Vitamin B12: unique metalorganic compounds and the most complex vitamins

Kinetic studies on the reaction of cob(II)alamin with hypochlorous acid: Evidence for one electron oxidation of the metal center and corrin ring destruction

Kinetic and mechanistic studies on the reaction of a major intracellular vitamin B₁₂ form, cob(II)alamin (Cbl(II)), with hypochlorous acid/hypochlorite (HOCl/OCl^-) have been carried out. Cbl(II) (Co(II)) is rapidly oxidized by HOCl to predominately aquacobalamin/hydroxycobalamin (Cbl(III), Co(III)) with a second-order rate constant of 2.4×10⁷M^-1s^-1 (25.0°C). The stoichiometry of the reaction is 1:1. UHPLC/HRMS analysis of the product mixture of the reaction of Cbl(II) with 0.9mol equiv. HOCl provides support for HOCl being initially reduced to Cl and subsequent H atom abstraction from the corrin macrocycle occurring, resulting in small amounts of corrinoid species with two or four H atoms fewer than the parent cobalamin. Upon the addition of excess (H)OCl further slower reactions are observed. Finally, SDS-PAGE experiments show that HOCl-induced damage to bovine serum albumin does not occur in the presence of Cbl(II), providing support for Cbl(II) being an efficient HOCl trapping agent.

Total Synthesis, Structure, and Biological Activity of Adenosylrhodibalamin, the Non-Natural Rhodium Homologue of Coenzyme B12

B12 is unique among the vitamins as it is biosynthesized only by certain prokaryotes. The complexity of its synthesis relates to its distinctive cobalt corrin structure, which is essential for B12 biochemistry and renders coenzyme B12 (AdoCbl) so intriguingly suitable for enzymatic radical reactions. However, why is cobalt so fit for its role in B12 -dependent enzymes? To address this question, we considered the substitution of cobalt in AdoCbl with rhodium to generate the rhodium analogue 5'-deoxy-5'-adenosylrhodibalamin (AdoRbl). AdoRbl was prepared by de novo total synthesis involving both biological and chemical steps. AdoRbl was found to be inactive in vivo in microbial bioassays for methionine synthase and acted as an in vitro inhibitor of an AdoCbl-dependent diol dehydratase. Solution NMR studies of AdoRbl revealed a structure similar to that of AdoCbl. However, the crystal structure of AdoRbl revealed a conspicuously better fit of the corrin ligand for Rh(III) than for Co(III) , challenging the current views concerning the evolution of corrins.

Photolytic properties of cobalamins: a theoretical perspective

This Perspective Article highlights recent theoretical developments, and summarizes the current understanding of the photolytic properties of cobalamins from a computational point of view. The primary focus is on two alkyl cobalamins, methylcobalamin (MeCbl) and adenosylcobalamin (AdoCbl), as well as two non-alkyl cobalamins, cyanocobalamin (CNCbl) and hydroxocobalamin (HOCbl). Photolysis of alkyl cobalamins involves low-lying singlet excited states where photodissociation of the Co-C bond leads to formation of singlet-born alkyl/cob(ii)alamin radical pairs (RPs). Potential energy surfaces (PESs) associated with cobalamin low-lying excited states as functions of both axial bonds, provide the most reliable tool for initial analysis of their photochemical and photophysical properties. Due to the complexity, and size limitations associated with the cobalamins, the primary method for calculating ground state properties is density functional theory (DFT), while time-dependent DFT (TD-DFT) is used for electronically excited states. For alkyl cobalamins, energy pathways on the lowest singlet surface, connecting metal-to-ligand charge transfer (MLCT) and ligand field (LF) minima, can be associated with photo-homolysis of the Co-C bond observed experimentally. Additionally, energy pathways between minima and seams associated with crossing of S1/S0 surfaces, are the most efficient for internal conversion (IC) to the ground state. Depending on the specific cobalamin, such IC may involve simultaneous elongation of both axial bonds (CNCbl), or detachment of axial base followed by corrin ring distortion (MeCbl). The possibility of intersystem crossing, and the formation of triplet RPs is also discussed based on Landau-Zener theory.

METALLOPROTEINS. A tethered niacin-derived pincer complex with a nickel-carbon bond in lactate racemase

Lactic acid racemization is involved in lactate metabolism and cell wall assembly of many microorganisms. Lactate racemase (Lar) requires nickel, but the nickel-binding site and the role of three accessory proteins required for its activation remain enigmatic. We combined mass spectrometry and x-ray crystallography to show that Lar from Lactobacillus plantarum possesses an organometallic nickel-containing prosthetic group. A nicotinic acid mononucleotide derivative is tethered to Lys(184) and forms a tridentate pincer complex that coordinates nickel through one metal-carbon and two metal-sulfur bonds, with His(200) as another ligand. Although similar complexes have been previously synthesized, there was no prior evidence for the existence of pincer cofactors in enzymes. The wide distribution of the accessory proteins without Lar suggests that it may play a role in other enzymes.

