Vitamin B« metabolism

Of 50 adult rats used in a reproductive/developmental study, 22% of those that received 6 mg/kg/day heptachlor in the diet developed lens cataracts 4.5-9.5 months following exposure. In addition, 6-8% of the pi offspring and 6% of the p2 offspring of these rats also developed cataracts 19-21 days after birth (Mestitzova 1967). The author of this study eliminated the possibility of a vitamin B deficiency or a recessive genetic trait as the cause of the cataracts. She could not rule out the possibility of altered vitamin B metabolism caused by heptachlor. 

In experimental animals, however, chronic dosing with isoniazid causes degeneration of the peripheral nerves. The biochemical basis for this involves interference with vitamin B metabolism. 

Other Changes in Vitamin B. Metabolism Associated with the Use of... 

L9. Luhby, A. L., Brin, M., Gordon, M., Davis, P., Murphy, M., and Spiegel, H., Vitamin B metabolism in users of oral contraceptive agents. I. Abnormal urinary xanthurenic acid excretion and its correction by pyridoxine. Amer. J. Clin. Nutr. 24, 684-693 (1971). 

Shultz, T. D. and Leklem, J. E., 1982, Effect of high dose ascorbic acid on vitamin B metabolism. Am. J. Clin. Nutr., 35 1400-1407. 

The generic descriptor vitamin includes six vitamers (see Figure 11.15) the alcohol pyridoxine, the aldehyde pyridoxal, the amine pyridoxamine and their 5 -phosphates. There is some confusion in the literature, because at one time pyridoxine was used as a generic descriptor, with pyridoxol as the specific name for the alcohol. The vitamers are metabolically interconvertible and have equal biological activity they are all converted in the body to the metabolically active form, pyridoxal phosphate. 4-Pyridoxic acid is a biologically inactive end-product of vitamin B metabolism. 

HPLC methods designed to separate and quantify the six major forms of vitamin Bg and 4-PA in biological material, are termed comprehensive methods in this review. These methods were usually applied in studies of human vitamin B metabolism or pharmacokinetics of pyridoxine supplementation. 

Figure 8 HPLC assay of enzymes involved in vitamin B metabolism. (A) Standard solution, containing PLP (226 nmol/L) and PL (147 nmol/L) (B) hemolysate (C-E) the same hemolysate to which PLP (C), PL (D), and PMP (E) were added to measure PLP phosphatase, PL kinase and PM (PN) 5 -phosphate oxidase activities, respectively. Peaks 1 = PLP 2 = PL. (From Ref. 123.)...

The availability of dideuterated PN and the application of GC-MS to B vitamer analysis is another major recent development which allows accurate assessment of vitamin B metabolism and turnover in experimental animals (156), humans (157), and tissue cultures (159). 

DB McCormick, AH Merril. Pyridoxamine (pyridoxine) 5 -phosphate oxidase. In GP Tryfiates, ed. Vitamin B metabolism in growth. Westport, CT Food Nutrition Press, 1980, pp 1-26. 

LR Solomon, RS Hillman. Vitamin B metabolism in idiopathic sideroblastic anaemia and related disorders. Br J Haematol 42 239-253, 1979. 

Metabolism and Mobilization. On entry of vitamin B 2 into the cell, considerable metaboHsm of the vitamin takes place. Co(III)cobalamin is reduced to Co(I)cobalamin, which is either methylated to form methylcobalamin or converted to adenosylcobalamin (coenzyme B>22)- The methylation requires methyl tetrahydrofolate. 

The water-soluble vitamins generally function as cofactors for metabolism enzymes such as those involved in the production of energy from carbohydrates and fats. Their members consist of vitamin C and vitamin B complex which include thiamine, riboflavin (vitamin B2), nicotinic acid, pyridoxine, pantothenic acid, folic acid, cobalamin (vitamin B12), inositol, and biotin. A number of recent publications have demonstrated that vitamin carriers can transport various types of water-soluble vitamins, but the carrier-mediated systems seem negligible for the membrane transport of fat-soluble vitamins such as vitamin A, D, E, and K. 

Vitamin Ba (pyridoxine, pyridoxal, pyridoxamine) like nicotinic acid is a pyridine derivative. Its phosphorylated form is the coenzyme in enzymes that decarboxylate amino acids, e.g., tyrosine, arginine, glycine, glutamic acid, and dihydroxyphenylalanine. Vitamin B participates as coenzyme in various transaminations. It also functions in the conversion of tryptophan to nicotinic acid and amide. It is generally concerned with protein metabolism, e.g., the vitamin B8 requirement is increased in rats during increased protein intake. Vitamin B6 is also involved in the formation of unsaturated fatty acids. 

ABC transporters involved in the uptake of siderophores, haem, and vitamin B]2 are widely conserved in bacteria and Archaea (see Figure 10). Very few species lack representatives of the siderophore family transporters. These species are mainly intracellular parasites whose metabolism is closely coupled to the metabolism of their hosts (e.g. mycoplasma), or bacteria with no need for iron (e.g. lactobacilli). In many cases, several systems of this transporter family can be detected in a single species, thus allowing the use of structurally different chelators. Most systems were exclusively identified by sequence data analysis, some were biochemically characterised, and their substrate specificity was determined. However, only very few systems have been studied in detail. At present, the best-characterised ABC transporters of this type are the fhuBCD and the btuCDF systems of E. coli, which might serve as model systems of the siderophore family. Therefore, in the following sections, this report will mainly focus on the components that mediate ferric hydroxamate uptake (fhu) and vitamin B12 uptake (htu). 

