Vitamin B12 sources and bioavailability

The usual dietary sources of vitamin B(12) are animal foods, meat, milk, egg, fish, and shellfish. As the intrinsic factor-mediated intestinal absorption system is estimated to be saturated at about 1.5-2.0 microg per meal under physiologic conditions, vitamin B(12) bioavailability significantly decreases with increasing intake of vitamin B(12) per meal. The bioavailability of vitamin B(12) in healthy humans from fish meat, sheep meat, and chicken meat averaged 42%, 56%-89%, and 61%-66%, respectively. Vitamin B(12) in eggs seems to be poorly absorbed (< 9%) relative to other animal food products. In the Dietary Reference Intakes in the United States and Japan, it is assumed that 50% of dietary vitamin B(12) is absorbed by healthy adults with normal gastro-intestinal function. Some plant foods, dried green and purple lavers (nori) contain substantial amounts of vitamin B(12), although other edible algae contained none or only traces of vitamin B(12). Most of the edible blue-green algae (cyanobacteria) used for human supplements predominantly contain pseudovitamin B(12), which is inactive in humans. The edible cyanobacteria are not suitable for use as vitamin B(12) sources, especially in vegans. Fortified breakfast cereals are a particularly valuable source of vitamin B(12) for vegans and elderly people. Production of some vitamin B(12)-enriched vegetables is also being devised.     

Vitamin B 12 plasma concentrations in Alzheimer disease

In this contribution we investigated the correlation between plasma vitamin B12 levels and cognitive impairment in Alzheimer's disease. We could demonstrate a significant inverse correlation when the MMSE scores of those patients with the lowest 10% Vitamin B12 plasma levels (<184 ng/ml) were compared with the upper 10% Vitamin B12 plasma levels (>598 ng/ml): p=0.008, Spearman-Rho= -0.36. MMSE in the upper percentile of plasma B12 levels was 20.0 +/- 4.6 and in the lower percentile 15.7 +/- 6.1, resulting in a difference of 4.3 MMSE points. We conclude, that vitamin B12 deficiency could aggravate or accelerate the course of Alzheimer disease as vitamin B12 possesses neuroprotective and anti-inflammatory properties.     

Vitamin B12 absorption from eggs.

The assimilation of 57Co B12 from in vivo labeled eggs was much inferior to that of a comparable amount of crystalline 57Co B12. Furthermore, the absorption varied with the form in which the eggs were served. Judged by the urinary excretion test and the plasma absorption of radioactivity the average absorption from boiled and fried eggs was more than twice that from scrambled whole eggs, but less than half that absorbed from crystalline 57Co B12.     

Vitamin B12 transport by rat liver lysosomal membrane vesicles

Vitamin B12 (hydroxycobalamin) is endocytosed by mammalian cells as a complex with transcobalamin II and then processed to free B12 in lysosomes. The mechanism by which free B12 becomes available for subsequent cellular metabolism has been uncertain. Lysosomal transport of cyanocobalamin (B12) was examined using membrane vesicles prepared from Percoll gradient purified lysosomes. B12 uptake by vesicles was dependent upon pH and was inhibited by the protonophore CCCP. Transport exhibited saturation kinetics with a Km of 3.5 microM and temperature dependence with a Q10 of 1.8. Uptake of B12 was dependent upon divalent cations and was inhibited by EDTA. Preparation of vesicles in the presence of 100 microM B12 resulted in stimulation of uptake consistent with a mechanism of countertransport. Excess cyanocobalamin, adenosylcobalamin, methylcobalamin, or cobinamide dicyanide inhibited uptake of B12. Trans-stimulation studies showed that only the first three compounds are actually transported species with cyanocobalamin as the preferred substrate. We conclude that lysosomes have a specific transport system for vitamin B12 that results in release of this enzyme cofactor to the cytoplasm.     

