Enzymatic Synthesis of the Methyl Group of Methionine

Investigating the effects of a novel rumen-protected folic acid supplement on feedlot performance and carcass characteristics of beef steers

Angus-crossbred steers (n = 180; 292 ± 18 kg) from a single ranch were used to investigate the effects of a novel rumen-protected folic acid (RPFA) supplement on feedlot performance and carcass characteristics. On d 0, steers were blocked by body weight to pens (5 steers/pen), and pens within a block were randomly assigned to dietary treatments (n = 6 pens/treatment): target intake of 0 (CON), 30 (RPFA-30), 60 (RPFA-60), 90 (RPFA-90), 120 (RPFA-120), or 150 (RPFA-150) mg RPFA·steer⁻¹·d⁻¹. Steers were weighed before feeding on d −1, 0, 55, 56, 86, 87, 181, and 182. Pen average daily gain (ADG), dry matter intake (DMI), and gain:feed (G:F) were calculated for growing (d 0 to 56), dietary transition (d 56 to 87), finishing (d 87 to 182), and overall (d 0 to 182). Liver and blood samples were collected from two steers/pen before trial initiation and at the end of growing and finishing. Steers were slaughtered on d 183, and carcass data were collected after a 48-h chill. Data were analyzed as a randomized complete block design using ProcMixed of SAS 9.4 (fixed effects of treatment and block; experimental unit of pen). Liver abscess scores were analyzed using the Genmod Procedure of SAS 9.4. Contrast statements assessed the polynomial effects of RPFA. Supplemental RPFA linearly increased plasma folate at the end of growing and finishing (P < 0.01), and linearly decreased plasma glucose at the end of growing (P = 0.01). There was a cubic effect of RPFA on liver folate at the end of growing (P = 0.01), driven by lesser concentrations for RPFA-30, RPFA-60, and RPFA-150. Growing period ADG and G:F were greatest for CON and RPFA-120 (cubic P ≤ 0.03). Transition period DMI was linearly increased due to RPFA (P = 0.05). There was a tendency for a cubic effect of RPFA on the percentage of livers with no abscesses (P = 0.06), driven by a greater percentage of non-abscessed livers in RPFA-30 and RPFA-60. Despite supplementing 1 mg Co/kg DM, and regardless of treatment, plasma vitamin B12 concentrations were low (<200 pg/mL), which may have influenced the response to RPFA as vitamin B12 is essential for recycling of folate.

Possible Involvement of Hydrosulfide in B12-Dependent Methyl Group Transfer

Evidence from several fields of investigation lead to the hypothesis that the sulfur atom is involved in vitamin B₁₂-dependent methyl group transfer. To compile the evidence, it is necessary to briefly review the following fields: methylation, the new field of sulfane sulfur/hydrogen sulfide (S°/H₂S), hydrosulfide derivatives of cobalamins, autoxidation of hydrosulfide radical, radical S-adenosylmethionine methyl transfer (RSMT), and methionine synthase (MS). Then, new reaction mechanisms for B₁₂-dependent methyl group transfer are proposed; the mechanisms are facile and overcome difficulties that existed in previously-accepted mechanisms. Finally, the theory is applied to the effect of S°/H₂S in nerve tissue involving the “hypomethylation theory” that was proposed 50 years ago to explain the neuropathology resulting from deficiency of vitamin B₁₂ or folic acid. The conclusions are consistent with emerging evidence that sulfane sulfur/hydrogen sulfide may be beneficial in treating Alzheimer’s disease.

A Phospholipid-Protein Complex from Antarctic Krill Reduced Plasma Homocysteine Levels and Increased Plasma Trimethylamine-N-Oxide (TMAO) and Carnitine Levels in Male Wistar Rats

Seafood is assumed to be beneficial for cardiovascular health, mainly based on plasma lipid lowering and anti-inflammatory effects of n-3 polyunsaturated fatty acids. However, other plasma risk factors linked to cardiovascular disease are less studied. This study aimed to penetrate the effect of a phospholipid-protein complex (PPC) from Antarctic krill on one-carbon metabolism and production of trimethylamine-N-oxide (TMAO) in rats. Male Wistar rats were fed isoenergetic control, 6%, or 11% PPC diets for four weeks. Rats fed PPC had reduced total homocysteine plasma level and increased levels of choline, dimethylglycine and cysteine, whereas the plasma level of methionine was unchanged compared to control. PPC feeding increased the plasma level of TMAO, carnitine, its precursors trimethyllysine and γ-butyrobetaine. There was a close correlation between plasma TMAO and carnitine, trimethyllysine, and γ-butyrobetaine, but not between TMAO and choline. The present data suggest that PPC has a homocysteine lowering effect and is associated with altered plasma concentrations of metabolites related to one-carbon metabolism and B-vitamin status in rats. Moreover, the present study reveals a non-obligatory role of gut microbiota in the increased plasma TMAO level as it can be explained by the PPC's content of TMAO. The increased level of carnitine and carnitine precursors is interpreted to reflect increased carnitine biosynthesis.

