Vitamin forms

CYP27A1 catalyzes the side chain oxidation (27-hydroxylation) in bile acid biosynthesis. Because bile acid synthesis is the only elimination pathway for cholesterol, mutations in the CYP27A1 gene lead to abnormal deposition of cholesterol and cholestanol in various tissues. This sterol storage disorder is known as cerebrotendinous xanthomatosis. CYP27B1 is the 1-alpha hydroxylase of vitamin D3 that converts it to the active vitamin form. The function of CYP27C1 is not yet known. 

Vitamin Bg and related compoimds (Figure 10.2) were quantitatively separated by preparative TLC on silica gel H. After elution, the pyridoxic acid lactone method was employed for fluorimetric determination of the concentration of the vitamin forms involved [8]. Table 10.2 shows Revalues obtained for various forms of vitamin Bg, using several solvent systems. The solvent selected, ethyl acetate/pyridine/water (2 1 2, v/v), gave excellent separation of pyridoxamine, pyridoxic acid, and pyri-doxine together with pyridoxal. 

Severe cyanocobalamin (vitamin B12) deficiency results in pernicious anemia that is characterized by megaloblastic anemia and neuropathies. The symptoms of this deficiency can be masked by high intake of folate. Vitamin B12 is recycled by an effective enterohep-atic circulation and thus has a very long half-hfe. Absorption of vitamin B12 from the gastrointestinal tract requires the presence of gastric intrinsic factor. This factor binds to the vitamin, forming a complex that... 

Dietary Deficiency of Niacin, the Vitamin Form of NAD and NADP, Causes Pellagra... 

Thiamine pyrophosphate (TPP) is the biologically active form of fre vitamin, formed by the transfer of a pyrophosphate group from ATP to thiamine (Figure 28.11). Thiamine pyrophosphate serves as a coen zyme in the formation or degradation of a-ketols by transketolase (Figure 28.12A), and in the oxidative decarboxylation of a-keto adds i (Figure 28.12B). 

Vitamins comprise a diverse group of organic chemicals that the body needs to facilitate specific metabolic and biosynthetic processes.4 In many cases, vitamins act as enzymatic cofactors that is, the vitamin works directly with the enzyme to catalyze a specific chemical reaction. In other situations, the vitamin forms an essential component of a chemical structure or species that is needed for a specific chemical reaction. The body typically needs small amounts of vitamins to promote normal growth and development, and to maintain optimal health throughout adulthood.32... 

In mammals, dietary vitamin forms, including pyridoxal, pyridoxol, and pyridoxamine, can all be converted to the respective coenzyme forms by phosphorylation. 

Foods may contain precursors of vitamin D as well as vitamin D. The precursors are converted to the vitamin following their absorption from the diet. T c body is a source of one of the precursors, 7-dehydnocholesteroL Conversion of the precursor to the vitamin form occurs in the skin. The major function of vitamin D is to maintain the concentration of calcium ions in the plasma. Maintenance of these concentrations is vital for normal functioning of the nervous system and for the growth of bones and the maintenance of bone mass. Although vitamin D is closely associated with bone growth, it is thought that the relationship is only indirect. T he vitamin stimulates the absorption of dietary calcium by the gut, but it seems not to stimulate the subsequent deposit of calcium in the bone. 

The oxidative product of ascorbic acid, dehydroascorbic acid, is the preferred form of the vitamin for uptake by neutrophils, erythrocytes, and lymphocytes (27). Once within the erythrocyte, dehydroascorbic acid is reduced to ascorbic acid by a glutathione-dependent, dehydro-ascorbic-acid-reducing enzyme (20,28). However, the reduced form of ascorbic acid is found in most other tissues, that is, liver, lungs, kidneys, skin, and pituitary and adrenal glands (20,29). From these studies, ascorbic acid is taken up by several tissues by an energy-dependent and Na -sensitive process, but the transport of the oxidized vitamin form follows the principles of diflFusion. 

