Phosphorus

Phosphorus (atomic weight 29.97) has 3 or 5 valences w ith oxidation states between —3 and +5. Phosphate (PO4 , molecular weight 94) is the form utilized by humans.

Abbreviations

Glvr-1 gibbon ape leukemia virus receptor (Pit-1. SLC20A1 )

PTH parathyroid hormone

TmP tubular maximum for phosphate

Nutritional summary

Function Phosphate is the constituent anion of bone, participates in energy metabolism and storage (as ATP. (¡TP. creatine phosphate, arginine phosphate etc.). and is an important buffer in most body compartments.

Requirements: Adults should consume at least 700mg day; pregnancy and lactation do not increase needs.

Sources: Phosphate content of foods correlates somewhat with protein content, but distinct differences exist. Milk and dairy products have the highest phosphate to protein ratio (near 30mg g). Chicken and tish arc at the low end (6 -7 mg/g). Processed cheese, many other processed foods as well as colas and some other sodas are also important sources. Much of the large amounts of phosphate in plant-derived foods is bound to inositol (phytatc and related forms) and is not absorbed. Food tables give the misleading impression that legumes, oats, rye, and other grains are major phosphate sources, which they are not.

Deficiency: Phosphate inadequacy is usually due to low food consumption or starvation, and is not rare in old people. Accelerated bone mineral loss causes osteoporosis and increases fracture risk.

Excessive intake: Phosphate intakes that significantly exceed calcium intakes (on a milligram basis) induce parathyroid gland hyperplasia and parathyroid hormone (PTII) secretion, impair vitamin D activation, and accelerate bone mineral loss and fracture risk. Extremely high intakes may cause calcification of extraosseous (non-hone) tissues, including arteries, kidney s, muscles, and tendons.

Dietary sources

It is important to recognize that foods contain three very different types of phosphate compounds. The first two types, inorganic phosphate salts and most organophosphates including phospholipids, are readily absorbed and profoundly impact human metabolism. In stark contrast, inositol polyphosphates are hardly absorbed at all. share virtually none of the metabolic characteristics of phosphate salts or other organophosphates, and have their own distinct properties. Current food tables and other sources of food composition data routinely provide only a single value for total phosphate and become virtually meaningless in the case of grains, legumes, fruits, and vegetables. Many (but not all) of these foods pro\ ¡de relatively little bioavailable phosphate, currently available food composition information and practice guidelines (ADA, 1998) notwithstanding.

h-poj

Inorganic phosphate Inositol hexaphosphate Phospholipid

Figure 11,13 Different type, of phosphate-containing cnmptiunck in faodi

Milk i l.Omg/g) and dairy products arc major phosphate sources. Particularly concentrated sources arc hard cheeses such as cheddar (5.1 mgg) and sv\ iss-type cheese (6.1 mg'g). Smaller amounts are in soft cheeses like cottage cheese (I -3 mg'g) or cream cheese 11 .Omg/g). An important measure is the ratio ofbioavailable phosphate to protein, which ranges from very high tn milk (29.6). hard cheese (around 201, and eggs (14,3), to much lower in pork (i 1.4), beef (9,5). chicken (6.7). and lish (6,1). This ratio can be an important tool for people who need to minimize phosphate intakes while maintaining adequate protein nutrition.

The phosphate content of plant-derived foods is much more difficult to assess, since much of it is bound to inositol as phytate (inositol bexaphosphatc), inositol pen-taphosphate and inositol tetraphosphate.

Americans continue to increase their consumption of phosphate-containing food additives and may have average intakes from this source alone that approach 500mg/day bv now (Calvo and Park, 19%), Total daily phosphate intakes of young men tend to be around I500mg, that of young women around I IKK) mg (Food and Nuirition Board. Institute of Medicine, 1997: Appendix D). Intakes vary greatly between indiv tdusls and lend to decline after young adulthood. The above-mentioned caveat about the possible lack of relevance of total phosphate intake estimates, especially for groups with presumably healthful diets (rich in whole grains, legumes, fruits, and vegetables), has to be emphasized, however.

