Calcium biologically active chelates

The human organism contains 1-1.4 kg calcium, and about 1% of this is in the extracellular fluid. The rest is largely in bone. The serum calcium concentration is 9-11.5 mg/dL, of which 4.5-5.0 mg/dL is in the free, ionized, biologically active form. The rest is protein bound or complexed with a variety of chelators, such as citrate. The daily dietary calcium requirement is 400-500 mg, and each day, 300-400 mg calcium is lost in the urine and an additional 150 mg in the feces. Inorganic phosphorus (largely as HP042 ) amounts to 2.7-4.5 mg/dL in adult serum. 

Despite the fact that a plethora of dietary factors could, and will, affect the absorption characteristics of phytochemicals, this area has not been systematically explored. One reason might be the complexity of dietary factors and their interactions that could affect absorption. A nonexhaustive list would include the volume and composition of the food consumed, pH, caloric density, viscosity, nutrients (carbohydrates, protein, fat, fibers), alcohol, caffeine, and the presence of other phytochemicals. Such dietary factors affect the functional status, motility, and acidity of the gastrointestinal tract in a complex manner and modify the physicochemical properties, formulation, and dissolution characteristics of the compound of interest. Calcium in dairy products, for example, has the potential to chelate tetracyclines and fluoroquinolones and, thereby, reduce their bioavailability and biological activity [31]. 

Calcium ions are mostly present in bones or chelated to biological molecules. In blood plasma, only 1% of the calcium ions present are unbound 78% is bound to albumin, 8% to citrate, and 13% to other plasma proteins. The free calcium ions are prevented from precipitating by plasma pyrophosphate. Calcium ions are also stored in the endoplasmic reticulum (ER), mostly chelated to ER-resident proteins and phosphatidylser-ine. Free calcium ions may be released through transient receptor potential channels to the cytosol where it activates numerous physiological processes. If the free calcium ion concentration of blood plasma falls, parathyroid hormone (PTH) is secreted by the parathyroid gland cells. PTH speeds up the transport of demineralized bone products by osteoclasts. In the kidney, it increases the excretion of phosphate and decreases the excretion of calcium. PTH also acts on kidney cells to make calcitriol from vitamin D, which induces calcium transporters in the intestine and osteoclasts. PTH mediates these effects by activating G-protein-coupled receptors in the kidney and osteoclasts. 

Transfer of calcium cations (Ca2 + ) across membranes and against a thermodynamic gradient is important to biological processes, such as muscle contraction, release of neurotransmitters or biological signal transduction and immune response. The active transport can be artificially driven (switched) by photoinduced electron transfer processes (Section 6.4.4) between a photoactivatable molecule and a hydroquinone Ca2 + chelator (405) (Scheme 6.194).1210 In this example, oxidation of hydroquinone generates a quinone to release Ca2+ to the aqueous phase inside the bilayer of a liposome, followed by reduction of the quinone back to hydroquinone to complete the redox loop, which results in cyclic transport of Ca2 +. The electron donor/acceptor moiety is a carotenoid porphyrin naphthoquinone molecular triad (see Special Topic 6.26). 

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