Water-soluble vitamins II

Vitamin Be consists of three substituted pyr-idines—pyridoxal, pyridoxoi, and pyrid- 

The active form of vitamin Be, pyridoxai phosphate, is the most important coenzyme in the amino acid metabolism (see p. 106). Almost all conversion reactions involving amino acids require pyridoxal phosphate, including transaminations, decarboxylations, dehydrogenations, etc. Glycogen phosphory-lase, the enzyme for glycogen degradation, also contains pyridoxal phosphate as a cofactor. Vitamin Be deficiency is rare. 

Vitamin B12 (cobalamine) is one of the most complex low-molecular-weight substances occurring in nature. The core of the molecule consists of a tetrapyrrol system (corrin), with cobalt as the central atom (see p. 108). The vitamin is exclusively synthesized by microorganisms. It is abundant in liver, meat, eggs, and milk, but not in plant products. As the intestinal flora synthesize vitamin B12, strict vegetarians usually also have an adequate supply of the vitamin. 

Cobalamine can only be resorbed in the small intestine when the gastric mucosa secretes what is known as intrinsic factor—a glycoprotein that binds cobalamine (the extrinsic factor) and thereby protects it from degradation. In the blood, the vitamin is bound to a special protein known as trans-cobalamin. The liver is able to store vitamin Bi2 in amounts suf cient to last for several months. Vitamin B12 deficiency is usually due to an absence of intrinsic factor and the resulting resorption disturbance. This leads to a disturbance in blood formation known as pernicious anemia. 

In animal metabolism, derivatives of cobalamine are mainly involved in rearrangement reactions. For example, they act as coenzymes in the conversion of methylmalonyl-CoA to succinyl-CoA (see p. 166), and in the formation of methionine from homocysteine (see p. 418). In prokaryotes, cobalamine derivatives also play a part in the reduction of ribonucleotides. 

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A. Amin, High-performance liquid chromatography of water-soluble vitamins. II. Simultaneous determination of vitamins B-l, B-2, B-6, and B-12 in pharmaceutical preparations, J. Chromatogr., 590 448 (1987). 

Water-soluble vitamins II pyridoxal phosphate (Be) Vitamins 117... 

II Water-soluble vitamins One index vitamin folic acid (if present)... 

II. Water soluble vitamins, includes B-complex group and vitamin C. 

The antioxidant activity of alizarin was established in four different assays (1) suppression of light emission in the p-iodophenol enhanced chemiluminescent assay, (2) scavenging of superoxide anion (02 -) in a hypoxanthine-xanthine oxidase system, (3) protection of rat liver microsomes from lipid peroxidation by ADP/iron(II) ions, and (4) protection of bromobenzene-intoxicated mice from liver injury in vivo [141]. Alizarin was compared with Trolox (water soluble vitamin E), the flavonoid baicalin and green tea proanthocyanidins. In assay (1) the activity of alizarin was 76% of that of Trolox. In assay (2) the inhibition of 02 -induced chemiluminescence was 40%, 32%, 23% and 14% for Trolox, alizarin, green tea polyphenols and baicalin respectively. Alizarin was not significantly active in the lipid peroxidation assay but after baicalin the most active compound in the in vivo assay. This shows again the difficulty in the evaluation of antioxidant activity and the differences between in vitro and in vivo assays [141]. 

The diffusion studies described in the above sections pertain to water-continuous and bicontinuous microemulsions. Chen and Georges [34] were the first to study diffusion in oil-continuous microemulsions using steady-state microelectrode voltammetry. Ferrocene was used to probe diffusion in an SDS-dodecane-1-heptanol-water system. The diffusion coefficient of the hydrophobic probe indicated the microviscosity of the oil rather than the bulk viscosity of the microemulsion. Owlia et al. [36] reported diffusion coefficient measurements of water droplets in an Aerosol OT [AOT, bis(2-ethylhexyl)sulfosuccinate] microemulsion using a microelectrode. Water-soluble cobalt(II) corrin complex (vitamin Bi2r) was used in an oil-continuous microemulsion containing 0.2 M AOT, 4 M water buffered at pH 3, and isooctane. The apparent diffusion coefficient decreased with the probe concentration in accordance with Eq. (13) as shown in Fig. 6 [36]. The water droplet size was... 

Plaut, G.W.E. Water soluble vitamins, part II (Folic acid, riboflavin, thiamine, vitamin Bi2)- Annu. Rev. Biochem. 30, 409-446 (1961)... 

With other quinones, the only olefin yielding stable complexes is 1,5-cyclooctadiene. The quinones employed have been trimethyl-p-benzoquinone, 2,5- and 2,6-dimethyl-p-benzoquinone (531), and vitamin E quinone (530). In general, these complexes show higher water solubility, higher dipole moments, and more marked paramagnetism than do the duroquinone complexes. The paramagnetism suggests that there is some electron transfer from nickel to quinone and that the nickel may indeed have an oxidation state midway between Ni(0) and Ni(II). 

Niacin is the antipellagra vitamin, although in achieving its effects consideration must be given to the presence in the diet of the amino acid, tryptophan. Niacin is water-soluble. In the body it is a component of coenzyme I and of coenzyme II, both of which are concerned with glycolysis and cell respiration. None of these reactions has been clearly related to pellagra, which presents a clinical picture of diarrhea, dermatitis, and sometimes dementia. 

The parent compound in the vitamin A group is called sdl-trans retinol (Fig. lA) [4]. Its aldehyde and acid forms are retinal (Fig. IB) and retinoic acid (Fig. 1C). The active form of vitamin A in vision is ll-cis retinal (Fig. ID), and a therapeutically useful form (accutane, isotretinoin) is l3-cis retinoic acid (Fig. IE). Retinyl palmitate (Fig. IF) is a major storage form, and retinoyl p-glucuronide is a biologically active, relatively non-toxic water-soluble metabolite (Fig. IG). A synthetic aromatic analog (etretin, acitretin), shows therapeutic usefulness (Fig. IH). Finally, p-carotene, a major provitamin A carotenoid, is shown in Figure II. 

Absorption of Lipides.—In order to be absorbed, simple and complex lipides must first undergo hydrolysis to aliphatic acids. These acids then combine with the bile salts to form water-soluble, diffusible complexes, which pass into the intestinal mucosa, where they interact with glycerophosphoric acid to regenerate the neutral fat, and thus enter the lymphatics. Hence, three factors are concerned in fat absorption (i.) the enzyme lipase and its activators (ii.) the bile salts, which act as carriers (iii.) the phosphorylation mechanism in the mucosa, which in turn, requires vitamin Bj (p. 257) and the hormone of the adrenal cortex (p. 415), as demonstrated by Verzar. This elaborate mechanism endows the organism with considerable power of discrimination in the absorption of lipides and lipoids, as shown by the preferential absorption of carotene. 

For most inorganic chemists the peripheral groups of the cobalamins, which play important roles in the enzymology of the natural systems, only add to the overall complexity of the molecule, as well as impose severe restrictions on their solubility, polarity, and volatility. All these properties can be dramatically modified by converting vitamin Bi2 into dicyanocobyrinic acid heptamethyl ester (II, R = R = —CN), which is exceedingly soluble in most organic solvents (compared to Bt2 which has a low solubility only in very polar solvent such as water, DMF, and DMSO) and is sufficiently volatile that, with care, useful mass spectra can be obtained using a direct insertion probe. Like B12 and the cobalox-... 

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