Phosphate uptake system

Due to its electronic configuration of 3d 4 s, vanadium is mostly present in the + 5 oxidation state that forms the oxyanion vanadate (VO4 Due to its structural similarity to phosphate P04 , 4 is taken up by phosphate uptake systems and treated like phosphate by the cell (Mahanty etal. 1991). However, since vanadate does not form stable molecules as does phosphate (Ivancsits et al. 2002), this leads to vanadate toxicity. This ability is also used for in-vitro and in-vivo vanadate-inhibition experiments (Rensing et al. 1997). 

Figure 3. Examples of major types of uptake mechanisms realised in prokaryotic outer membranes (a to c) and cytoplasmic membranes (a, and d to 1). The solutes to be transported are shown by filled circles x symbolises another solute which is transported in the same or in the opposite direction. In systems h-k, uptake is driven by the cleavage of ATP to ADP and phosphate. One type of uptake system, 1, depends on the energy-rich molecule phosphoenolpyruvate shown as PEP .

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. 

Vanadate is reduced in red cells to vanadyl ion by intracellular glutathione following uptake through a phosphate transport system.1072,1073 Free vanadyl is normally unstable with respect to oxidation, but appears to be stable when complexed with intracellular proteins or smaller molecules.1074 Vanadyl ion is a relatively powerful inhibitor of (Na+, K+)-ATPase. For pure fractions of the enzyme, inhibition was nearly complete at less than 5 pmol dm-3 vanadyl ion.1075 The state of vanadyl ion at pH 7 is somewhat uncertain, but may involve a hydroxylated species. Vanadyl ion also inhibits alkaline phosphatase more effectively than does vanadate.1076... 

Bowman, B.J. 1983. Vanadate uptake in Neurospora crassa occurs via phosphate transport system II. J. Bacteriol. 153 286-291. 

In this chapter, we indicate that the metabolic error (i.e., deficiency of phosphotransferase activity in I-cell disease) gives rise to a secondary phenotype of generalized diminished lysosomal enzyme activity. What other metabolic defects in the mannose 6-phosphate-mediated uptake system could result in such a phenotype, and how would you confirm the defect ... 

Phosphate uptake was studied in phosphate-limited chemostat cultures of Methanobacterium thermoautotrophicum. In cells adapted to nmolar phosphate concentrations the presence of a high-affinity transport system (A m = 25 nM Fmax = 60nmol/min x mg) was demonstrated [273]. 

One important problem, particularly if one wants to involve mitochondria in the regulation of intracellular Ca, is the affinity and the maximal rate of the uptake system. The latter is now accepted to be as high as 10 nmol mg protein s at 25 ° C in the presence of phosphate [36], (see Ref. 14 for a comprehensive discussion), but considerably lower in its absence. At the physiological temperature of about 37 ° C, this rate is approximately doubled. As mentioned before, phosphate increases the total amount of Ca that can be accumulated, but its stimulating effect on the initial rate of uptake must evidently be explained differently. It may be mentioned here that the penetration of phosphate into mitochondria lowers the in-tramitochondrial pH, which increases after the penetration of Ca ". This may remove a hypothetical limiting step in the Ca uptake process, but it must be emphasized here that the precipitation of Ca " " and phosphate within the matrix as amorphous hydroxyapatite involves net acidification, and thus complicates the matter considerably. 

In an attempt to exploit the mannose-6-phosphate receptor system to facilitate the uptake of nucleotides, nucleoside adducts of D-mannose-6-pyrophosphate have been prepared. P -(5-Iodo-2 -deoxyuridine)-5, P -D-... 

Brzoska, P., Rimmele, M., Brzostek, K. and Boos, W. (1994a) The pho regulon-dependent Ugp uptake system for glycerol-3-phosphate in Escherichia coli is frans-inhibited by P. Journal of Bacteriology 1 76, 1 5-20. 

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