Energy cascades, excited state dynamics, and photochemistry in cob(III)alamins and ferric porphyrins

Porphyrins and the related chlorins and corrins contain a cyclic tetrapyrrole with the ability to coordinate an active metal center and to perform a variety of functions exploiting the oxidation state, reactivity, and axial ligation of the metal center. These compounds are used in optically activated applications ranging from light harvesting and energy conversion to medical therapeutics and photodynamic therapy to molecular electronics, spintronics, optoelectronic thin films, and optomagnetics. Cobalt containing corrin rings extend the range of applications through photolytic cleavage of a unique axial carbon-cobalt bond, permitting spatiotemporal control of drug delivery. The photochemistry and photophysics of cyclic tetrapyrroles are controlled by electronic relaxation dynamics including internal conversion and intersystem crossing. Typically the electronic excitation cascades through ring centered ππ* states, ligand to metal charge transfer (LMCT) states, metal to ligand charge transfer (MLCT) states, and metal centered states. Ultrafast transient absorption spectroscopy provides a powerful tool for the investigation of the electronic state dynamics in metal containing tetrapyrroles. The UV-visible spectrum is sensitive to the oxidation state, electronic configuration, spin state, and axial ligation of the central metal atom. Ultrashort broadband white light probes spanning the range from 270 to 800 nm, combined with tunable excitation pulses, permit the detailed unravelling of the time scales involved in the electronic energy cascade. State-of-the-art theoretical calculations provide additional insight required for precise assignment of the states. In this Account, we focus on recent ultrafast transient absorption studies of ferric porphyrins and corrin containing cob(III)alamins elucidating the electronic states responsible for ultrafast energy cascades, excited state dynamics, and the resulting photoreactivity or photostability of these compounds. Iron tetraphenyl porphyrin chloride (Fe((III))TPPCl) exhibits picosecond decay to a metal centered d → d* (4)T state. This state decays on a ca. 16 ps time scale in room temperature solution but persists for much longer in a cryogenic glass. The photoreactivity of the (4)T state may lead to novel future applications for these compounds. In contrast, the nonplanar cob(III)alamins contain two axial ligands to the central cobalt atom. The upper axial ligand can be an alkyl group as in the two biologically active coenzymes or a nonalkyl ligand such as -CN in cyanocobalamin (vitamin B12) or -OH in hydroxocobalamin. The electronic structure, energy cascade, and bond cleavage of these compounds is sensitive to the details of the axial ligand. Nonalkylcobalamins exhibit ultrafast internal conversion to a low-lying state of metal to ligand or ligand to metal charge transfer character. The compounds are generally photostable with ground state recovery complete on a time scale of 2-7 ps in room temperature aqueous solution. Alkylcobalamins exhibit ultrafast internal conversion to an S1 state of d/π → π* character. Most compounds undergo bond cleavage from this state with near unit quantum yield within ∼100 ps. Recent theoretical calculations provide a potential energy surface accounting for these observations. Conformation dependent mixing of the corrin π and cobalt d orbitals plays a significant role in the observed photochemistry and photophysics.

Interactive performances of betaine on the metabolic processes of Pseudomonas denitrificans

The performances of betaine on the metabolic processes of vitamin B12-producing Pseudomonas denitrificans were investigated in this paper. The results showed that betaine was an indispensable methyl-group donor for vitamin B12 biosynthesis, but large amounts of the extracellular glycine accompanied by betaine metabolism would impose a severe restriction on the cell growth of P. denitrificans. By further using a comparative metabolomics approach coupled with intracellular free amino acids analysis for the fermentation processes with betaine addition (10 g/l) or not, it was found that betaine could highly strengthen the formation of some key precursors and intermediates facilitating vitamin B12 biosynthesis, such as δ-aminolevulinic acid (ALA, the first precursor of vitamin B12), glutamate (an intermediate of ALA via C5 pathway), glycine (an intermediate of ALA via C4 pathway), and methionine (directly participating in the methylation reaction involved in vitamin B12 biosynthetic pathway). Therefore, the performances of betaine on P. denitrificans metabolic processes were not only serving as a decisive methyl-group donor for vitamin B12 biosynthesis, but also playing a powerfully promoting role in the generation of vitamin B12 precursors and intermediates.

Industrial vitamin B12 production by Pseudomonas denitrificans using maltose syrup and corn steep liquor as the cost-effective fermentation substrates

The aerobic Pseudomonas denitrificans is widely used for industrial and commercial vitamin B12 fermentation, due to its higher productivity compared to the anaerobic vitamin B12-producing microorganisms. This paper aimed to develop a cost-effective fermentation medium for industrial vitamin B12 production by P. denitrificans in 120,000-l fermenter. It was found that maltose syrup (a low-cost syrup from corn starch by means of enzymatic or acid hydrolysis) and corn steep liquor (CSL, a by-product of starch industry) were greatly applicable to vitamin B12 production by P. denitrificans. Under the optimal fermentation medium performed by response surface methodology, 198.27 ± 4.60 mg/l of vitamin B12 yield was obtained in 120,000-l fermenter, which was close to the fermentation with the refined sucrose (198.80 mg/l) and was obviously higher than that obtained under beet molasses utilization (181.75 mg/l). Therefore, maltose syrups and CSL were the efficient and economical substrates for industrial vitamin B12 fermentation by P. denitrificans.

