Form of the vitamin

Infants maybe sensitive to doses of vitamin A [11103-57-4] in the range of 75,000—200,000 lU (22.5—60 mg), although the toxic dose in adults is probably 2—5 million lU (90.6—1.5 g). Intakes in this range from normal food suppHes without oral supplements are simply beyond imagination (79). Vitamin D [1406-16-2] toxicity is much more difficult to substantiate clinically. Humans can synthesize active forms of the vitamin in the skin upon irradiation of 7-dehydrocholesterol. Toxic symptoms are relatively nonspecific, and dangerous doses seem to He in the range of 1000—3000 lU/kg body wt (25—75 flg/kg body wt) (80). Cases of toxicity of both vitamins E and K have been reported, but under ordinary circumstances these vitamins are considered relatively innocuous (81). 

Nicotinamide is incorporated into NAD and nicotinamide is the primary ckculating form of the vitamin. NAD has two degradative routes by pyrophosphatase to form AMP and nicotinamide mononucleotide and by hydrolysis to yield nicotinamide adenosine diphosphate ribose. 

Vitamin A acetate [11098-51-4] (2) is the commercially significant form of the vitamin and is mainly produced by Hoffmann-La Roche, BASF, and Rhc ne-Poulenc (Fig. 4). AH of these processes have P ionone (18) as their key intermediate and in this regard are based on work performed in the 1940s... 

Vitamin A palmitate [79-81-2] (3), a commercially important form of the vitamin, is produced from vitamin A acetate (2) via a transesterification reaction with methyl palmitate. En2ymatic preparation of the palmitate from the acetate has also been described (22). 

As described ia the USP, phytoaadioae is a mixture of the cis- and trans-isomers of vitamin IQ. This mixture should not contain mote than 103% and not less than 97.0% of total vitamin K content. The amount of the cis-isomer is also specified and is not to exceed 21%. In addition to the pure substance, the USP also describes methods for the analysis of parental as well as tableted forms of the vitamin (21). 

Vitamin B2 or riboflavin is chemically defined as 7,8-dimethyl-10-(lY-D-tibityl)isoalloxazine. Figure 1 shows the oxidized and reduced form of the vitamin. The ending flavin (from the latin word flavus—yellow) refers to its yellowish color. 

The B vitamin niacin is also known as nicotinic acid. The body converts various forms of the vitamin into forms it can use. 

Figure 45-13. Vitamin B,2(cobalamin). R may be varied to give the various forms of the vitamin, eg,...

Liver tissue B12 is assayed by suspending 50 mg of lyophilized human liver in 100 ml of aconitate buffer to which 100 mg sodium metabisulfite is added. The suspension is autoclaved for 30 minutes at 16 psi to liberate the bound forms of the vitamin. Aliquots are diluted 1 10 after autoclaving to reduce metabisulfite toxicity on microorganism growth. Each milliliter now contains 50 pg of liver (w/v) 1.0, 1.5, and 2.0 ml of this hydrolyzate is added. After dilution to a volume of 5 ml, this represents an assay of 10, 15, and 20 ig of liver per milliliter of final solution. Normal liver contains between 2-14 mpg of vitamin B /mg liver powder. 

There are several factors which underlie what might seem to an impartial observer to be an extremely backward situation. These factors, in the author s opinion, are (1) inadequacy of provisions for clinical research in general, (2) minor difficulty because of the existence in nature of more than one form of the vitamin, and (3) serious difficulty in establishing human needs because these needs are probably highly variable from individual to individual. 

The principal circulating form of vitamin D is 25-hydroxy vitamin D and it is this form of the vitamin that is measured in efforts to determine your vitamin D status. 

Vitamin Bg is a mixture of six interrelated forms pyridoxine (or pyridoxol) (Figure 19.23), pyri-doxal, pyridoxamine, and their 5 -phosphates derivatives. Interconversion is possible between all forms. The active form of the vitamin is pyridoxal phosphate, which is a coenzyme correlated with the function of more than 60 enzymes involved in transamination, deamination, decarboxylation, or desulfuration reactions. 

Pyridoxine is present in food in the free form and as a glucoside, which may undergo partial hydrolysis in the gut lumen, or may be absorbed intact. Although pyridoxine is associated with the enzyme glycogen phosphorylase in muscles, it is not released in response to a dietary deficiency therefore it cannot be regarded as a storage form of the vitamin. 

Vitamin B12 consists of a porphyrin-like ring with a central cobalt atom attached to a nucleotide. Various organic groups may be covalently bound to the cobalt atom, forming different cobalamins. Deoxyadenosylcobalamin and methylcobalamin are the active forms of the vitamin in humans. Cyanocobalamin and hydroxocobalamin (both available for therapeutic use) and other cobalamins found in food sources are converted to the active forms. The ultimate source of vitamin Bi2 is from microbial synthesis the vitamin is not synthesized by animals or plants. The chief dietary source of vitamin Bi2 is microbially derived vitamin B12 in meat (especially liver), eggs, and dairy products. Vitamin Bi2 is sometimes called extrinsic factor to differentiate it from intrinsic factor, a protein normally secreted by the stomach that is required for gastrointestinal uptake of dietary vitamin B12. 

