REABSORPTION OF PROTEINS IN RENAL TUBULES

Abstract

In order to investigate the mechanism of the reabsorption of a protein in renal tubules of rats, we employed OCT-7000, which is a recombinant variant of natural human interleukin-1α with a molecular mass of approximately 18, 000. In this study, OCT-7000 uptake in renal tubules was examined using immunoelectron microscopic technique with immunogold staining. The effects of various proteins or synthetic polypeptides on the urinary excretion of OCT-7000 were also investigated. Immunoelectron microscopic observations showed that OCT-7000 was taken up into the endocytic vesicle close to the brush border membrane located in segment 2 of the proximal tubules, followed by accumulation of secondary lysosomes. Urinary excretion of OCT-7000 after systemic administration was extremely low, accounting for 0.014% of the dose. Human serum albumin had no effect on the excretion of OCT-7000, while increases in the urinary excretion of OCT-7000 were found in rats treated with a trypsin inhibitor, myoglobin and trypsinogen, in a dose-dependent manner. The order of potency for urinary excretion of OCT-7000 was trypsinogen > myoglobin > trypsin inhibitor. Poly-L-lysine, a synthetic polypeptide dose-dependently increased the urinary excretion of OCT-7000, whereas poly-L-glutamic acid had no effect on excretion. Specifically, the data reveal that reabsorption of OCT-7000 in the proximal tubules was inhibited by trypsinogen, myoglobin, trypsin inhibitor or poly-L-lysine, resulting in an increase of urinary excretion of OCT-7000. Furthermore, it was considered that negative charges on the brush border membrane in the proximal tubules were involved in the reabsorption of OCT-7000 because the inhibitory potency of proteins or synthetic polypeptides on the reabsorption of OCT-7000 was increased with a high isoelectric point. From the above, the mechanisms of reabsorption of protein in renal tubule are speculated as follows. (1) The reabsorption site of proteins is located in segment 1 and/or 2 of the proximal tubules in the kidneys. (2) Binders with negative charges likely exist on the surface of the brush border membrane, glomerular filtrated proteins are bound to the binder. The binder has an unusually broad binding specificity for filtered proteins, and the binding affinity between them is involved in only molecular charges. (3) After binding of filtered proteins to the binder on the membrane, the membrane invaginates into the cell, and then the endocytic vesicle including filtered proteins is incorporated into the cell. (4) Primary lysosome and endocytic vesicle are fused to constitute a secondary lysosome.