Metabolism of bone

No requirement for boron in mammals is proven, although evidence is accumulating suggesting that boron may be an essential nutrient. Boron is related to normal energy utilization, immune function, and metabolism of bone, minerals, and lipids (Penland 1998). Boron deficiency (<0.04... 

Radiation Personnel Accident. Radiation exposure in accidents of personnel in nuclear facilities and non-destructive detection have been evaluated with ESR dosimetry. For example, the amputated medium fibre bone of a worker in Peru, who tried to solve the problem by directly touching the source of 60Co unit for a few seconds, was measured four years later.The dose was measured by 6.4 + 0.5 Gy, while 20 + 3 Gy was obtained from fluorescence in situ hybridization method. The result would suggest diagenic decay of the signal due to the metabolism of bone.96 Thus, ESR gives the retrospective dose for radiation accidents. 

Disturbances of the metabolism of bone tissue that usually exhibit not very specific clinical signs and symptoms are induced by the effects of both systemic and local agents. The role of hormones has already been described and defined many times, but news and new views keep appearing all the time. Without doubt, practically all systemic agents, hormones, but also drugs act on the bone tissue only through mediators. Their effect is provided by local, paracrine, and autocrine substances effective on the level of the cell, and by cellular components and on molecular level. Some of the hormones and local agents will be mentioned below. 

In spite of these inherent shortcomings, the measurement of urinary hydroxyproline offers a useful index of bone matrix metabolism to add to the spate of techniques of bone mineral measurement used to study the endocrinology and metabolism of bone (B8). 

No requirement for boron in mammals is proven, although evidence is accumulating suggesting that boron may be an essential nutrient. Boron is related to normal energy utilization, immune function, and metabolism of bone, minerals, and lipids. Boron deficiency (<0.04 mg B/kg ration of dams) impairs early embryonic development in rodents these effects were not observed at 2.0 mg B/kg ration. Boron deprivation in animals and humans results in decreased brain electrical activity similar to that observed in nonspecific malnutrition, and reduced cognitive and psychomotor function. Learning performance (manual dexterity, eye-hand coordination, memory, attention, perception) in humans was... 

Urinary excretion (Table 3) of phosphate is dependent on (1) the amount that is filtered through the glomeruli and the fraction thereof that is reabsorbed in the (proximal) tubular system, (2) the intake of phosphate by food, and (3) the metabolism of bone. 

Metabolism of Bone. The huge reservoir of calcium contained in the bones plays a unique role in the metabolism of calcium. More than 1 kg is bound there, whereas the daily intake is only around 1 gm. Bound calcium nevertheless performs a vital task Ca++ salts essential for the supportive tissue are mainly the minerals hydroxyapatite and carbonate apatite, and secondarily apatites with anions of organic acids. Their structure is illustrated in the diagram below. 

Little is known about the biochemical mechanism of vitamin D action. It promotes the absorption of Ca++-ions in the gastro-intestinal tract and influences the metabolism of bone tissue. Deficiency of vitamin D results in the clinical syndrome rachitis (also in animal experiments), characterized by a softenii (inadequate calcification) of the bones. Excessive administration of vitamin D produces a hyper-vitaminosis, during which calcium is again mobilized out of the bone structure this hypervitaminosis resembles the action of the parathyroid hormone. 

Metabolic Functions. Bones act as a reservoir of certain ions, in particular Ca " and which readily exchange between bones and blood. 

Bone metabolism comprises the processes of bone formation and bone resorption, the key actions by which skeletal mass, structure and quality are accrued and maintained throughout life. In the mature skeleton, anabolic and catabolic actions are mostly balanced due to the tight regulation of the activity of bone forming ( osteoblast) and bone resorbing ( osteoclast) cells through circulating osteotropic hormones and locally active cytokines. 

Boskey A (2006) Mineralization, Structure and Function of Bone. In Dynamics of Bone and Cartilage Metabolism, 2nd edn. Seibel MJ, Robins SP, Bilezikian JP (eds), San Diego, Academic Press,201-212... 

Systemic regulators of osteoblast, osteocyte and osteoclast functions, and therefore of bone metabolism. The major bone-seeking hormones are parathyroid hormone (PIH), 1,25-dihydroxy vitamin D3 (calcitriol) and the various ex hormones. 