The formulator of liquid multivitamin pharmaceutical products such as baby drops, syrups, elixirs, and injectables encounters numerous problems in attempting to develop products having adequate physical and chemical stability as well as suitable taste, odor, color, and freedom from bacterial contamination. Many of these problems arise from the diflFering solubility and stability characteristics of the individual vitamins, particularly as these relate to the pH of the solutions and potential interactions. Despite these numerous problems, various ways have been devised for producing multivitamin combinations in liquid form containing L-ascorbic acid that have acceptable stability characteristics. Successful development of such products requires a knowledge of (i) the fundamental aspects of the physical and chemical properties of the vitamin forms available (ii) the use of adequate techniques of manufacture and (iii) the employment of suitable overages based on critical stability studies. 

The term niacin refers to nicotinic acid (pyridine-3-carboxyhc acid), its amide nicotinamide, and derivatives that show the same biological activity as nicotinamide. A distinction between the two primary vitamin forms has to be considered, however, when considering some aspects of their metabolism and especially their different pharmacological actions at high doses. Structures of both vitamers and the two coenzyme forms containing the nicotinamide moiety are given in Figure 30-23. 

Which of the following water-soluble vitamins forms part of the structure of CoA ... 

There is only few published data concerning vitamin content and bioavailability of vitamin forms contained in edible seaweeds. Generally, seaweeds contain both water- and fat-soluble vitamins, B-complex vitamins, vitamin C, provitamin A, and vitamin E, but some of them only in relatively low content. 

Most of the water-soluble vitamins form parts of phosphate ester coenzymes as in Equation 11.108 and thus have a dual function (Section 11.2). They are found in a variety of plants, animal tissues and microorganisms. 

About the middle of the nineteenth century, the use of certain phosphorus compounds (rather than the element) became the vogue. The laxative properties of sodium phosphates were recognised and in the same period hypophosphites (as Na, Mg or Ca salts) were introduced for the treatment of neurasthenia and other nervous deficiencies. These were all considerably less toxic than the element itself and were followed by glycerophosphates (as Na, Ca and other salts) which were believed to function as brain nutrients or nerve tonics . The use of hypophosphites and glycerophosphates persisted well into the twentieth century, but they are now considered to be of very limited value for these purposes. A syrup of phosphorus , described in British Pharmacopea 1885, contained phosphoric acid, sodium phosphate and iron sulphate among other ingredients. Commercial exploitation of products of this kind was inevitable (Hgure 12.22). Modem dietary supplements are often based on collections of vitamins which usually include Bg and B,2. These two vitamins form part of phosphate ester enzymes (e.g. Equation ll.lOSj). 

The use of cyclodextrins in the cosmetic industry is very similarly widespread. Most vitamins form stable complexes with cyclodextrins. These complexes have been found to increase the bioavailability of compounds such as vitamin A and retinol. The added advantage is the protection of vifamin A and retinol against oxidation. Again, as with the food indusfry, emulsions are easily prepared. Cyclodextrins have also been incorporated into many sun-block lotions. Complex-ation increases the photostability of the cream, increases its protective properties, reduces odour and decreases the staining of fabrics. 

The purified human factor has a molecular weight of 60,000 and may exist in the form of a dimer (mol wt 120,000). The factor is a mucoprotein with great affinity for vitamin Bi2- The factor and the vitamin form a complex in the presence of calcium at pH values larger than 6. At least three forms of vitamin malabsorption resulting from the distorted elaboration and use of the intrinsic factor have been described [83-86]. These include congenital absence of intrinsic factors secretion of adequate quantities of immuno-... 

Efficient absorption of cobalamin requires that the vitamin form a macromolecular complex with the glycoprotein secreted by the stomach. In patients with pancreatic insufficiency, cobalamin absorption is poor, suggesting that pancreatic proteases are necessary for this process, although the mechanism responsible has not yet been identified. For efficient absorption, the distal small bowel must be intact, and in its absence, the absorption of cobalamin is... 

