Calcium, absorption protein-bound

Most potentiometric electrodes are selective for only the free, uncomplexed analyte and do not respond to complexed forms of the analyte. Solution conditions, therefore, must be carefully controlled if the purpose of the analysis is to determine the analyte s total concentration. On the other hand, this selectivity provides a significant advantage over other quantitative methods of analysis when it is necessary to determine the concentration of free ions. For example, calcium is present in urine both as free Ca + ions and as protein-bound Ca + ions. If a urine sample is analyzed by atomic absorption spectroscopy, the signal is proportional to the total concentration of Ca +, since both free and bound calcium are atomized. Analysis with a Ca + ISE, however, gives a signal that is a function of only free Ca + ions since the protein-bound ions cannot interact with the electrode s membrane. 

Calcium exists in the human body as Ca(II) protein-bound and free Ca (II) ions (Dilana et al. 1994). For total extracellular Ca in plasma, serum and urine a definitive isotope dilution-mass spectrometry (ID-MS) method exist. Free Ca(II) in plasma/serum can be determined with PISE, but no definitive and reference methods exist. For Ca in faeces, tissue and blood flame atomic absorption (FAAS) is used widely. 

Vitamin D-binding protein and its associated vitamin are lost in nephrotic urine. Biochemical abnormalities in nephrotic patients (children and adults) include hypocalcemia, both total (protein-bound) and ionized hypocalciuria, reduced intestinal calcium absorption and negative calcium balance reduced plasma 25-hydroxycholecalciferol and 24,25-dihydroxycholecalciferol and, surprisingly, also 1,25-dihydroxycholecalciferol and blunted response to parathormon (PTH) administration and increased PTH levels. Clinically, both osteomalacia and hyperparathyroidism have been described in nephrotic patients, more commonly in children than in adults, but bone biopsies are commonly normal, and clinically significant bone disease is very rare in nephrotic subjects. There is, however, evidence that patients with renal failure accompanied by nephrotic range proteinuria may be particularly prone to develop renal osteodystrophy. 

Clinical trials have shown that raloxifene, in combination with oral calcium supplementation, decreases the risk of vertebral fracture and promotes bone formation, albeit to a lesser extent than with estrogen. Raloxifene has been shown to have a beneficial effect on lipid profiles (11). Raloxifene should not be administered in combination with cholestyramine (decreased absorption), coumadin (prothrombin times and international normalized ratios must be monitored more closely), and those drugs that are highly protein bound, such as clofibrate, diazepam, ibuprofen, indomethacin, and naproxen. 

B. Pharmacokinetics. Fluoride is a weak acid (pKa 3.4) that is passively absorbed from the stomach and small intestine. In an acidic environment more fluoride is present as HF, which is more rapidly absorbed. Fasting peak absorption occurs in 30-60 minutes. The volume of distribution is 0.5-0.7 Ukg. Fluoride Is not protein bound, but binds readily to magnesium and calcium in blood, tissues, and bone (most fluoride in the body is bound to bone). The elimination half-life is 2.4-4.3 hours and is prolonged in patients with renal failure. 

Calcium that is protein-bound and inorganically complexed is detectable by FAAS only after dissociation fixim these complexes. Acid is used for the dissociation of protein-bound calcium and lanthanum or strontium ions are added, usually in the sample diluent, to displace calcium from phosphate, oxalate, citrate, and other complexes (10). Interference from magnesium and other elements is reduced by using a narrow bandpass, diffraction-grating spectrophotometer set at the specific absorption line of calcium (422.7 nm). Sodium and potassium interference is eliminated by the addition of physiological concentrations of sodium and potassium to the calcium standards (111-... 

In the case of a hydrophilic compound that enters the cell by carrier-mediated diffusion, a net increase in concentration inside the cell can sometimes be achieved by binding it to an intracellular protein. Only material in free solution can equilibrate across the membrane, not that which is protein bound. Such binding proteins are important, for example, in the intestinal absorption of calcium (section 4.6.1) and iron (section 4.6.2). 

Meat and such high protein plant foods as soy are excellent sources of phosphorus as well as protein. The phosphorus in meat is readily absorbed from the gastrointestinal tract however, much of the phosphorus in plant products is in a bound form which may inhibit the absorption of calcium as well as phosphorus. This study was designed to determine the effect of different levels of calcium and phosphorus with plant protein or animal protein on bone breaking strength and calcium and phosphorus utilization of weanling mice. 

Calcium is present in three forms e.g., as free calcium ion, bound to plasma protein albumin and in diffusable complexes. The endocrine system, through parathyroid hormone and calcitonin, helps in keeping the concentration of ionized plasma calcium in normal level. Decrease in plasma levels of ionized calcium leads to increased parathyroid hormone secretion. Parathyroid hormone tends to increase plasma calcium level by increasing bone resorption, increasing intestinal absorption and increasing reabsorption of calcium in kidney. Vitamin D acts by stimulating... 

Vitamin B12 can be absorbed when present in physiological amounts only if it is first bound to a specific protein—the so-called intrinsic factor—that tightly binds to the vitamin. The complex then passes through the jejunum to the ileum, which contains receptor sites for the vitamin B12/intrinsic factor complex. Calcium ions are required for the reaction between ileal receptors and the intrinsic factor/vitamin B12 complex. The reaction is inhibited by EDTA and reduced by a pH below 5.4. The vitamin appears to be separated from intrinsic factor at the ileal receptor sites and is then bound to another protein carrier, transcobalamin II, which transports the vitamin and permits its uptake by a number of tissues. The subject has been well reviewed by Jacob and her colleagues (Jl). Removal of 60 cm of ileum may impair vitamin B12 absorption and with the loss of 180 cm absorption is almost always affected. 

The BS I lectin requires bound calcium for activity.131 Two moles of calcium and 1.25 moles of magnesium per mole of protein were found by atomic absorption spectroscopy. Inactive, metal-free lectin, obtained by exhaustive dialysis, could be reconstituted by addition of calcium, cadmium, or strontium (magnesium restored 80% of the activity). Although bound-calcium was not removed by dialysis against EDTA, inclusion of this chelating agent in the precipitin reaction resulted in complete inhibition. 

The results of dietary zinc analysis need to be considered in terms of the availability of the zinc in the food for intestinal absorption. The zinc content of whole meals and the total daily zinc intake are not sufficient information on their own, without knowledge of factors which inhibit or promote intestinal absorption (O Dell, 1984). Free ionic zinc probably does not exist in the intestinal tract, zinc being bound to molecular species such as protein, amino acids, phytic acid, citrate and others. The bioavailability of the metal is determined by the nature of these zinc binding ligands. When the zinc complex is insolubie as in Zn-phytate, the uptake from diet is poor, whereas zinc-protein or zinc-amino acid complexes are more easily dissociated and are a good source of available zinc. Other dietary components affect zinc absorption such as the amount of iron, calcium and phosphate. 

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. 

A major role of vitamin D is to maintain the concentrations of calcium and phosphorus in the blood, primarily by enhancing the absorption of dietary calcium from the alimentary tract and regulating the interchange of calcium between blood and bone. It is likely that there are other cellular roles as yet little understood. The active form of vitamin D responsible for the hormone-like functions described above is 1,25-dihydroxycholecalciferol. Cholecalciferol, whether absorbed from the diet or synthesized in the skin, is first hydroxylated in the liver to 25-hydroxycholecalciferol. This is the main form of the hormone circulating in the blood, bound to a sterol binding protein. A further hydroxylation occurs in the kidneys to give 1,25-... 

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