 20°C were a K _m of 15.0±1.5 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 M, while zinc competitively inhibited ¹⁰⁹Cd uptake by isolated cells. Cysteine and histidine stimulated zinc transport at an amino acidzinc molar ratio ranging from 11 to 81. This stimulation was not observed in the absence of a sodium gradient. At a molar ratio greater than 161 (i.e., 400 M cysteine or histidine and 25 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.