Vitamin physiological role

The most important physiological role of CODH in the metabolism of acetogenic bacteria was unknown until 1985, when it was shown that the enzyme is bifunctional and has acetyl-CoA synthase activity (121). It was previously thought that acetyl-CoA was synthesized at the cobalt center of a vitamin-Bi2-containing protein. In the same paper, it was proposed that nickel is the active site of CO oxidation and acetyl-CoA synthesis. 

Compounds showing vitamin K activity are substituted naphthoquinones. The parent compound, 2-methyl-1,4-naphthoquinone, does show some biological activity as do other similar but synthetic compounds. The production of the complete naturally active forms is thought to depend upon the addition of an isoprene chain at position 3 on the aromatic ring. Differences in this side chain produce the various K vitamins (Figure 12.10). A most important physiological role of vitamin K is in the synthesis of the blood clotting factors, II (prothrombin), VII, IX and X. 

The physiological role of vitamin K is in blood clotting and is essential for the synthesis of at least four of the proteins (including prothrombin) involved in this process. Vitamin K also plays a role in the synthesis of a protein (osteocalcin) in bone. Vitamin K deficiency is rare but can result from impaired absorption of fat. Vitamin K levels in the body are also reduced if the intestinal flora is killed (e.g. by antibiotics). Vitamin K toxicity is rare but can be caused by excessive intake of vitamin K supplements. Symptoms include erythrocyte haemolysis, jaundice, brain damage and reduced effectiveness of anticoagulants. 

Some elements found in hody tissues have no apparent physiological role, hut have not been shown in he toxic. Examples arc rubidium, strontium, titanium, niobium, germanium, and lanthanum. Other elements are toxic when found in greater than trace amounts, and sometimes in trace amounts. These taller elements include arsenic, mercury, lead, cadmium, silver, zirconium, beryllium, and thallium. Numerous irlhcr elements are used in medicine in non-nulrieni roles. These include lithium, bismuth, antimony, bromine, platinum, and gold. The interactions of mineral nutrients with carbohydrates, fats, and proteins, minerals with vitamins (qv). and mineral nutrients with toxic elements are areas of active investigation. 

The principal physiological role of vitamin D is in the maintenance of the plasma concentration of calcium. Calcitriol acts to increase intestinal absorption of calcium, to reduce its excretion by increasing reabsorption in the distal renal tubule, and to mobilize the mineral from bone - of the 25 mol of calcium in the adult body, 99% is in bone. The daily intake of calcium is around 25 mmol, and intestinal secretions add an additional 7 mmol to the intestinal contents 10 to 14 mmol of this is normally absorbed, with 18 to 22 mmol excreted in feces. Bone turnover accounts for exchange of 10 mmol of calcium between bone and plasma daily. The kidneys filter some 240 mmol of calcium daily, almost all of which is reabsorbed urinary excretion of calcium is about 3 to 7 mmol per day. 

The coenzyme B12 molecule consists of several unique stractural units the corrin ligand, the intramolecularly coordinating nucleotide function, the corrin-bound Co -ion, and the organometallically bound 5 -deoxyadenosyl ligand (Figure 3). The cyanide derivative, vitamin B12 (1), is an isolation artifact that appears not to have a physiological role itself ... 

Luminal thiol oxidation is facilitated by ascorbate (vitamin C) (45) or FAD (12, 13), so the physiologic role of their transport has been proposed. ER membrane is permeable selectively to dehydroascorbate, the oxidized form of ascorbate (10, 11). Luminal reduction of dehydroascorbate to ascorbate is associated with thiol oxidation and leads to ascorbate entrapment (46). 

Xhe brilliant success of ascorbic acid research in the 1930s led to the commercial production of inexpensive ascorbic acid in large quantities. The wide distribution of ascorbic acid, and its incorporation into many food products, so completely solved the problem of scurvy in both general and special populations that pressure for a more complete scientific understanding of this vitamin was sharply reduced. As a result many questions concerning ascorbic acid s chemistry, biochemistry, physiological roles and kinetics, and its nutritional requirements were deferred for more pressing scientific problems. 

The physiological role of vitamin D is to maintain calcium homeostasis. Phosphate metabolism is also affected. Vitamin D accomplishes its role by enhancing the absorption of calcium and phosphate from tte small intestines, promoting their mobilization from bone, and decreasing their excretion by the kidney. Also involved are parathyroid hormone and edeitonin. 

OC contains three specific glutamyl residues at amino acid positions 17, 21, and 24, which may be converted to y-carboxyglutamyl residues by a posttranslational, vitamin K-dependent enzymatic carboxylation. This unique car-boxylated amino acid binds calcium ions and is found in various proteins involved in blood coagulation and in calcium transport, deposition, and homeostasis. Undercar-boxylated OC, which has been reported in serum in some conditions, may be related to decreased bone density and may respond to administration of vitamin K. Although OC binds calcium and hydroxyapatite, its physiological role is unknown. 

Pantothenate kinase phosphorylates Vitamin B6, the first and rate controlling step in Coenzyme A biosynthesis. Due to the lack of pantothenate kinase inhibitors and activators, the exact physiological role of this enzyme in metabolism and disease is not yet known. In a recent study aimed at the discovery of antimicrobial agents, a set of novel inhibitors of S. aureus pantothenate kinase were disclosed that also inhibited mammalian (murine) pantothenate kinase with high micromolar IC50 values.235 A team at St. Jude Children s Hospital studied a library of known bioactive compounds to find inhibitors of pantothenate kinase. Several inhibitors with IC50 values below 10 pM were identified.236... 

Beyond its physiologic role in the intestinal digestion of lipids and lipid-soluble vitamins, the bUe also plays an important role in the excretion of xenobio-tics, including drugs and their metabolites. This includes a diverse array of compounds, both polar and lipophilic, including, anions, cations, and neutral molecules. In humans, the molecular threshold is approximately 500 to 600, with renal excretion being the primary route of excretion for smaller molecules. 

Many of the coenzymes are closely related to vitamins, as Table 11.5 shows. In the table, the portion of the coenzyme molecule that must be provided in the human diet as a vitamin is identified in blue. Keep in mind, though, that other vitamins (such as vitamin A for example) play essential physiological roles but are not associated with coenzymes. 

Recall List the fat-soluble vitamins, and give a physiological role for each. 

Dietary supplements may be grouped into three major categories related to dietary function or origin (1) substances with established nutritional function, such as vitamins and minerals, (2) other substances with a wide variety of origins and physiologic roles (comprising ergogenic supplements), and (3) botanical products and their concentrates and extracts. " ... 

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