The phosphate transporter

A particularly reactive sulfhydryl group (or groups) on the phosphate transporter react with relatively low concentrations of maleimide derivatives and organic mercurials to inhibit the function of the transporter [17]. This sulfhydryl group is probably near, or a part of, the substrate binding site because phosphate protects the carrier activity from inhibition by A-ethylmaleimide. Fonyo has written a comprehensive review on the subject of sulfhydryl sensitivity of the phosphate carrier and its implications concerning the mechanism of transport [192]. 

A number of workers have been able to isolate a protein, covalently linked to radioactive Af-ethylmaleimide, identified as the mitochondrial phosphate transporter [122,193-196]. The isolation and identification of the transporter was based for the most part on the maleimide and mersalyl reactivity of the protein. The molecular weight of the protein isolated from different sources varies from 27000 to 34000. Because of the covalent linkage to the irreversible inhibitor, reconstitution of transport was not feasible. 

Three laboratories have been able to isolate a protein free of maleimide labelling which will catalyze phosphate transport in reconstituted proteoliposomes 

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As mentioned in Section II, arsenate is present in seawater at a fairly uniform concentration (about 0.5-2 ng As/liter), and in nutrient-deficient waters its concentrations may exceed that of the essential phosphate (146). In oxygenated seawater, the species exist predominantly as H2PO4 and H2As04, and algae may absorb arsenate because it is similar to the phosphate anion (147). In terrestrial plants and other organisms, arsenic is taken up by the phosphate transport mechanism... 

Vanadate is taken up by N. crassa when the cells are phosphate limited and growing in higher pH values. It takes place by the phosphate transport system II, which has high affinity for phosphate. Phosphate is a competitive inhibitor of vanadate uptake.1071 This confirms the competitive relationship between phosphate and vanadate. 

In mitochondrial research the phosphate-transporting protein from rat liver mitochondria has been labeled with ° Hg-mersalyl At For protein labeling with astatine (alpha emitter) the following procedures may be u reaction of the protein with p-astatobenzoic acid < ndensation reaction with peptide bond and protein acetylation While labeling by the above procedures seems to be sufficiently stable a remarkable instability of the At-label was obserwd when astatinated protein was prepared electrophoretically 202) jjjg results of these authors indicate that the tyrosine-astatine bond is unstable. The conclusion of Vau an et al. that astatinated proteins lore as much as 50% of their biological activity and, in addition, are extremely toxic, is very serious. 

The pyruvate transporter [201] and the carnitine translocase [202] have both been isolated but not characterized in any detail. The pyruvate transporter and the carnitine translocase, like the phosphate transporter, are inhibited by maleimide derivatives and mercurials, although at higher concentrations of the sulfhydryl reagents. The pyruvate transporter has been isolated in inactive form covalently linked to phenyl maleimide. Identification was based on the correlation of labelling of the protein with inhibition of transport, and by the fact that mercurials prevented the labelling. The molecular weight of the isolated monomeric protein is surprisingly low, approximately 15000. 

The Phosphate Transporter Proteins and Pyrophosphate in Matrix Vesicles... 

The phosphate transporter catalyzes the import of one HP04 coupled to the export of one OH . Likewise, the ATP/ADP antiporter allows one molecule of ADP to enter only if one molecule of ATP exits simultaneously. The ATP/ADP antiporter, a dimer of two 30,000-Da subunits, makes up 10-15 percent of the protein in the inner membrane, so it is one of the more abundant mitochondrial proteins. Functioning of the two antiporters together produces an Influx of one ADP and one Pj and efflux of one ATP together with one OH . Each OH transported outward combines with a proton, translocated during electron transport to the Intermembrane space, to form H2O. This drives the overall reaction in the direction of ATP export and ADP and Pj Import. 

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