Spectroscopic studies of the Salmonella enterica adenosyltransferase enzyme SeCobA: molecular-level insight into the mechanism of substrate Cob(II)alamin activation

CobA from Salmonella enterica (SeCobA) is a member of the family of ATP:Co(I)rrinoid adenosyltransferase (ACAT) enzymes that participate in the biosynthesis of adenosylcobalamin by catalyzing the transfer of the adenosyl group from an ATP molecule to a reactive Co(I)rrinoid species transiently generated in the enzyme active site. This reaction is thermodynamically challenging, as the reduction potential of the Co(II)rrinoid precursor in solution is far more negative than that of available reducing agents in the cell (e.g., flavodoxin), precluding nonenzymic reduction to the Co(I) oxidation state. However, in the active sites of ACATs, the Co(II)/Co(I) redox potential is increased by >250 mV via the formation of a unique four-coordinate (4c) Co(II)rrinoid species. In the case of the SeCobA ACAT, crystallographic and kinetic studies have revealed that the phenylalanine 91 (F91) and tryptophan 93 (W93) residues are critical for in vivo activity, presumably by blocking access to the lower axial ligand site of the Co(II)rrinoid substrate. To further assess the importance of the F91 and W93 residues with respect to enzymatic function, we have characterized various SeCobA active-site variants using electronic absorption, magnetic circular dichroism, and electron paramagnetic resonance spectroscopies. Our data provide unprecedented insight into the mechanism by which SeCobA converts the Co(II)rrinoid substrate to 4c species, with the hydrophobicity, size, and ability to participate in offset π-stacking interactions of key active-site residues all being critical for activity. The structural changes that occur upon Co(II)rrinoid binding also appear to be crucial for properly orienting the transiently generated Co(I) "supernucleophile" for rapid reaction with cosubstrate ATP.

Models for B12-conjugated radiopharmaceuticals. Cobaloxime binding to new fac-[Re(CO)3(Me2bipyridine)(amidine)]BF4 complexes having an exposed pyridyl nitrogen

New mononuclear amidine complexes, fac-[Re(CO)3(Me2bipy)(HNC(CH3)(pyppz))]BF4 [(4,4'-Me2bipy (1), 5,5'-Me2bipy (2), and 6,6'-Me2bipy (3)] (bipy = 2,2'-bipyridine), were synthesized by treating the parent fac-[Re(I)(CO)3(Me2bipy)(CH3CN)]BF4 complex with the C2-symmetrical amine 1-(4-pyridyl)piperazine (pyppzH). The axial amidine ligand has an exposed, highly basic pyridyl nitrogen. The reaction of complexes 1-3 with a B12 model, (py)Co(DH)2Cl (DH = monoanion of dimethylglyoxime), in CH2Cl2 yielded the respective dinuclear complexes, namely, fac-[Re(CO)3(Me2bipy)(μ-(HNC(CH3)(pyppz)))Co(DH)2Cl]BF4 [(4,4'-Me2bipy (4), 5,5'-Me2bipy (5), and 6,6'-Me2bipy (6)]. (1)H NMR spectroscopic analysis of all compounds and single-crystal X-ray crystallographic data for 2, 3, 5, and 6 established that the amidine had only the E configuration in both the solid and solution states and that the pyridyl group is bound to Co in 4-6. Comparison of the NMR spectra of 1-3 with spectra of 4-6 reveals an unusually large "wrong-way" upfield shift for the pyridyl H2/6 signal for 4-6. The wrong-way H2/6 shift of (4-Xpy)Co(DH)2Cl (4-Xpy = 4-substituted pyridine) complexes increased with increasing basicity of the 4-Xpy derivative, a finding attributed to the influence of the magnetic anisotropy of the cobalt center on the shifts of the (1)H NMR signals of the pyridyl protons closest to Co. Our method of employing a coordinate bond for conjugating the fac-[Re(I)(CO)3] core to a vitamin B12 model could be extended to natural B12 derivatives. Because B12 compounds are known to accumulate in cancer cells, such an approach is a very attractive method for the development of (99m)Tc and (186/188)Re radiopharmaceuticals for targeted tumor imaging and therapy.

Treatment of vitamin B12 deficiency-methylcobalamine? Cyancobalamine? Hydroxocobalamin?-clearing the confusion

Vitamin B12 (cyancobalamin, Cbl) has two active co-enzyme forms, methylcobalamin (MeCbl) and adenosylcobalamin (AdCbl). There has been a paradigm shift in the treatment of vitamin B12 deficiency such that MeCbl is being extensively used and promoted. This is despite the fact that both MeCbl and AdCbl are essential and have distinct metabolic fates and functions. MeCbl is primarily involved along with folate in hematopiesis and development of the brain during childhood. Whereas deficiency of AdCbl disturbs the carbohydrate, fat and amino-acid metabolism, and hence interferes with the formation of myelin. Thereby, it is important to treat vitamin B12 deficiency with a combination of MeCbl and AdCbl or hydroxocobalamin or Cbl. Regarding the route, it has been proved that the oral route is comparable to the intramuscular route for rectifying vitamin B12 deficiency.