Vitamin B12b enhances the cytotoxicity of dithiothreitol

It has been found previously that vitamin B12b amplifies significantly the cytotoxic effects of ascorbic acid by catalyzing the formation of reactive oxygen species, and the antioxidant dithiothreitol (DTT), in contrast to catalase, does not prevent the cytotoxicity. Therefore, in this study we examined whether B12b is able to enhance the cytotoxicity of DTT. It was revealed that B12b strongly increases the cytotoxic effect of DTT. Vitamin B12b added to DTT catalyzed the generation and drastic accumulation of hydrogen peroxide in culture medium to a concentration of 260 microM within 7 min. The extracellular oxidative burst induced by the combination of B12b and DTT (DTT + B12b) was accompanied by intracellular oxidative stress, the destabilization of lysosomes, and damage to DNA. The accumulation of DNA lesions led to the initiation of apoptotic cell death, including the activation of caspase-3 and the release of cytochrome c. The antioxidants pyruvate and catalase completely prevented the DTT + B12b-induced oxidative stress and cell death. The iron chelators desferrioxamine and phenanthroline prevented the geno- and cytotoxic action of the combination although they did not reduce the exogenous oxidative burst, indicating a key role for intracellular iron in the cytotoxicity of the combination. Thus, vitamin B12b dramatically enhances the cytotoxicity of DTT, catalyzing the generation of hydrogen peroxide and inducing extra- and intracellular oxidative stress, early destabilization of lysosomes, and iron-dependent DNA damage.     

Vitamin B12-binding proteins of the horseshoe crab Limulus polyphemus

Vitamin B12-binding proteins were detected in the body fluids and/or tissues of horseshoe crabs (Limulus polyphemus), clams and sponges. Among the biological specimens tested the Limulus plasma was especially rich in free B12-binding proteins. Gel filtration experiments revealed that Limulus plasma contains two classes of B12-binding proteins. One class of proteins, molecular weight in excess of 100,000, bind B12 preferentially with affinity constant of 5 X 10(11)M-1. The second type of proteins, molecular weights around 50,000, bind B12 with specificity approaching that of mammalian intrinsic factors. The binding constant of these proteins for B12 is around 10(11)M-1.     

Transport of Vitamin B12 in Escherichia coli: Common Receptor Sites for Vitamin B12 and the E Colicins on the Outer Membrane of the Cell Envelope

The first step in the transport of cyanocobalamin (CN-B(12)) by cells of Escherichia coli was shown previously to consist of binding of the B(12) to specific receptor sites located on the outer membrane of the cell envelope. In this paper, evidence is presented that these B(12) receptor sites also function as the receptors for the E colicins, and that there is competition between B(12) and the E colicins for occupancy of these sites. The cell strains used were E. coli KBT001, a methionine/B(12) auxotroph, and B(12) transport mutants derived from strain KBT001. Colicins E1 and E3 inhibited binding of B(12) to the outer membrane B(12) receptor sites, and CN-B(12) protected cells against these colicins. Half-maximal protection was given by CN-B(12) concentrations in the range of 1 to 6 nM, depending upon the colicin concentration used. Colicin E1 competitively inhibited the binding of (57)Co-labeled CN-B(12) to isolated outer membrane particles. Functional colicin E receptor sites were found in cell envelopes from cells of only those strains that possessed intact B(12) receptors. Colicin K did not inhibit the binding of B(12) to the outer membrane receptor sites, and no evidence was found for any identity between the B(12) and colicin K receptors. However, both colicin K and colicin E1 inhibited the secondary phase of B(12) transport, which is believed to consist of the energy-coupled movement of B(12) across the inner membrane.     

Production of Vitamin B12 by Micromonospora

Various species of Micromonospora produced yields of vitamin B(12) activity as high as 11 mug/ml under conditions of shaken flask fermentation.     