Undoubtedly this vitamin Bi2 coenzyme is the naturally occurring form of the vitamin, and apparently in the previous isolations of the vitamin the nucleotide was removed and replaced by cyanide. The nature of the attachment of the adenosine, in the form of a carbanion, to the cobalt, is most unusual. It might have been expected that the cobalt would be more likely to bind to one of the nitrogen atoms on the adenine. The oxidation state of the cobalt, which is +3 in the cyanide form of the vitamin (13), appears at present to be unknown in the coenzyme (7), although magnetic evidence suggests that it is +2 (6). 

It is of considerable interest to understand under what conditions such an unusual organometallic bond is produced. Not surprisingly, such a bond does not result from the addition of adenosine to the vitamin. It has been demonstrated that the formation of the bond, starting with the hydroxo form of the vitamin, requires ATP (7,51) (and a metal ion), and that the triphosphate is split from the ATP in two steps (7) (Figure 11). In the first step the ATP loses orthophosphate, yielding ADP, which probably becomes bound to the cobalt through the pyro-... 

L-dehydroascorbic acid, are active forms of the vitamin. However, once the dehydro form is further oxidized to diketogiuconic acid, no vitamin C activity is retained. The thermal half life of dehydroascorbic acid is less than 1 minute at lOO C (pH 6), and 2 minutes at 7O C Vitamin C is also... 

Vitamin B12 consists of a porphyrin-like ring structure, with an atom of Co chelated at its centre, linked to a nucleotide base, ribose and phosphoric acid (6.34). A number of different groups can be attached to the free ligand site on the cobalt. Cyanocobalamin has -CN at this position and is the commercial and therapeutic form of the vitamin, although the principal dietary forms of B12 are 5 -deoxyadenosylcobalamin (with 5 -deoxyadeno-sine at the R position), methylcobalamin (-CH3) and hydroxocobalamin (-OH). Vitamin B12 acts as a co-factor for methionine synthetase and methylmalonyl CoA mutase. The former enzyme catalyses the transfer of the methyl group of 5-methyl-H4 folate to cobalamin and thence to homocysteine, forming methionine. Methylmalonyl CoA mutase catalyses the conversion of methylmalonyl CoA to succinyl CoA in the mitochondrion. 

Formation of 1,25-diOH D3 Vitamins D2 and D3 are not biologically active, but are converted in vivo to the active form of the D vitamin by two sequential hydroxylation reactions (Figure 28.23). The first hydroxylation occurs at the 25-position, and is catalyzed by a specific hydroxylase in the liver. The product of the reaction, 25-hydroxycholecalciferol (25-OH D3), is the predominant form of vitamin D in the plasma and the major storage form of the vitamin. 25-OH D3 is further hydroxylated at the one position by a specific 25-hydroxycholecalciferol 1 -hydroxylase found primarily in the kidney, resulting in the formation of 1,25-dihydroxycholecalciferol j (1,25-diOH D3). [Note This hydroxylase, as well as the iver 25-hydroxylase, employ cytochrome P450, molecular oxygen, and NADPH.]... 

Renal rickets (renal osteodystrophy) This disorder results from chronic renal failure and, thus, the decreased ability to form the active form of the vitamin. 1,25-diOH cholecalciferol (calcitriol) administration is effective replacement therapy. 

Which one of the following statements concerning vitamin D is correct A. Chronic renal failure requires the oral administra tion of 1,25-dihydroxycholecalciferol. B. It is required in the diet of individuals exposed to sunlight. C. 25-Hydroxycholecalciferol is the active form of the vitamin. D. Vitamin D opposes the effect of parathyroid hor mone. E. A deficiency in vitamin D results in an increased secretion of calcitonin. Correct answer = A. Renal failure results in the decreased ability to form the active form of the vitamin, which must be supplied. The vitamin is not required in individuals exposed to sunlight. 1,25-dihydroxycholecalciferol is the active form of the vitamin. Vitamin D and parathyroid hormone both increase serum calcium. A deficiency of vitamin D decreases the secretion of calcitonin. 

To replace losses, oxaloacetate can be synthesized from pyruvate and C02 in a reaction that uses ATP as an energy source. This is indicated by the heavy gray line leading downward to the right from pyruvate in Fig. 10-1 and at the top center of Fig. 10-6. This reaction depends upon yet another coenzyme, a bound form of the vitamin biotin. Pyruvate is formed from breakdown of carbohydrates such as glucose, and the need for oxaloacetate in the citric acid cycle makes the oxidation of fats in the human body dependent on the concurrent metabolism of carbohydrates. 

Nicotinic acid was prepared in 1867 by oxidation of nicotine. Although it was later isolated by Funk and independently by Suzuki in 1911-1912 from yeast and rice polishings, it was not recognized as a vitamin. Its biological significance was established in 1935 when nicotinamide was identified as a component of NAD+ by von Euler and associates and of NADP+ by Warburg and Christian.3 Both forms of the vitamin are stable, colorless compounds highly soluble in water. 