Spectral and electronic properties of nitrosylcobalamin

Nitrosylcobalamin (NOCbl) is readily formed when Co(II)balamin reacts with nitric oxide (NO) gas. NOCbl has been implicated in the inhibition of various B12-dependent enzymes, as well as in the modulation of blood pressure and of the immunological response. Previous studies revealed that among the known biologically relevant cobalamin species, NOCbl possesses the longest bond between the Co ion and the axially bound 5,6-dimethylbenzimidazole base, which was postulated to result from a strong trans influence exerted by the NO ligand. In this study, various spectroscopic (electronic absorption, circular dichroism, magnetic circular dichroism, and resonance Raman) and computational (density functional theory (DFT) and time-dependent DFT) techniques were used to generate experimentally validated electronic structure descriptions for the "base-on" and "base-off" forms of NOCbl. Further insights into the principal Co-ligand bonding interactions were obtained by carrying out natural bond orbital analyses. Collectively, our results indicate that the formally unoccupied Co 3dz(2) orbital engages in a highly covalent bonding interaction with the filled NO π* orbital and that the Co-NO bond is strengthened further by sizable π-backbonding interactions that are not present in any other Co(III)Cbl characterized to date. Because of the substantial NO(-) to Co(III) charge donation, NOCbl is best described as a hybrid of Co(III)-NO(-) and Co(II)-NO(•) resonance structures. In contrast, our analogous computational characterization of a related species, superoxocobalamin, reveals that in this case a Co(III)-O2(-) description is adequate due to the larger oxidizing power of O2 versus NO. The implications of our results with respect to the unusual structural features and thermochromism of NOCbl and the proposed inhibition mechanisms of B12-dependent enzymes by NOCbl are discussed.

Analyses of cobalt-ligand and potassium-ligand bond lengths in metalloproteins: trends and patterns

Cobalt and potassium are biologically important metal elements that are present in a large array of proteins. Cobalt is mostly found in vivo associated with a corrin ring, which represents the core of the vitamin B12 molecule. Potassium is the most abundant metal in the cytosol, and it plays a crucial role in maintaining membrane potential as well as correct protein function. Here, we report a thorough analysis of the geometric properties of cobalt and potassium coordination spheres that was performed with high resolution on a representative set of structures from the Protein Data Bank and complemented by quantum mechanical calculations realized at the DFT level of theory (B3LYP/ SDD) on mononuclear model systems. The results allowed us to draw interesting conclusions on the structural characteristics of both Co and K centers, and to evaluate the importance of effects such as their association energies and intrinsic thermodynamic stabilities. Overall, the results obtained provide useful data for enhancing the atomic models normally applied in theoretical and computational studies of Co or K proteins performed at the quantum mechanical level, and for developing molecular mechanical parameters for treating Co or K coordination spheres in molecular mechanics or molecular dynamics studies.

A Stability-Indicating HPLC Method for the Determination of Nitrosylcobalamin (NO-Cbl), a Novel Vitamin B12 Analog

Nitrosylcobalamin (NO-Cbl), a novel vitamin B12 analog and anti-tumor agent, functions as a biologic 'Trojan horse', utilizing the vitamin B12 transcobalamin II transport protein and cell surface receptor to specifically target cancer cells. a stability-indicating HPLC method was developed for the detection of NO-Cbl during forced degradation studies. This method utilized an ascentis® RP-amide (150 mm × 4.6 mm, 5 μm) column at 35 °C with a mobile phase (1.0 mL min-1) combining a gradient of methanol and an acetate buffer at pH 6.0. Detection wavelengths of 450 and 254 nm were used to detect corrin and non-corrin-based products, respectively. NO-Cbl, synthesized from hydroxocobalamin and pure nitric oxide gas, was subjected to degradative stress conditions including oxidation, hydrolysis and thermal and radiant energy challenge. The method was validated by assessing linearity, accuracy, precision, detection and quantitation limits and robustness. The method was applied successfully for purity assessment of synthesized NO-Cbl and for the determination of NO-Cbl during kinetic studies in aqueous solution and in solid-state degradation assessments. This HPLC method is suitable for the separation of cobalamins in aqueous and methanolic solutions, for routine detection of NO-Cbl and for purity assessment of synthesized NO-Cbl. additionally, this method has potential application in identification and monitoring of diseases involving altered nitric oxide homeostasis where vitamin B12 therapy is utilized to scavenge excess nitric oxide, subsequently resulting in the in vivo production of NO-Cbl.

Improved vitamin B12 fermentation process by adding rotenone to regulate the metabolism of Pseudomonas denitrificans

Our previous research had revealed that the dissolved oxygen limitation was more favorable for vitamin B12 fermentation, due to its inducement to the increased glycolytic flux in Pseudomonas denitrificans. In this paper, a novel strategy was implemented to further investigate the metabolic characteristics of P. denitrificans under different oxygen supply levels, by exogenously adding rotenone (a respiratory chain inhibitor interfering with the oxygen consumption) to the fermentation broths. Compared to the fermentation process without rotenone treatment, it was observed that 5 mg/L rotenone treatment could significantly strengthen the glycolytic flux of P. denitrificans via activating the key glycolytic enzymes (phosphofructokinase and pyruvate kinase), resulting in the accelerated generations of anterior precursors (glutamate and 5-aminolevulinic acid) for vitamin B12 biosynthesis. Although 5 mg/L rotenone treatment had a negative effect on cell growth of P. denitrificans, the vitamin B12 yield was increased from 48.28 ± 0.62 mg/L to 54.70 ± 0.45 mg/L, which further proved that an increased glycolytic flux in P. denitrificans was a consequence of higher vitamin B12 production.

