The effect of histidine and cysteine on zinc influx into rat and human erythrocytes.

1. The effect of histidine and cysteine on the initial rate of zinc influx into rat and human erythrocytes in the presence of bovine serum albumin has been investigated. 2. The L-enantiomers of both amino acids promoted zinc influx into rat erythrocytes in a dose-dependent manner. L-Histidine, but not L-cysteine, also promoted zinc uptake into human erythrocytes. D-Histidine did not promote zinc uptake in either rat or human erythrocytes. In rat erythrocytes D-cysteine was significantly less effective than L-cysteine. 3. The stimulation of zinc influx into rat erythrocytes by 20 mM L-histidine was approximately 4.1 times that seen with human erythrocytes. 4. The influx of zinc in the presence of varying concentrations of L-histidine was linearly related to the calculated concentration of the zinc-bis-histidine complex but not to that of the zinc-mono-histidine complex or the free ionic zinc concentration. 5. These results are discussed in relation to the nature of the transport mechanisms involved.