The choice of fhe LC mode for the analysis of water-soluble vitamins depends on the extraction procedure employed and the vitamin form to be quantified (Figure 18.1). The most popular LC modes are normal-phase (NP), reversed-phase (RP), ion-pair RP, ion-suppression RP, and ion-exchange chromatography. 

Several LC modes have been used for the separation of fat-soluble vitamins. The choice depends on the vitamin forms to be determined, the nature of the food matrix, and the sample treatment. Adsorption chromatography has two main advantages (a) geometric and positional isomers are generally resolved on silica stationary phases [86,87] (b) relatively high loads of lipoidal material can be tolerated by this type of column. The latter feature allows the direct injection of extracts obtained by means of direct liquid extraction [88] or sample dissolution in hexane (e.g., vitamin E from oils) [89], when the recourse to saponification is not essential for the analyte isolation. In these cases, fluorescence detection is... 

Milk is a very compKcated dispersed system (see Section 7.6.3.2.1) in which casein molecules form a micellar dispersion, globular whey proteins a colloidal dispersion, fat present in the form of fat droplets (mflk microsomes) forms an emulsion, particles of lipoproteins form a colloidal suspension and low molecular weight substances (lactose and other sugars, amino acids, minerals and water soluble vitamins) form true solutions. Milk s typical white colour is related to the scattering and absorption of light by the fat particles and casein micelles. The yellowish colour that is sometimes seen is caused by carotenoid compounds dissolved in the fat phase (cream and butter), while the greenish colour of whey is caused by riboflavin. 

The antioxidant activity of vitamin E in emulsions depends on the structure of the emulsions and the presence of other antioxidants, such as 3,5-di-tert-butyl-4-hydroxytoluene (BHT) and ascorbyl pahnitate. Temperature plays an important part, as does, particularly, the presence of oxygen and the stability of the radicals of tocopherols produced as intermediates in reactions with oxidised lipids. At 80 °C in the presence of air, 5-tocopherol, for example, is the only vitamin form which partially withstands heating for 6 h, when used as an antioxidant to protect linoleic acid against autoxidation. In an atmosphere containing only 10% oxygen (hah of the amount of oxygen in air), P- and y-tocopherols are also present, but a-tocopherol and aU tocotrienols are absent. At 60 °C in the absence of oxygen, all tocopherols and tocotrienols are present. 

Raw meat contains pyridoxal phosphate as the main vitamin form, whereas cooked meat predominantly contains a product of its transamination pyridoxamine phosphate. For example, the main form of vitamin Bg in raw chicken meat is pyridoxal phosphate (56%), followed by pyridoxamine phosphate (42%) and pyridoxamine (2%). The main form in roasted chicken meat is pyridoxamine phosphate (70%), followed by pyridoxal phosphate (21%), pyridoxol (7%) and pyridoxamine (2%). Vitamin retention in roasted meat is about 45-65%. 

Sodium ascorbate and ascorbic acid esters, such as ascorbyl 6-palmitate (5-111) and ascorbyl 2-phosphate (5-113), are fully bioavailable, while ascorbyl 2-sulfate (5-114) is a completely inactive vitamin form. Phosphate and sulfate are about 20 times more stable to oxidation than the free acid. D-lsoascorbic acid (5-107) shows only 5-20% activity, 6-deoxy-L-ascorbic acid (5-115), found in fungi, has about 30% activity and the bound ascorbic acid form ascorbigen has 15-20% of the activity of ascorbic acid. Ascorbic acid 2-0-P-D-glucoside, systematic name 2-0-(P-D-glucopyranosyl)-L-ascorbic acid (5-116), has the same biological activity as ascorbic acid and is also stable against oxidation. 

Folic acid was initially distinguished from vitamin Bi2 as a dietary anti-anemia factor by Wills in the 1930s. The subsequent chemical isolation of folic acid and the identification of its role as a cofactor in one-carbon metabolism led to the elucidation of deficiency diseases at the molecular level. The term folate encompasses the entire group of folate vitamin forms, comprising the naturally occurring... 

---