Beyond catalysis: vitamin B12 as a cofactor in gene regulation

Vitamin B12 is well known as an enzyme cofactor in the catalysis of many important biological reactions, and the role of B12 in regulation of bacterial gene expression as a ligand of riboswitches is well established. Only recently evidence has emerged that B12 can also affect bacterial gene expression by acting as a cofactor of regulatory proteins. In 2011 a role of B12 as a cofactor of the transcriptional repressor of carotenogenesis, CarH, in Myxococcus xanthus was reported. B12 is required for light-dependent DNA binding by CarH, which can therefore be considered to be a new type of photoreceptor. Cheng et al. (2014) report the identification of B12 as a cofactor of the AerR protein in Rhodobacter capsulatus. AerR acts as an antirepressor of the CrtJ protein, which represses photosynthesis genes when binding to its target promoters. As in Myxococcus B12 may have the role of a chromophore in photoreception, but it is suggested that a main function of AerR is the sensing of B12. The co-regulation of the pathways is beneficial because the syntheses of B12 , haem and bacteriochlorophylls share common precursors and the accumulation of the free molecules is toxic.

Planar substrate-binding site dictates the specificity of ECF-type nickel/cobalt transporters

The energy-coupling factor (ECF) transporters are multi-subunit protein complexes that mediate uptake of transition-metal ions and vitamins in about 50% of the prokaryotes, including bacteria and archaea. Biological and structural studies have been focused on ECF transporters for vitamins, but the molecular mechanism by which ECF systems transport metal ions from the environment remains unknown. Here we report the first crystal structure of a NikM, TtNikM2, the substrate-binding component (S component) of an ECF-type nickel transporter from Thermoanaerobacter tengcongensis. In contrast to the structures of the vitamin-specific S proteins with six transmembrane segments (TSs), TtNikM2 possesses an additional TS at its N-terminal region, resulting in an extracellular N-terminus. The highly conserved N-terminal loop inserts into the center of TtNikM2 and occludes a region corresponding to the substrate-binding sites of the vitamin-specific S components. Nickel binds to NikM via its coordination to four nitrogen atoms, which are derived from Met1, His2 and His67 residues. These nitrogen atoms form an approximately square-planar geometry, similar to that of the metal ion-binding sites in the amino-terminal Cu(2+)- and Ni(2+)-binding (ATCUN) motif. Replacements of residues in NikM contributing to nickel coordination compromised the Ni-transport activity. Furthermore, systematic quantum chemical investigation indicated that this geometry enables NikM to also selectively recognize Co(2+). Indeed, the structure of TtNikM2 containing a bound Co(2+) ion has almost no conformational change compared to the structure that contains a nickel ion. Together, our data reveal an evolutionarily conserved mechanism underlying the metal selectivity of EcfS proteins, and provide insights into the ion-translocation process mediated by ECF transporters.

Long-term intermittent hypoxia elevates cobalt levels in the brain and injures white matter in adult mice

STUDY OBJECTIVES

Exposure to the variable oxygenation patterns in obstructive sleep apnea (OSA) causes oxidative stress within the brain. We hypothesized that this stress is associated with increased levels of redox-active metals and white matter injury.

DESIGN

Participants were randomly allocated to a control or experimental group (single independent variable).

SETTING

University animal house.

PARTICIPANTS

Adult male C57BL/6J mice.

INTERVENTIONS

To model OSA, mice were exposed to long-term intermittent hypoxia (LTIH) for 10 hours/day for 8 weeks or sham intermittent hypoxia (SIH).

MEASUREMENTS AND RESULTS

Laser ablation-inductively coupled plasma-mass spectrometry was used to quantitatively map the distribution of the trace elements cobalt, copper, iron, and zinc in forebrain sections. Control mice contained 62 ± 7 ng cobalt/g wet weight, whereas LTIH mice contained 5600 ± 600 ng cobalt/g wet weight (P < 0.0001). Other elements were unchanged between conditions. Cobalt was concentrated within white matter regions of the brain, including the corpus callosum. Compared to that of control mice, the corpus callosum of LTIH mice had significantly more endoplasmic reticulum stress, fewer myelin-associated proteins, disorganized myelin sheaths, and more degenerated axon profiles. Because cobalt is an essential component of vitamin B12, serum methylmalonic acid (MMA) levels were measured. LTIH mice had low MMA levels (P < 0.0001), indicative of increased B12 activity.

CONCLUSIONS

Long-term intermittent hypoxia increases brain cobalt, predominantly in the white matter. The increased cobalt is associated with endoplasmic reticulum stress, myelin loss, and axonal injury. Low plasma methylmalonic acid levels are associated with white matter injury in long-term intermittent hypoxia and possibly in obstructive sleep apnea.