Transport of Vitamin B12 in Escherichia coli: Genetic Studies

The chromosomal location of two genetic loci involved in the transport of cyanocobalamin (B(12)) in Escherichia coli K-12 was determined. One gene, btuA, is believed to code for the transport protein in the cytoplasmic membrane, because a mutant with an alteration in this gene has lost the ability to accumulate B(12) within the cell although normal levels of the surface receptors for B(12) are present. The other locus, btuB, apparently codes for the surface receptor on the outer membrane. These mutants have lost the ability to bind B(12) and have greatly reduced transport activity, although growth experiments have shown that they can utilize B(12) for growth, but with decreased efficiency. This surface receptor for B(12) also appears to function as the receptor for the E colicins, because btuB mutants are resistant to the E colicins, and mutants selected for resistance to colicin E1 are defective in B(12) binding and transport. The gene order was determined by transduction analysis to be cyc-argH-btuA-btuB-rif-purD. In addition, mutations in metH, the gene for the B(12)-dependent homocysteine methylating enzyme, were obtained in this study. This gene was localized between metA and malB.     

维生素B12的光解研究及发酵液中维生素B12检测新方法的建立

采用HPLC检测法研究5,脱氧腺苷钴胺素和甲基基钴胺素的水溶液在不同光源及不同照度下的光解情况,结果表明随着光能量的增加光解速度加快。利用脱氧腺苷钴胺素的光解性质,探索了一种检测发酵液中维生素B12含量的新方法。将含有钴胺素的细胞破碎液完全光解后,测定光解后的产物羟基钴胺素,以此来确定维生素B12产量。此方法简便快速、重复性好,可应用于发酵生产维生素B12的各个环节。     

大鹏湾维生素B1、B12与赤潮

海湾及近海水域 ,因某些浮游生物爆发性繁殖引起的赤潮异常现象时有发生 ,已构成水产资源的严重破坏。为弄清赤潮发生的机理 ,已有许多作者进行过研究 ,现已知水温、盐度、溶解氧、ｐＨ、Ｎ、Ｐ ,以及Ｆｅ、Ｍｎ、Ｓｅ等微量元素都是发生赤潮的重要环境因子[1 ]。近年来通过生态实验研究 ,发现维生素Ｂ1 、Ｂ1 2 (以下简称Ｂ1 、Ｂ1 2 )亦有促进赤潮生物生长繁殖的作用[2 ]。本文报道 1 991年对大鹏湾海域水样中Ｂ1 、Ｂ1 2 含量的测定结果 ,并对赤潮发生与其相关性进行探讨。1　采样布点和分析方法1 .1　采样布点大鹏湾盐田海域的Ｓ0 、Ｓ1…     

Vitamin B12 accelerates re-entrainment of activity rhythms in rats.

The effects of methyl vitamin B12 (5-6 mg/kg, p.o.) on the entrainment of circadian running wheel activity rhythm to a new lighting schedule were measured in rats. After the light-dark (LD) cycle was abruptly reversed, rats given vitamin B12 took less time to entrain their circadian locomotor activity rhythm to the new cycle than did controls. This result indicates that vitamin B12 accelerates the reentrainment of the mammalian circadian activity rhythm following an abrupt change in the environmental LD cycle.     

Vitamin B12 Production by a Methanol-Utilizing Bacterium

Vitamin B(12) production by a newly isolated strain of a methanol-utilizing bacterium was studied. The maximal yield of the vitamin, 2.6 mg/liter of medium was attained by optimization.     

Magnetic Nanoparticle-Based Solid-Phase Extraction of Vitamin B12 from Pharmaceutical Formulations