In Silico Analysis of Sequence-Structure-Function Relationship of the Escherichia coli Methionine Synthase

The molecular evolution of various metabolic pathways in the organisms can be employed for scrutinizing the molecular aspects behind origin of life. In the present study, we chiefly concerned about the sequence-structure-function relationship between the Escherichia coli methionine synthase and their respective animal homologs by in silico approach. Using homology prediction technique, it was observed that only 79 animal species showed similarity with the E. coli methionine synthase. Also, multiple sequence alignment depicted only 25 conserved patterns between the E. coli methionine synthase and their respective animal homologs. Based on that, Pfam analysis identified the protein families of 22 conserved patterns among the attained 25 conserved patterns. Furthermore, the 3D structure was generated by HHpred and evaluated by corresponding Ramachandran plot specifying 93% of the ϕ and ψ residues angles in the most ideal regions. Hence, the designed structure was established as a good quality model for the full length of E. coli methionine synthase.

In silico analysis of sequence-structure-function relationship of the Escherichia coli methionine synthase

The molecular evolution of various metabolic pathways in the organisms can be employed for scrutinizing the molecular aspects behind origin of life. In the present study, we chiefly concerned about the sequence-structure-function relationship between the E. coli methionine synthase and their respective animal homologs by In-silico approach. Using homology prediction technique, it was observed that only 79 animal species showed similarity with the E. coli methionine synthase. Also, multiple sequence alignment depicted only 25 conserved patterns between the E. coli methionine synthase and their respective animal homologs. Based on that, Pfam analysis identified the protein families of 22 conserved patterns amongst the attained 25 conserved patterns. Furthermore, the 3D structure was generated by HHpred and evaluated by corresponding Ramachandran plot specifies 93% of the ϕ and □ residues angles in the most ideal regions. Hence, the designed structure was established as good quality model for the full length of E. coli methionine synthase.

Recognition of age-damaged (R,S)-adenosyl-L-methionine by two methyltransferases in the yeast Saccharomyces cerevisiae

The biological methyl donor S-adenosylmethionine (AdoMet) can exist in two diastereoisomeric states with respect to its sulfonium ion. The S configuration, (S,S)-AdoMet, is the only form that is produced enzymatically as well as the only form used in almost all biological methylation reactions. Under physiological conditions, however, the sulfonium ion can spontaneously racemize to the R form, producing (R,S)-AdoMet. As of yet, (R,S)-AdoMet has no known physiological function and may inhibit cellular reactions. In this study, we found two Saccharomyces cerevisiae enzymes that are capable of recognizing (R,S)-AdoMet and using it to methylate homocysteine to form methionine. These enzymes are the products of the SAM4 and MHT1 genes, identified previously as homocysteine methyltransferases dependent upon AdoMet and S-methylmethionine, respectively. We found here that Sam4 recognizes both (S,S)- and (R,S)-AdoMet, but that its activity is much higher with the R,S form. Mht1 reacts with only the R,S form of AdoMet, whereas no activity is seen with the S,S form. R,S-Specific homocysteine methyltransferase activity is also shown here to occur in extracts of Arabidopsis thaliana, Drosophila melanogaster, and Caenorhabditis elegans, but has not been detected in several tissue extracts of Mus musculus. Such activity may function to prevent the accumulation of (R,S)-AdoMet in these organisms.

Human methionine synthase reductase is a molecular chaperone for human methionine synthase

Sustained activity of mammalian methionine synthase (MS) requires MS reductase (MSR), but there have been few studies of the interactions between these two proteins. In this study, recombinant human MS (hMS) and MSR (hMSR) were expressed in baculovirus-infected insect cells and purified to homogeneity. hMSR maintained hMS activity at a 1:1 stoichiometric ratio with a K(act) value of 71 nM. Escherichia coli MS, however, was not activated by hMSR. Moreover, hMS was not significantly active in the presence of E. coli flavodoxin and flavodoxin reductase, which maintain the activity of E. coli MS. These results indicate that recognition of MS by their reductive partners is very strict, despite the high homology between MS from different species. The effects of hMSR on the formation of hMS holoenzyme also were examined by using crude extracts of baculovirus-infected insect cells containing hMS apoenzyme (apoMS). In the presence of MSR and NADPH, holoenzyme formation from apoMS and methylcobalamin was significantly enhanced. The observed stimulation is shown to be due to stabilization of human apoMS in the presence of MSR. Apoenzyme alone is quite unstable at 37 degrees C. MSR also is able to reduce aquacobalamin to cob(II)alamin in the presence of NADPH, and this reduction leads to stimulation of the conversion of apoMS and aquacobalamin to MS holoenzyme. Based on these findings, we propose that MSR serves as a special chaperone for hMS and as an aquacobalamin reductase, rather than acting solely in the reductive activation of MS.