The role of B vitamins in marine biogeochemistry

The soluble B vitamins (B1, B7, and B12) have long been recognized as playing a central metabolic role in marine phytoplankton and bacteria; however, the importance of these organic external metabolites in marine ecology has been largely disregarded, as most research has focused on inorganic nutrients and trace metals. Using recently available genomic data combined with culture-based surveys of vitamin auxotrophy (i.e., vitamin requirements), we show that this auxotrophy is widespread in the marine environment and occurs in both autotrophs and heterotrophs residing in oligotrophic and eutrophic environments. Our analysis shows that vitamins originate from the activities of some bacteria and algae and that taxonomic changes observed in marine phytoplankton communities could be the result of their specific vitamin requirements and/or vitamin availability. Dissolved vitamin concentration measurements show that large areas of the world ocean are devoid of B vitamins, suggesting that vitamin limitation could be important for the efficiency of carbon and nitrogen fixation in those regions.

Water-soluble vitamins in people with low glomerular filtration rate or on dialysis: a review

People with low glomerular filtration rate and people on dialysis are spontaneously at risk for vitamin deficiency because of the potential for problems with decreased appetite and decreased sense of smell and taste, leading to decreased intake, and because decreased energy or decreased cognitive ability results in difficulties in shopping and cooking. Imposed dietary restrictions because of their renal dysfunction and because of comorbidities such as hypertension and diabetes exacerbate this problem. Finally, particularly for water-soluble vitamins, loss may occur into the dialysate. We did not identify any randomized trials of administering daily doses close to the recommended daily allowances of these vitamins. In people who are eating at all, deficiencies of B5 and B7 seem unlikely. It is unclear whether supplements of B2 and B3 are necessary. Because of dialyzability and documented evidence of insufficiency in dialysis patients, B1 supplementation is likely to be helpful. B6, B9, and B12 are implicated in the hyperhomocysteinemia observed in patients on dialysis. These vitamins have been studied in combinations, in high doses, with the hope of reducing cardiovascular outcomes. No reductions in patient-important outcomes were seen in adequately powered randomized trials. Because of their involvement in the homocysteine pathway, however, supplementation with lower doses, close to the recommended daily allowances, may be helpful. Vitamin C deficiency is common in patients on dialysis who are not taking supplements: low-dose supplements are warranted. Vitamins for dialysis patients contain most or all of the B vitamins and low-dose vitamin C. We are not aware of any medical reasons to choose one over another.

Water-soluble vitamins in people with low glomerular filtration rate or on dialysis: a review

People with low glomerular filtration rate and people on dialysis are spontaneously at risk for vitamin deficiency because of the potential for problems with decreased appetite and decreased sense of smell and taste, leading to decreased intake, and because decreased energy or decreased cognitive ability results in difficulties in shopping and cooking. Imposed dietary restrictions because of their renal dysfunction and because of comorbidities such as hypertension and diabetes exacerbate this problem. Finally, particularly for water-soluble vitamins, loss may occur into the dialysate. We did not identify any randomized trials of administering daily doses close to the recommended daily allowances of these vitamins. In people who are eating at all, deficiencies of B5 and B7 seem unlikely. It is unclear whether supplements of B2 and B3 are necessary. Because of dialyzability and documented evidence of insufficiency in dialysis patients, B1 supplementation is likely to be helpful. B6, B9, and B12 are implicated in the hyperhomocysteinemia observed in patients on dialysis. These vitamins have been studied in combinations, in high doses, with the hope of reducing cardiovascular outcomes. No reductions in patient-important outcomes were seen in adequately powered randomized trials. Because of their involvement in the homocysteine pathway, however, supplementation with lower doses, close to the recommended daily allowances, may be helpful. Vitamin C deficiency is common in patients on dialysis who are not taking supplements: low-dose supplements are warranted. Vitamins for dialysis patients contain most or all of the B vitamins and low-dose vitamin C. We are not aware of any medical reasons to choose one over another.

Structural basis for universal corrinoid recognition by the cobalamin transport protein haptocorrin

Cobalamin (Cbl; vitamin B12) is an essential micronutrient synthesized only by bacteria. Mammals have developed a sophisticated uptake system to capture the vitamin from the diet. Cbl transport is mediated by three transport proteins: transcobalamin, intrinsic factor, and haptocorrin (HC). All three proteins have a similar overall structure but a different selectivity for corrinoids. Here, we present the crystal structures of human HC in complex with cyanocobalamin and cobinamide at 2.35 and 3.0 Å resolution, respectively. The structures reveal that many of the interactions with the corrin ring are conserved among the human Cbl transporters. However, the non-conserved residues Asn-120, Arg-357, and Asn-373 form distinct interactions allowing for stabilization of corrinoids other than Cbl. A central binding motif forms interactions with the e- and f-side chains of the corrin ring and is conserved in corrinoid-binding proteins of other species. In addition, the α- and β-domains of HC form several unique interdomain contacts and have a higher shape complementarity than those of intrinsic factor and transcobalamin. The stabilization of ligands by all of these interactions is reflected in higher melting temperatures of the protein-ligand complexes. Our structural analysis offers fundamental insights into the unique binding behavior of HC and completes the picture of Cbl interaction with its three transport proteins.

