Charge-dependent renal uptake of beta2-microglobulin in conscious rats

The influence of molecular charge on the tubular reabsorption of proteins was studied in conscious rats injected intravenously with beta2-microglobulins of different isoelectric points (pI). Native human beta2-microglobulin (pI 5.8), two anionized (pI 4.85 and 5.55) and three cationized derivatives (pI 7.2, 8.35 and 8.7) were used. The six forms of beta2-microglobulin had a molecular radius between 15.7 and 15.9 Å. The renal uptake was estimated by measuring the amount excreted in urine with a sensitive immunoassay. The ability of rat kidney to reabsorb beta2-microglobulin was clearly related to the net charge of the protein. Increasing the pI of the protein significantly reduced the urinary excretion, whereas lowering it had the opposite effect. Anionization was particularly effective in reducing the beta2-microglobulin uptake, since a decrease of the pi of one unit enhanced the urinary output by two orders of magnitude. This charge-dependency persisted when the tubular reabsorption of proteins was partly inhibited by lysozyme. By contrast, it was practically abolished by lysine, probably because the inhibitory effect of this amino acid on protein tubular reabsorption is not competitive. The administration of ammonium chloride in rats produced an immediate and transient elevation of rat beta2-microglobulinuria. This phenomenon, which was partly inhibited by the subsequent administration of sodium bicarbonate, presumably results from a competition between the NH+4 ion and beta2-microglobulin for tubular binding sites. These data support the hypothesis that proteins bind to the luminal membrane of tubular cells mainly via positively charged amino groups.