Interactions between the histidine stimulation of cadmium and zinc influx into human erythrocytes

1. Histidine (2-40 mM) stimulated cadmium uptake into human erythrocytes incubated in the presence of 1% bovine serum albumin to ensure that the free, ionic cadmium concentration was low. 2. The histidine-stimulated cadmium uptake correlated with the calculated concentration of the cadmium-bis-histidine complex rather than the cadmium-mono-histidine complex or free ionic cadmium. 3. The histidine stimulation of cadmium uptake was saturable and stereospecific. D-Histidine (10 mM) had no effect. 4. Cadmium and zinc were both able to inhibit 65Zn2+ uptake into erythrocytes incubated in the presence of 40 mM L-histidine. The relationships between the percentage inhibition of 65Zn2+ uptake and the calculated concentrations of cadmium-bis-histidine and zinc-bis-histidine were very similar, which suggests that the metal histidine complexes compete for a common transport mechanism. 5. Pretreatment of the erythrocytes with N-ethylmaleimide using a protocol which is known to inhibit the system y+ amino acid transport mechanism had no effect on the histidine stimulation of metal transport.