Hydroxocobalamin association during cell culture results in pink therapeutic proteins

Process control of protein therapeutic manufacturing is central to ensuring the product is both safe and efficacious for patients. In this work, we investigate the cause of pink color variability in development lots of monoclonal antibody (mAb) and Fc-fusion proteins. Results show pink-colored product generated during manufacturing is due to association of hydroxocobalamin (OH-Cbl), a form of vitamin B12. OH-Cbl is not part of the product manufacturing process; however we found cyanocobalamin (CN-Cbl) in cell culture media converts to OH-Cbl in the presence of light. OH-Cbl can be released from mAb and Fc-fusion proteins by conversion with potassium cyanide to CN-Cbl, which does not bind. By exploiting the differential binding of CN-Cbl and OH-Cbl, we developed a rapid and specific assay to accurately measure B12 levels in purified protein. Analysis of multiple products and lots using this technique gives insight into color variability during manufacturing.

Two-step activation prodrugs: transplatin mediated binding of chemotherapeutic agents to vitamin B12

Clinically approved organic chemotherapeutic drugs such as cytarabine, dacarbazine and anastrozole were attached to B12via a {CN-trans-Pt(NH3)2}-bridge to yield [{Co}-CN-{trans-Pt(NH3)2}-{drug}](2+). The active organic drugs are protected by the platinum complex and by B12, which represents at the same time the targeting vector. We refer to these bioconjugates as two-step activation prodrugs since two reactions are finally required to liberate the actual organic drugs. All three prodrugs are soluble and stable in water. The physiological stability and the therapeutic efficiency of [{Co}-CN-{trans-Pt(NH3)2}-{cytarabine}](2+) (2) were studied. Under physiological conditions, 2 is stable for 3 days. Its affinity to the cobalamin transport proteins (haptocorrin, intrinsic factor and transcobalamin) is not substantially affected despite the introduction of a bulky group in the β-axial position. The cleavage of the [trans-CN-Pt(NH3)2-{cytarabine}](+) complex was observed upon chemical reduction of Co(III)→ Co(II) with Zn(0). Cytarabine was subsequently released from the cleaved complex to exhibit its cytotoxicity. 2 displayed a reduced cytotoxicity (IC50 = 230 ± 62 nM) as compared to cytarabine (IC50 = 30 ± 5 nM). However, cytarabine released from 2 showed comparable cytotoxicity (IC50 = 30 ± 11 nM).

Cobalt derivatives as promising therapeutic agents

Inorganic complexes are versatile platforms for the development of potent and selective pharmaceutical agents. Cobalt possesses a diverse array of properties that can be manipulated to yield promising drug candidates. Investigations into the mechanism of cobalt therapeutic agents can provide valuable insight into the physicochemical properties that can be harnessed for drug development. This review presents examples of bioactive cobalt complexes with special attention to their mechanisms of action. Specifically, cobalt complexes that elicit biological effects through protein inhibition, modification of drug activity, and bioreductive activation are discussed. Insights gained from these examples reveal features of cobalt that can be rationally tuned to produce therapeutics with high specificity and improved efficacy for the biomolecule or pathway of interest.

Structural insights into the mechanism of four-coordinate Cob(II)alamin formation in the active site of the Salmonella enterica ATP:Co(I)rrinoid adenosyltransferase enzyme: critical role of residues Phe91 and Trp93

ATP:co(I)rrinoid adenosyltransferases (ACATs) are enzymes that catalyze the formation of adenosylcobalamin (AdoCbl, coenzyme B(12)) from cobalamin and ATP. There are three families of ACATs, namely, CobA, EutT, and PduO. In Salmonella enterica, CobA is the housekeeping enzyme that is required for de novo AdoCbl synthesis and for salvaging incomplete precursors and cobalamin from the environment. Here, we report the crystal structure of CobA in complex with ATP, four-coordinate cobalamin, and five-coordinate cobalamin. This provides the first crystallographic evidence of the existence of cob(II)alamin in the active site of CobA. The structure suggests a mechanism in which the enzyme adopts a closed conformation and two residues, Phe91 and Trp93, displace 5,6-dimethylbenzimidazole, the lower nucleotide ligand base of cobalamin, to generate a transient four-coordinate cobalamin, which is critical in the formation of the AdoCbl Co-C bond. In vivo and in vitro mutational analyses of Phe91 and Trp93 emphasize the important role of bulky hydrophobic side chains in the active site. The proposed manner in which CobA increases the redox potential of the cob(II)alamin/cob(I)alamin couple to facilitate formation of the Co-C bond appears to be analogous to that utilized by the PduO-type ACATs, where in both cases the polar coordination of the lower ligand to the cobalt ion is eliminated by placing that face of the corrin ring adjacent to a cluster of bulky hydrophobic side chains.

Hydroxocobalamin in cyanide poisoning

INTRODUCTION

On theoretical grounds, hydroxocobalamin is an attractive antidote for cyanide poisoning as cobalt compounds have the ability to bind and detoxify cyanide. This paper reviews the pharmacokinetic and pharmacodynamic aspects of hydroxocobalamin, its efficacy in human cyanide poisoning and its adverse effects.

