Reconstitution of the renal brush-border membrane sodium/phosphate co-transporter

Molecular and ionic mimicry and the transport of toxic metals

Despite many scientific advances, human exposure to, and intoxication by, toxic metal species continues to occur. Surprisingly, little is understood about the mechanisms by which certain metals and metal-containing species gain entry into target cells. Since there do not appear to be transporters designed specifically for the entry of most toxic metal species into mammalian cells, it has been postulated that some of these metals gain entry into target cells, through the mechanisms of ionic and/or molecular mimicry, at the site of transporters of essential elements and/or molecules. The primary purpose of this review is to discuss the transport of selective toxic metals in target organs and provide evidence supporting a role of ionic and/or molecular mimicry. In the context of this review, molecular mimicry refers to the ability of a metal ion to bond to an endogenous organic molecule to form an organic metal species that acts as a functional or structural mimic of essential molecules at the sites of transporters of those molecules. Ionic mimicry refers to the ability of a cationic form of a toxic metal to mimic an essential element or cationic species of an element at the site of a transporter of that element. Molecular and ionic mimics can also be sub-classified as structural or functional mimics. This review will present the established and putative roles of molecular and ionic mimicry in the transport of mercury, cadmium, lead, arsenic, selenium, and selected oxyanions in target organs and tissues.

Factors affecting the stability of the renal sodium/phosphate symporter during its solubilization and reconstitution

Phosphate is reabsorbed across the brush-border membrane of the proximal tubule by a specific sodium-dependent symporter. Like the other brush-border membrane transport proteins of the kidney, the phosphate carrier remains to be isolated in a functional state. To establish a set of parameters that allow to preserve its biological activity, the phosphate carrier was solubilized under systematically varied conditions and reconstituted into proteoliposomes. Successful reconstitution was achieved only when the extraction buffer contained lipids extracted from the renal brush-border membrane. Glycerol, an osmolyte which reduces the water activity of the solution, was also required. It could however be replaced by 150 mM sodium or potassium phosphate. Below this concentration and in the presence of glycerol, the ionic strength of the solution had little effect on the stability of the transporter, but sodium phosphate could not be replaced by sodium chloride. Phosphate transport in reconstituted vesicles depended on the concentration of detergent and pH of the extraction buffer. Finally, transport activity was increased when solubilization was carried out in the presence of a reducing agent, dithiothreitol. These results should be helpful during the purification and further characterization of the renal phosphate symporter.

Molecular size of the renal sodium/phosphate symporter in native and reconstituted systems

The size of the renal sodium/phosphate symporter was estimated with the radiation inactivation technique in isolated bovine brush border membrane vesicles and after reconstitution in proteoliposomes. The functional unit of the native phosphate carrier had a radiation inactivation size of 172 +/- 17 kDa. Identical values were obtained for the reconstituted carrier whether it was irradiated before or after the formation of the proteoliposomes (161 +/- 9 and 159 +/- 11 kDa, respectively). The sodium-independent uptake of phosphate was not affected significantly by radiation doses up to 10 Mrad. This activity is therefore not due to the reconstitution of a large phosphate-binding protein such as alkaline phosphatase. Furthermore, bromotetramisole, a specific inhibitor of phosphate binding to this enzyme, had no significant effect on the uptake of phosphate by the proteoliposomes.