Plasma and intracellular Inorganic phosphate (Pi) is essential for various cellular metabolic functions and signal transport. Pi levels are hormonally regulated that affects the physiological activity of bone, kidney, and small intestine. Majority of the Pi is absorbed in the small intestine and reabsorbed in the proximal tubules in the kidney. Given the negative electrochemical potential across the cell membrane, limited solubility of Pi in complex with Ca+, the anionic Pi cannot be accumulated inside the cell by simple diffusion. Therefore, cellular uptake of Pi is actively coupled to a downhill movement of H+ and Na+ depending upon the cell type and transporter employed.

A group of membrane located phosphate transporters have been cloned and characterized from various species: Type I-related NaPi transporters designated NPT1, Npt1, and NaPi-1 respectively in humans, mouse, and rabbit are expressed in the kidney and liver. Its expression and activity are not regulated by Pi deprivation or parathyroid hormone (PTH) and its role in Pi-homeostasis is not clear. Type IIa-related cotransporters, designated NaPi-2 in rat, NaPi-3 or NPT2 in humans, NaPi-4 in opposum, NaPi-5 in flounder vessel, NaPi-6 or Npt-2 in mouse, and NaPi-7 in rabbit, is the primary target for Pi regulation by dietary, hormonal, and tubular Pi reabsorption. Deletion of Npt2 gene produces severe Pi wasting. Type II transporters are expressed in kidney, brain, lung, bone and small intestine. Type IIb, designated as NaPi-IIB or NaPi-2b in rat/mouse, and NaPi-3b in human, is closely related isoform of the NaPi-2 family. It is expressed in small intestine and lung. Type III NaPi transporters, originally described as a family of cell surface receptors for gibbon ape leukemia virus (GALV) and murine amphotropic retrovirus (A-MuLv), share very low (<20%) sequence homology with Type I and II proteins, and are found in most tissues. Human homolog are now desganted as PiT1 (also known as GALV receptors or GLVR1) and PiT2 (A-MuLV receptors or GLVR2 or Ram1). Pit1 and Pit2 are ~62% related and predicted to contain 10 TM domains that is in contrast with Type 1/2 transporters (6-8 TM). The N and C-termini are predicted to be cytoplasmic.

 

M= Mouse; R=Rat; H=Human; Ha=Hamster; Rb=Rabbit; B=Bovine; CT= near C-terminus; NT=near N-terminus; Ch=Chicken; c=canine, o=opossum, b=bovine; zf=zebra fish, f=frog; IC=intracellular and EC=extracellualr regions. *

** Expected antibody crossreactivity information is mostly based upon high (>70%) sequence conservation of antigenic/control peptides in various species. When antibody crossreactivity has actually been experimentally confirmed in various species, it will be mentioned in the appropriate data sheets.
" Antisera or neat antisera" are the unpurified antiserum and it is suitable for ELISA and Western.
"Affinity pure" IgG is more suitable for immunohistochemical (IHC) applications and to reduce background in most immunological applications including ELISA and Western.
"Control peptides" are the actual antigenic peptides and cannot be used for Western as they are very short peptides. They are intended for ELISA or antibody blocking studies to establish antibody specificity.
Western blot +ve protein controls, where available, are semi-pure, pure or recombinant proteins that are formulated in SDS-PAGE sample buffer. They are recommended to be used for Western (load 10 ul/lane) for visualization with antibodies.

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