Inorganic phosphate absorption

Phytic acid solutions were prepared by titrating sodium phytate (Sigma Chemical Company) with HCl the concentration was determined by analyzing for inorganic phosphate after wet ashing with HoSOu-HNOo (3 2) for 45 minutes. The concentration of CaCl stock solutions was measured by atomic absorption spectrometry. 

Walton, J., and T.K. Gray. 1979. Absorption of inorganic phosphate in the human small intestine. Clin Sci (Lond) 56 407. 

It is believed that the major part of inorganic phosphor absorption in the intestine occurs via Na+-dependent phosphate cotransporter NaPi-Iib (SLC34A2, Xu et al. 1999). Small drugs like phosphocarbonic acid and foscamet are transported by this transporter (Swaan et al. 1995 Tsuji and Tamai 1996). However, the substrate range is rather limited including only inorganic phosphate compounds. 

A wide range of plasma phosphorus concentration has been observed by other workers in primary hyperparathyroidism (C7) and explained in terms of diet and renal excretion. Unlike the calcium concentration, which is normally very constant regardless of dietary intake and urinary excretion, the concentration of inorganic phosphate in plasma is the resultant of the rate of phosphorus absorption from the gut and protein catabolism, on the one hand, and of renal excretion, on the other. Although the parathyroid hormone promotes phosphorus excretion, this is only one of the factors governing plasma phosphate concentration. Plasma phosphate in cases of hyperparathyroidism on a relatively high phosphorus intake may therefore not be distinguishable from that in normal subjects on a lower intake. 

As the inorganic phosphate content of the blood drops during absorption of these sugars, there is a theory that the specific-rate absorption of glucose, galactose, and fructose involves a temporary combination with phosphate. The hypothesis is supported, though not proved, by the fact that several enzyme poisons can lower the rate of absorption of these sugars (69). 

Figure 21. P Is NEXAFS spectra of inorganic phosphate in different protonation states. The inset shows the close up of the absorption edge (from Myneni and Martinez 1999 Myneni, unpublished results).

Sodium cellulose phosphate (SCP) is an insoluble, non-absorbable ester of cellulose containing 34% inorganic phosphate and 11% sodium. It is capable of binding calcium in the intestinal tract, reducing absorption of this ion, as well as magnesium. SCP is indicated only for the treatment of absorptive hypercalciuria type I with recurrent calcium oxalate or calcium phosphate nephrolitMasis. 

Zaugg, W.S. and Knox, R.J., Indirect determination of inorganic phosphate by atomic absorption spectrophotometric determination of molybdenum. Analytical Chemistry 38,1759,1966. 

Phosphorus is more readily absorbed from the intestinal tract than calcium. Approximately two-thirds of the phosphorus excreted is found in the feces, one-third in the urine. Between 10 and 20 % of phosphorus is found in tissues other than bone, and this phosphorus appears to have metabolic priority. The mechanisms which regulate deposition and release of phosphorus from bone are the same as those for calcium. Phosphorus in soft tissues plays a very vital role in many metabolic processes. The importance of adenosine triphosphate (ATP) in energy transfer systems has been discussed previously (p. 203, Chapter 16). Phosphorylation appears to be essential for the absorption of a number of nutrients, e.g., fatty acids and glucose. The phosphate radical is bound to proteins, fatty acids, carbohydrates, and enzymes. Phosphate is the chief inorganic anion of intracellular fluid, and phosphates of extracellular fluid participate in acid-base regulation. The inorganic phosphate level of blood ranges from 2 to 4 mg. per 100 ml. in adults and 3 to 5 mg. per 100 ml. in children. 

The total phosphoms content in the body is about 700 g. The daily requirement is about 0.8—1.2 g. The Ca/P ratio in food should be about 1. Phosphoms, in the form of phosphate, liee or bound as an ester or present as an anhydride, plays an important role in metabolism and, as such, is an essential nutrient. The organic forms of phosphoms in food are cleaved by intestinal phosphatases and, thereby, absorption occurs mostly in the form of inorganic phosphate. Polyphosphates, used as food additives, are absorbed only after prior hydrolysis into orthophosphate. The extent of hydrolysis is influenced by the degree of condensation of the polyphosphates. Table 7.2 includes a compilation of the phosphoms content of some foods. 

Other biochemical changes include depressed serum concentrations of calcium and inorganic phosphate, largely because of insufficient intestinal absorption directly relating to too Httle of these ions in the usual diet. Serum alkaline phosphatase, especially bone-specific alkaline phosphatase, is elevated because of osteoblastic cell overproduction when these cells attempt to form new bone tissue. 

Tablets and other drug formulations often comprise the active pharmaceutical ingredient (API) and pharmacologically inert carrier substances, or excipients, which bind, stabilize, and assist absorption of the drug by the body, or simply add bulk to APIs in low dosages. Common excipients are inorganic phosphates and organic compounds such as lactose, cellulose, and polyethylene glycol. Particle size reduction by micronization or milling can provide the energy to transform an API into undesirable polymorphs or...

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