No specific antidote has been shown to be effective in treating 1,2-dibromoethane intoxication once absorption into the bloodstream has occurred (Ellenhorn and Barceloux 1988). Intravenous infusions of glucose may limit the hepatotoxicity of 1,2-dibromoethane (ERA 1989b). During the recovery phase, a diet rich in vitamin B and carbohydrates may limit liver damage (Dreisbach and Robertson 1987 Lawrence and Michaels 1984). Hemodialysis may be needed to regulate extracellular fluid and electrolyte balance and to remove metabolic waste products if renal failure occurs (ERA 1989b). 

The number of vitamin B 12-dependent reactions is not large. Most of these involve rearrangements of the carbon skeletons of metabolites. Such reactions are important in linking some aspects of fatty acid metabolism to the citric acid cycle. In another form, a vitamin Bi2-derived coenzyme is involved, along with folic acid coenzymes, in the metabolism of one-carbon fragments, including the biosynthesis of methionine. 

Jackson, W. B. Ashton, A. D. 8th Steenbock Symp., Vitamin K metabolism and Vitamin K-dependent proteins. University of Wisconsin, Madison, 1979... 

Cyanocoba/am/n- Cyanocobalamin is rapidly absorbed from IM and subcutaneous injection sites the plasma level peaks within 1 hour. Once absorbed, it is bound to plasma proteins, stored mainly in the liver and is slowly released when needed to carry out normal cellular metabolic functions. Within 48 hours after injection of 100 to 1,000 meg of vitamin B-12, 50% to 98%... 

The name vitamin B12 indicates a group of cobalt-containing corrinoids, also described as cobala-mins. Hydroxycobalamin (HOCbl), adenosylcobalamin (AdoCbl), and methylcobalamin (MeCbl) are the natural occurring forms. Instead, cyanocobalamin (Figure 19.20) is the commercially available form used for supplements and food fortification, thanks to its greater relative stability. Occasionally, sulfitocobalmin can occur in processed foods. Vitamin B,2 functions as a coenzyme and it is linked to human growth, cell development, and is involved in metabolism of certain amino acids. Vitamin B12 is present mainly in meat and diary foods, therefore a deficiency can occur in... 

Vitamins are usually classified as either fat soluble (vitamins A, D, E, and K) or water soluble (vitamins B and C). The fat-soluble vitamins are generally metabolized slowly and are stored in the liver. In contrast, the water-soluble vitamins are rapidly metabolized and are readily excreted in the urine. 

The rates of metabolism are also impaired in vitamin deficiency states (especially vitamin A, vitamin B, C and E). Starvation in mice leads to decrease in the rates of metabolism of certain drugs like pethidine, acetanilide, hexobarbitone etc. Ethanol increases the hepatic content of monooxygenase enzymes and cytochrome P450 on chronic ingestion. 

Thiamine, biotin and pyridoxine (vitamin B) coenzymes are grouped together because they catalyze similar phenomena, i.e., the removal of a carboxyl group, COOH, from a metabolite. However, each requires different specific circumstances. Thiamine coenzyme decarboxylates only alpha-keto acids, is frequently accompanied by dehydrogenation, and is mainly associated with carbohydrate metabolism. Biotin enzymes do not require the alpha-keto configuration, are readily reversible, and are concerned primarily with lipid metabolism. Pyridoxine coenzymes perform nonoxidative decarboxylation and are closely allied with amino acid metabolism. 

The most clearly documented role lor selenium is as a necessary component of glutathione peroxidase. Selenium is also involved in the functions of additional enzymes, e.g.. type I iodoihvronine deiodinase. leukocyte acid phosphatase, and glucuronidases. A role for selenium in electron transfer has been suggested as has involvement in nonheme iron proteins. Selenium and vitamin b appear to be necessary lor proper functioning of lysosomal membranes. A role for selenium in metabolism of thyroid hormone has been continued. 

VITAMIN B (Pyridoxine). Infrequently called adermine or pyridoxol, this vitamin participates in protein, carbohydrate, and lipid metabolism. The metabolically active form of B6 is pyridoxal phosphate, the structures of which are ... 

Vitamin B, [thiamin) Men 1.2 mg/d Women 1.1 mg/d Coenzyme in the metabolism of carbohydrates and certain amino acids prevents beriberi No adverse effects have been reported ... 

As shown in the review of the homocysteine metabolism, vitamin B 2, vitamin B6, and folate are important cofactors in the metabolic pathways for homocysteine elimination, and consequently, deficiencies of these vitamins are characterized by elevated plasma concentrations of tHcy. Hyperhomocysteinemia is also frequently found in diseases such as renal failure, rheumatic and auto-immune diseases, hypothyroidism, and malignancies. Several drugs are also known to increase plasma tHcy concentrations (16-24). 

Y. Nishikawa, B. Dmochowska, J. Madaj, M. Satake, P. L. Rinaldi, and V. M. Monnier, Impairment of vitamin C metabolism in STZ diabetic rats revealed with 6-deoxy-6-flu-oroascorbic acid, in G, 2002, 417 -18. 

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