In the present study, a novel quantitative method, namely magnetic nanoparticle-based solid-phase extraction (MSPE), was applied to extract vitamin B(12) from pharmaceutical formulations. The technique involves the use of Fe(3)O(4) nanoparticles modified by sodium dodecyl sulfate (SDS) as an efficient adsorbent for solid-phase extraction of vitamin B(12). Collection of magnetic nanoparticles (MNPs) from aqueous solution was simply achieved by applying external magnetic field. The analyte was desorbed from MNPs using alkali 1-propanol. The extracted analyte was analyzed by using flow injection inductively coupled plasma-optical emission spectrometry. Factors affecting the extraction efficiency were investigated and optimized. Under the optimum conditions, enhancement factor of 184, linear dynamic range of 2.5-500 μg L(-1) with correlation of determination (R(2) > 0.999), and limit of detection of 1.0 μg L(-1) were obtained for vitamin B(12). The percent relative standard deviation based on five-replicate determination was less than 6.2%. The method was successfully applied for extraction and determination of vitamin B(12) in different types of pharmaceutical samples such as multivitamin tablet, effervescent tablet, and injection sample. The results showed that the proposed method based on SDS-Fe(3)O(4) MSPE was a simple, accurate, and highly efficient approach for analysis of vitamin B(12).     

Vitamin B12 absorption: Mammalian physiology and acquired and inherited disorders

Background

Vitamin B12 (cobalamin) is a cobalt-containing compound synthesized by bacteria and an essential nutrient in mammals, which take it up from diet. The absorption and distribution of dietary vitamin B12 to the organism is a complex process involving several gene products including carrier proteins, plasma membrane receptors and transporters. Disturbed cellular entry, transit or egress of vitamin B12 may lead to low vitamin B12 status or deficiency and eventually hematological and neurological disorders.

Objective

The aim of this review is to summarize the causes leading to vitamin B12 deficiency including decreased intake, impaired absorption and increased requirements. Under physiological conditions, vitamin B12 bound to the gastric intrinsic factor is internalized in the ileum by a highly specific receptor complex composed by Cubilin (Cubn) and Amnionless (Amn). Following exit of vitamin B12 from the ileum, general cellular uptake from the circulation requires the transcobalamin receptor CD320 whereas kidney reabsorption of cobalamin depends on Megalin (Lrp2).Whereas malabsorption of vitamin B12 is most commonly seen in the elderly, selective pediatric, nondietary-induced B12 deficiency is generally due to inherited disorders including the Imerslund-Gräsbeck syndrome and the much rarer intrinsic factor deficiency. Biochemical, clinical and genetic research on these disorders considerably improved our knowledge of vitamin B12 absorption.This review describes basic and recent findings on the intestinal handling of vitamin B12 and its importance in health and disease.     

Vitamin B 12 formation by a gram-variable methanol-utilizing bacterium

Summary A methanol-utilizing bacterium isolated from soil was studied for ability to produce vitamin B 12. The isolated strain XF is gram-variable, pink-pigmented and rod-shaped. Effects of growth conditions on vitamin B 12 productivity were examined. The maximal yield of vitamin B 12 was 280 g/l in shaked flasks and 236 g/l in fermentation jars.     

Vitamin B12 Production and Depletion in a Naturally Occurring Eutrophic Lake

The distribution of vitamin B(12) within Upper Klamath Lake was surveyed at approximately monthly intervals during a period from September 1968 to November 1969. High concentrations (up to 1.8 mug/g of dry sediment) characteristically occurred at the water-sediment interface, with a sharp decline below this area. A heavy bloom of Aphanizomenon flos-aquae occurred from the latter part of May through October 1969. B(12) concentrations of the uppermost sediments, from all but one sampling site, increased gradually through the bloom, followed by a drastic increase during the die-off period. B(12) is probably not a limiting factor for primary productivity, since sufficient levels of this vitamin were found to occur throughout the year. Of 42 cultures isolated from Upper Klamath Lake water and sediments, 20 were found capable of producing 50 pg or more of B(12)/ml of medium. Phytoplankton samples were found to contain up to 5 mug of B(12)/g of dry material. Degradation of B(12) occurred in sterilized as well as fresh sediment samples.     

Microbiological Assay of Vitamin B12 in the Presence of Tetracycline

Treatment of a pharmaceutical preparation of vitamin B(12) and tetracycline with an ion-exchange resin by the batch method differentially adsorbed tetracycline, making possible a statistically reproducible microbiological assay of vitamin B(12) by the standard Escherichia coli plate method.