Renal transport systems

It seems likely that the prooxidant actions of ascorbate are of relatively little importance in vivo. Except in cases of iron overload, there are almost no transition metal ions in free solution. They are all bound to proteins, and because the renal transport system is readily saturated, plasma and tissue concentrations of ascorbate are unlikely to rise to a sufficient extent to lead to radical formation (Halliwell, 1996 Carr and Frei, 1999a). 

Lin JH, Chen IW, Deluna FA, Hichens M. Renal handling of alendronate in rats. An uncharacterized renal transport system. Drug Metab Dispos 1992 20 608-613. 

Much that is known about renal reabsorption mechanisms has been learned from the study of various forms of aminoaciduria. Three types of aminoaciduria have been identified (1) overflow aminoaciduria occurs when the plasma level of one or more amino acids exceeds the renal threshold (tubular capacity for reabsorption) (2) renal aminoaciduria occurs when plasma levels are normal but the renal transport system has a congenital or acquired defect and (3) no-threshold aminoaciduria occurs when excessive amounts of an amino acid, arising from an inherited metabolic block, are present in urine, but plasma levels are essentially normal because ah the amino acid is excreted. The no-threshold aminoacidurias, such as homocystinuria, are not due to congenital or acquired kidney defects but solely to saturation of the normal renal tubular reabsorption mechanisms. 

Other important renal transport systems include the peptide transporters, which are involved in the uptake of peptide-like drugs including /3 -lactam antibiotics and angiotensin-converting enzyme inhibitors, and nucleoside transporter proteins, which are involved in uptake of nucleosides and nucleoside analogs (e.g., zidovudine and dideoxyinosine). ... 

In the normal human kidney 98-100% of the filtered urate is reabsorbed. Substrate regulated secretion and postsecretory reabsorption of urate are considered to be the quantitatively most important renal transport systems for the regulation of the urinary excretion of urate (7). 

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