In most organisms reduced forms of the vitamin folic acid serve as carriers for one-carbon groups at three... 

A closely related and important family of chro-manols are the tocopherols or vitamins E (Fig. 15-24, Box 15-G). Tocopherols are plant products found primarily in plant oils and are essential to proper nutrition of humans and other animals. a-Tocopherol is the most abundant form of the vitamin E family smaller amounts of the P, 8, and y forms occur, as do a series of tocotrienols which contain unsaturated isoprenoid units.495 The configuration of a-tocopherol is 2R,4 R,8 R as indicated in Fig. 15-24. When a-toco-pherol is oxidized, e.g., with ferric chloride, the ring can be opened by hydrolysis to give tocopherolquinones (Fig. 15-24), which can in turn be reduced to tocopherol-hydroquinones. Large amounts of the tocopherolquinones have been found in chloroplasts. 

Cyanocobalamin, the form of vitamin B12 isolated initially, contains cyanide attached to cobalt. It occurs only in minor amounts, if at all, in nature but is generated through the addition of cyanide during the isolation. Hydroxocobalamin (vitamin B12a) containing OH in place of CN"does occur in nature. However, the predominant forms of the vitamin are the coenzymes in which an alkyl group replaces the CN of cyanocobalamin. 

The all-trams forms of the vitamins A predominate but 11-c/s-retinal is the lightabsorbing chromophore of the visual pigments... 

VITAMIN A. This substance also has been referred to as retinol, axerophthol, biosterol, vitamin Ai, anti-xerophthalmic vitamin, and anti-infective vitamin. The physiological forms of the vitamin include Retinol (vitamin A ) and esters 3-dehydroretinol (vitamin A2) and esters 3-dehydroretinal (retinme-2) retinoic acid neovitamin A neo-b-vitamin Ai. The vitamin is required by numerous animal species. All vertebrates and some invertebrates convert plant dietary carotenoids in gut to vitamin Ai. which is absorbed. Most animal species store appreciable amounts... 

The structure of vitamin Bi2 shown in Section 30-6B with a cyanide ion coordinated with cobalt is not the active form of the vitamin but is a particularly stable form, convenient to isolate and handle. The active form is a coenzyme that is remarkable in having a carbon-cobalt bond to an essentially alkyl-type carbon. The carbon-cobalt bond is to a 5 -deoxyadenosyl group, and if we abbreviate vitamin B12 coordinated to cyanide as 23, the coenzyme can be written, in the same style, as 24. (You will notice that 23 is an abbreviation of the formula of Section 30-6B turned 180°)... 

Certain human populations depend on dietary sources of vitamin D because of insufficient biosynthesis of the vitamin due to inadequate skin exposure to sunlight. The classic symptoms of vitamin D deficiency are rickets in children and osteomalacia in adults. 25-Hydroxyvitamin D3 is the major circulating metabolite in the blood, but the hormonally active form of the vitamin is 1,25-dihydroxyvitamin D3. The latter metabolite stimulates the intestine to absorb calcium and phosphate by two independent mechanisms and acts with parathyroid hormone to mobilize calcium, accompanied by phosphate, from the bone fluid compartment into the bloodstream. 1,25-dihydroxyvitamin D 3 is also involved in the formation of osteoclasts—giant cells that are solely responsible for the resorption of bone matrix (33). Resorption is an essential process for the development, growth, maintenance, and repair of bone. 

Reversed-phase chromatography also separates, isocratically, vitamin D2 or D3 from their respective previtamins and inactive isomers (207), but, unlike normal-phase chromatography, it can separate vitamin D2 from D3 using nonendcapped stationary phases (198). The 25-hydroxylated metabolites of vitamins D2 and D3 can be separated from one another using a Vydac 201 TP column (37). The separation of vitamin D2 from vitamin D3, and 25-hydroxyvitamin D2 from 25-hydroxy vitamin D3, allows the D2 form of the vitamin or its metabolite to be used as an internal standard for quantifying the corresponding D3 form. 

Phosphopantetheine coenzymes are the biochemically active forms of the vitamin pantothenic acid. In figure 10.11, 4 -phosphopantetheine is shown as covalently linked to an adenylyl group in coenzyme A or it can also be linked to a protein such as a serine hydroxyl group in acyl carrier protein (ACP). It is also found bonded to proteins that catalyze the activation and polymerization of amino acids to polypeptide antibiotics. Coenzyme A was discovered, purified, and structurally characterized by Fritz Lipmann and colleagues in work for which Lipmann was awarded the Nobel Prize in 1953. 

Saccharomyces cerevisiae). Vitamin D3 is not itself the active form of the vitamin, and in the body it is hydroxylated first to calcidiol and then to calcitriol (Figure 5.106). Colecalciferol and calcitriol have also been found in several plant species. 

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