Digestion and absorption

Various forms of dietary phosphates, including phosphate salts, nucleotides, and phospholipids, are absorbed with high efficiency (60-70%) from the small intestine. Many organophosphatcs arc cleaved prior to absorption of the phosphate ion. The brush border enzyme alkaline phosphatase (FC3.1.3.1). for instance, cleaves creatine phosphate. Both inorganic pyrophosphatase (P.C3.6.1.1) and alkaline phosphatase (EC3.1.3.1) can hydrolyzc pyrophosphate.

Type I (SLCI7A1) and type lib (NaPi3B. SLC34A2) sodium/phosphate cotrans-porters move inorganic phosphate and three sodium tons across the brush border membrane (Murer et til.. 2001). Transport with the type I transporter is constant, white expression and transport rate with type lib transporter depends on phosphate dose and 1.25-dihydroxy-vitamin D status. An additional, sodium-independent, phosphate transporter may operate at the intestinal brush border membrane.

The type III sodium eotransporters Glvr-I (gibbon ape leukemia virus receptor. Pit-1. SLC20A1) and Ram-I (SIX number not assigned) arc thought lo provide for phosphate influx from the basolateral side for the enterocytc's own needs (Tencnhouse el al.. 1998). In addition to phospholipid consumed with foods, large amounts arc-secreted with bile and form mixed micelles with bile acids, fatty acids, cholesterol, and Other minor lipids. Phospholipids are cleaved by phospholipase A2 IEC3.1.1.4) from pancreas and the resulting iysolecithin is taken up from the micelle across the brush border membrane.

The liver-type fatty acid binding protein (L-FABP) helps to move lysophospho-lipids to intracellular compartments. A fatty acid is linked to most lysophospholipids tyso-

Pnosp1 Jp onosphwiDK) lipas« A2

Phospholipid

Inorganic Phosphate

Frylate lost with

Phospholipid

Inorganic Phosphate

Frylate

Capillary lumen

Brush border membrane

Baso I ate ra I membrane

Capillary endothelium

Capillary lumen

Brush border membrane

Baso I ate ra I membrane

Capillary endothelium

Figur« 11.14 Intestinal absorption of phosphate and the phospholipid is exported with chylomicrons. Alternatively, lysophospholipase (EC3.1,1.5) can hydrolyzc lysophosphatidyl choline to 3-phosphorylcholinc, w hich is then exported and carried to the liver via the portal bloodstream.

Transport and cellular uptake

Blood circulation- Most phosphate in circulation is contained in the phospholipids of' lipoproteins and blood cell membranes. A smaller amount circulates as inorganic phosphate ion. Plasma concentration of inorganic phosphate is around 1 mmol/I. if' intakes are adequate, much lower in deficiency, and slightly higher with increasing intake (Food and Nutrition Board Institute of Medicine. 1997). Phosphate enters cells via sodium eotransport, largely via the ubiquitous type 111 sodium «transporters. A slightly different use is seen in some bone where the type III sodium.phosphate entransporter Glvr-l (SLC20AI) pumps phosphate into the extracellular matrix during early bone mineralization (Palmer ct a!.. 1999). Glvr-l was also found to play a role in vascular and soft tissue calcification in response to hyperphosphatemia (Giaehelli et al„ 2(J0l). Ram-1 (SIX number not assigned) is another type 111 cotransponcr in many tissues. Band 3 of red cell membrane (SLC4A11 facilitates phosphate uptake in exchange for bicarbonate.

Most ATP is utilized outside the mitochondria (e.g. for ion pumping), hut needs to he reconstituted again by oxidative phosphorylation in the mitochondria. The mitochondrial phosphate carrier (SLC25A3), a proton-dependent symporter, and the adenine nucleotide translocators (SI.C25A4, SLC25A5. and SLC25A6) ferry the precursors for ATP synthesis into mitochondria.

Blood brain barrier: Phosphate transport into and from the brain is not well understood. It is likely that much of the transfer occurs through an anion antiporter that exchanges phosphate for bicarbonate (Dallaire and Bcliveau. 1992), Materno fetal transfer Phosphate, which has to be supplied in significant quantities to the fetus for tissue expansion and bone mineralization, crosses the maternal-facing brush border membrane of the syntrophoblast via sodium cotransport (Lajcuncsse and Brunette. 1488). This process is aided by the negative charge of the syntrophoblast layer relative to its outside.

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