Vitamin Protein synthesis

In addition to its role in regulating calcium homeostasis, vitamin D is required for the intestinal absorption of calcium. Synthesis of the intracellular calciumbinding protein, calbindin, required for calcium absorption, is induced by vitamin D, which also affects the permeability of the mucosal cells to calcium, an effect that is rapid and independent of protein synthesis. 

Vitamin K is an essential factor in the production of coagulation proteins within the liver. Elevated clotting times from decreased protein synthesis are indistinguishable from those produced by low vitamin K levels caused by malnutrition or poor intestinal absorption. Vitamin K (phytonadione) 10 mg subcutaneously daily for 3 days can help to establish whether the prolonged bleeding time results from loss of synthetic function in the liver or vitamin K deficiency. 

Many vitamins or their metabolic derivatives are essential coenzymes for multiple metabolic reactions of life others or their metabolic derivatives are transcription factors, regulating gene expression and, therefore, protein synthesis. 

The mechanism of action of the vitamin D metabolites remains under active investigation. However, calcitriol is well established as the most potent agent with respect to stimulation of intestinal calcium and phosphate transport and bone resorption. Calcitriol appears to act on the intestine both by induction of new protein synthesis (eg, calcium-binding protein and TRPV6, an intestinal calcium channel) and by modulation of calcium flux across the brush border and basolateral membranes by a means that does not require new protein synthesis. The molecular action of calcitriol on bone has received less attention. However, like PTH, calcitriol can induce RANK ligand in osteoblasts and proteins such as osteocalcin, which may regulate the mineralization process. The metabolites 25(OH)D and 24,25(OH)2D are far less... 

FIGURE 12-40 General mechanism by which steroid and thyroid hormones, retinoids, and vitamin D regulate gene expression. The details of transcription and protein synthesis are discussed in Chapters 26 and 27. At least some steroids also act through plasma membrane receptors by a completely different mechanism. 

Effect of vitamin D on bone 1,25-diOH D3 stimulates the mobi lization of calcium and phosphate from bone by a process that requires protein synthesis and the presence of PTH. The result is an increase in plasma calcium and phosphate. Thus, bone is an important reservoir of calcium that can be mobilized to maintain plasma levels. 

Inhibitors are substances that tend to decrease the rate of an enzyme-catalysed reaction. Although some act on the substrate, the discussion here will be restricted to those inhibitors which combine directly with the enzyme. Inhibitors have many uses, not only in the determination of the characteristics of enzymes, but also in aiding research into metabolic pathways where an inhibited enzyme will allow metabolites to build up so that they are present in detectable levels. Another important use is in the control of infection where drugs such as sulphanilamides competitively inhibit the synthesis of tetrahydrofolates which are vitamins essential to the growth of some bacteria. Many antibiotics are inhibitors of bacterial protein synthesis (e.g. tetracyclin) and cell-wall synthesis (e.g. penicillin). 

The water-soluble salt of vitamin K3 (menadione) should never be used in therapeutics. It is particularly ineffective in the treatment of warfarin overdosage. Vitamin K deficiency frequently occurs in hospitalized patients in intensive care units because of poor diet, parenteral nutrition, recent surgery, multiple antibiotic therapy, and uremia. Severe hepatic failure results in diminished protein synthesis and a hemorrhagic diathesis that is unresponsive to vitamin K. 

With the isolated perfused duodenum, there is a rapid increase in calcium transport in response to the addition of calcitriol to the perfusion medium. Isolated enterocytes and osteoblasts also show a rapid increase in calcium uptake in response to calcitriol. It is not associated with changes in mRNA or protein synthesis, but seems to be because of recruitment of membrane calcium transport proteins from intracellular vesicles to the cell surface. It is inhibited by the antimicrotubule compound colchicine. It can only be demonstrated in tissues from animals that are adequately supplied with vitamin D in vitamin D-deficient animals, the increase in intestinal calcium absorption occurs only more slowly, together with the induction of calbindin. 

Estimation of the vitamm Be requirements of infants presents a problem, and there is a clear need for further research. Human mUk, which must be assumed to be adequate for infant nutrition, provides only 2.5 to 3.5 //g of vitamin Be per g of protein-lower than the requirement for adults. Although their requirement for catabolism of amino acids may be lower than in adults (because they have net new protein synthesis), they must also increase their body content of the vitamin as they grow. Coburn (1994) noted that the requirement for growth in a number of animal species was less than that to maintain saturation of transaminases or rniriimum excretion of tryptophan metabolites after a test dose and was about 15 nmol per g of body weight gain across a range of species. 

Weimann BI and Hermann D (1999) Studies on wound healing effects of calcium D-pantothenate on the migration, proliferation and protein synthesis of human dermal fibroblasts in culture. International Journal of Vitamin and Nutrition Research 69, 113-19. 

Vitamin D, apart from its availability in the diet, is produced in the body, in response to sunlight striking the skin, and really acts like a hormone. It helps to raise the blood levels of calcium and phosphorus in several ways. It stimulates calcium and phosphorus absorption in the gastrointestinal tract promotes transfer of calcium and phosphorus from bone to blood promotes calcium retention by the kidney. As a hormone it acts 1 entering the cell nucleus and interacting with DNA to regulate protein synthesis. 

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