METHODS

PubMed was searched for the period 1952 to April 2012. A total of 71 papers were identified in this way; and none was excluded. PHARMACOKINETICS AND PHARMACODYNAMICS: Pharmacokinetic studies in dogs and humans suggest a two-compartment model, with first order elimination kinetics. Pharmacodynamic studies in animals suggest that hydroxocobalamin would be a satisfactory antidote for human cyanide poisoning. EFFICACY IN HUMAN POISONING: There is limited evidence that hydroxocobalamin alone is effective in severe poisoning by cyanide salts. The evidence for the efficacy of hydroxocobalamin in smoke inhalation is complicated by lack of evidence for the importance of cyanide exposure in fires and the effects of other chemicals as well as confounding effects of other therapeutic measures, including hyperbaric oxygen. Evidence that hydroxocobalamin is effective in poisoning due to hydrogen cyanide alone is lacking; extrapolation of efficacy from poisoning by ingested cyanide salts may not be valid. The rate of absorption may be greater with inhaled hydrogen cyanide and the recommended slow intravenous administration of hydroxocobalamin may severely limit its clinical effectiveness in these circumstances.

ADVERSE EFFECTS

Both animal and human data suggest that hydroxocobalamin is lacking in clinically significant adverse effects. However, in one human volunteer study, delayed but prolonged rashes were observed in one-sixth of subjects, appearing 7 to 25 days after administration of 5 g or more of hydroxocobalamin. Rare adverse effects have included dyspnoea, facial oedema, and urticaria.

CONCLUSIONS

Limited data on human poisonings with cyanide salts suggest that hydroxocobalamin is an effective antidote; data from smoke inhalation are less clear-cut. Although clinically important reactions to hydroxocobalamin have not been seen, some, non-life threatening, adverse reactions can occur.

Cobalamin status in sickle cell disease

INTRODUCTION

Some studies comparing serum cobalamin in individuals with and without sickle cell disease (SCD) have suggested a higher prevalence of cobalamin deficiency in SCD but others have not. Our aim was to prospectively compare cobalamin status in African-Americans with and without SCD.

METHODS

We analyzed blood samples from 86 subjects in two groups: SCD (n = 29) and non-SCD (n = 57). Serum cobalamin, folate, homocysteine, methylmalonic acid (MMA), anti-intrinsic factor antibody, Helicobacter pylori antibody, and gastrin were measured and compared.

RESULTS

The median cobalamin was 235 pM in the SCD group vs. 292 pM in the non-SCD group (P-value = 0.014). No significant differences in MMA or homocysteine were seen. Using the criteria of a low cobalamin and an elevated MMA or an elevated MMA alone, cobalamin deficiency was suggested in 4 (13.8%) in the SCD group and 6 (10.5%) in the non-SCD group. Two of these SCD patients and four of these control subjects had chronic renal disease, which may lead to elevated MMA in the absence of cobalamin deficiency. The remaining four met criteria for cobalamin deficiency, 2 (6.9%) in the SCD group and 2 (3.5%) in the non-SCD group (P = 0.6).

CONCLUSION

A lower cobalamin was observed in SCD patients without a higher prevalence of cobalamin deficiency. The inclusion of haptocorrin and holotranscobalamin measurement in future studies may provide a better assessment of cobalamin status in this patient group.

Tris{2-[(furan-2-meth-yl)imino-meth-yl]-4-methyl-phenolato}cobalt(III)

In title compound, [Co(C(13)H(12)NO(2))(3)], the Co(III) ion is six-coordinated by three bidentate Schiff base ligands in a distorted octa-hedral environment. Adjacent complex mol-ecules are linked through C-H⋯O hydrogen bonds.

Bis{2,4-dibromo-6-[(2-phenyl-eth-yl)imino-meth-yl]phenolato-κN,O}cobalt(II)

In the title complex, [Co(C₁₅H₁₂Br₂NO)₂], the Co^II atom is four-coordinated by two N,O-bidentate chelate Schiff base ligands, displaying a flattened tetra­hedral coordination environment. The Co^II atom occupies a special position on a twofold rotation axis. In the crystal, mol­ecules are linked via weak C—H⋯Br inter­actions.

Mechanistic studies on the reaction between cob(II)alamin and peroxynitrite: evidence for a dual role for cob(II)alamin as a scavenger of peroxynitrous acid and nitrogen dioxide

Peroxynitrite/peroxynitrous acid (ONOO(-)/ONOOH; pK(a(ONOOH)) =6.8) is implicated in multiple chronic inflammatory and neurodegenerative diseases. Both mammalian B(12)-dependent enzymes are inactivated under oxidative stress conditions. We report studies on the kinetics of the reaction between peroxynitrite/peroxynitrous acid and a major intracellular vitamin B(12) form, cob(II)alamin (Cbl(II)), using stopped-flow spectroscopy. The pH dependence of the reaction is consistent with peroxynitrous acid reacting directly with Cbl(II) to give cob(III)alamin (Cbl(III)) and (.)NO(2) , followed by a subsequent rapid reaction between (.)NO(2) and a second molecule of Cbl(II) to primarily form nitrocobalamin. In support of this mechanism, a Cbl(II)/ONOO(H) stoichiometry of 2:1 is observed at pH 7.35 and 12.0. The final major Cbl(III) product observed (nitrocobalamin or hydroxycobalamin) depends on the solution pH. Analysis of the reaction products in the presence of tyrosine-a well-established (.)NO(2) scavenger-reveals that Cbl(II) reacts with (.)NO(2) at least an order of magnitude faster than tyrosine itself. Given that protein-bound Cbl is accessible to small molecules, it is likely that enzyme-bound and free intracellular Cbl(II) molecules are rapidly oxidized to inactive Cbl(III) upon exposure to peroxynitrite or (.)NO(2).