A major regulator of bone metabolism and calcium homeostasis, parathyroid hormone (PTH) is stimulated through a decrease in plasma ionised calcium and increases plasma calcium by activating osteoclasts. PTH also increases renal tubular calcium re-absorption as well as intestinal calcium absorption. Synthetic PTH (1-34) has been successfully used for the treatment of osteoporosis, where it leads to substantial increases in bone density and a 60-70% reduction in vertebral fractures. 

Recognition among bone-chemistry researchers that strontium enters bone in proportion to dietary levels has resulted in widely accepted yet erroneous inferences about the relationships among various elements in bone and past diet. One such inference is that more of any element in the diet translates directly to more of that element in bone. If an element is not biogenically incorporated within bone, or if biological levels are metabolically controlled, then that element will not reflect diet. A second erroneous inference is that strontium can be used to measure the dietary plant/meat ratio. Sr/Ca ratios in meat are generally lower than those of plants, but meat is also low in calcium and hence has little effect on the composition of bone. Plants, on the other hand, contribute substantially to bone composition. Variations in the strontium levels of bone thus more likely reflect differential consumption of plants rather than trophic position. Although efforts to determine plant/meat ratios from strontium and to draw dietary inferences from elements other than strontium and barium have not been successful, this failure has been due to inappropriate expectations, not to a failure of bone strontium to reflect diet. 

This paper will address three subjects a) carbon isotopic composition of bone carbonate b) the possible role of hpid metabolism in determining frae-tionation between bone carbonate and collagen and its apparent trophic signature and c) concepts of routing of carbon isotopes in the body, especially from protein foods to collagen. The discussion will be limited to carbon isotope ratios C C/ C), although we appreciate that N/ N ratios, which are also very useful in determining trophic levels and somces of protein, present similar biochemical problems. 

On the other hand, the scrambled model of carbon sourcing does not seem to be applicable when we consider the metabolic fate of fatty acids. We find that there are partial barriers to the movement of FA-derived carbon atoms into the synthesis of proteins. This partial restriction leads us to expect a trophic level effect in the fractionation between collagen and bone apatite or respired CO2 of which apatitic carbonate is a sample. The magnitude of the fractionation depends on two separate fractionation factors which cannot be disentangled by analyses of bone samples alone. 

Many factors are involved in the regulation of bone metabolism, only a few of which will be mentioned here. Some stimulate osteoblasts (eg, parathytoid hormone and 1,25-dihydroxycholecalciferol) and others inhibit them (eg, corticosteroids). Parathyroid hormone and 1,25-dihydroxycholecalciferol also stimulate osteoclasts, whereas calcitonin and estrogens inhibit them. 

Transferrin (Tf) is a Pj-globulin with a molecular mass of approximately 76 kDa. it is a glycoprotein and is synthesized in the liver. About 20 polymorphic forms of transferrin have been found, it plays a central role in the body s metabolism of iron because it transports iron (2 mol of Fe + per mole of Tf) in the circulation to sites where iron is required, eg, from the gut to the bone marrow and other organs. Approximately 200 billion red blood cells (about 20 mL) are catabolized per day, releasing about 25 mg of iron into the body—most of which will be transported by transferrin. 

The geometry and structure of a bone consist of a mineralised tissue populated with cells. This bone tissue has two distinct structural forms dense cortical and lattice-like cancellous bone, see Figure 7.2(a). Cortical bone is a nearly transversely isotropic material, made up of osteons, longitudinal cylinders of bone centred around blood vessels. Cancellous bone is an orthotropic material, with a porous architecture formed by individual struts or trabeculae. This high surface area structure represents only 20 per cent of the skeletal mass but has 50 per cent of the metabolic activity. The density of cancellous bone varies significantly, and its mechanical behaviour is influenced by density and architecture. The elastic modulus and strength of both tissue structures are functions of the apparent density. 

KHALIL D A, LUCAS E A, lUMA S, SMITH B J, PAYTON M E and ARJMANDI B H (2002) Soy prOteiu supplementation increases serum insulin-like growth factor-1 in young and old men but does not affect markers of bone metabolism. J Nutr 132, 2605-8. 

LUCAS E A, WILD R D, HAMMOND L J, KHALIL D A, JUMA S, DAGGY B P, STOECKER B J and ARJMANDI B H (2002) Flaxseed improves lipid profile without altering biomarkers of bone metabolism in postmenopausal women. J Clin Endocrinol Metab. 87 (4) 1527-32. 

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