Assays of methylenetetrahydrofolate reductase and methionine synthase activities by monitoring 5-methyltetrahydrofolate and tetrahydrofolate using high-performance liquid chromatography with fluorescence detection

We developed a method for assays of methylenetetrahydrofolate reductase and methionine synthase activities by monitoring their products of 5-methyltetrahydrofolate (5-CH(3)-H(4)folate) and tetrahydrofolate (H(4)folate) directly, using high-performance liquid chromatography with fluorescence detection. Folate derivatives and enzymes were stable in the assay process. No reagents in the assay mixture were found to disturb the separation and detection of both H(4)folate and 5-CH(3)-H(4)folate in our assay system. The detection limit of this method was less than 20 nM H(4)folate or 5-CH(3)-H(4)folate in the enzyme assay system. This analytical method, therefore, has a sensitivity high enough to obtain accurate parameters of Michaelis-Menten kinetics and for assays of crude extracts from various biological samples. In addition, the analytical procedure is very simple and economical; it may be a useful tool for studying methylenetetrahydrofolate reductase and methionine synthase activities.

Radioenzymatic assay for reductive catalysis of N(5)N(10)-methylenetetrahydrofolate by methylenetetrahydrofolate reductase

Methylenetetrahydrofolate reductase catalyzes the reduction of N(5), N(10)-methylenetetrahydrofolate to N(5)-methyltetrahydrofolate. Because this substrate is unstable and dissociates spontaneously into formaldehyde and tetrahydrofolate, the customary method to assay the catalytic activity of this enzyme has been to measure the oxidation of [14C]N(5)-methyltetrahydrofolate to N(5), N(10)-methylenetetrahydrofolate and quantify the [14C]formaldehyde that dissociates from this product. This report describes a very sensitive radioenzymatic assay that measures directly the reductive catalysis of N(5),N(10)-methylenetetrahydrofolate. The radio-labeled substrate, [14C]N(5),N(10)-methylenetetrahydrofolate, is prepared by condensation of [C(14)]formaldehyde with tetrahydrofolate and the stability of this substrate is maintained for several months by storage at -80 degrees C in a pH 9.5 buffer. Partially purified methylenetetrahydrofolate reductase from rat liver, incubated with the radio-labeled substrate and the cofactors, NADPH and FAD at pH 7. 5, generates [14C]N(5)-methyltetrahydrofolate, which is stable and partitions into the aqueous phase after the assay is terminated with dimedone and toluene. A K(m) value of 8.2 microM was obtained under conditions of increasing substrate concentration to ensure saturation kinetics. This method is simple, very sensitive and measures directly the reduction of N(5), N(10)-methylenetetrahydrofolate to N(5)-methyltetrahydrofolate, which is the physiologic catalytic pathway for methylenetetrahydrofolate reductase.

Homocysteine: a history in progress

This paper shows that the linkage between basic science and clinical research has characterized the field of sulfur amino acid metabolism since 1810, when Wollaston isolated cystine from a human bladder stone. The nature and consequences of this relationship are discussed.

The structure of the C-terminal domain of methionine synthase: presenting S-adenosylmethionine for reductive methylation of B12

BACKGROUND

In both mammalian and microbial species, B12-dependent methionine synthase catalyzes methyl transfer from methyltetrahydrofolate (CH3-H4folate) to homocysteine. The B12 (cobalamin) cofactor plays an essential role in this reaction, accepting the methyl group from CH3-H4folate to form methylcob(III)alamin and in turn donating the methyl group to homocysteine to generate methionine and cob(I)alamin. Occasionally the highly reactive cob(I)alamin intermediate is oxidized to the catalytically inactive cob(II)alamin form. Reactivation to sustain enzyme activity is achieved by a reductive methylation, requiring S-adenosylmethionine (AdoMet) as the methyl donor and, in Esherichia coli, flavodoxin as an electron donor. The intact system is controlled and organized so that AdoMet, rather than methyltetrahydrofolate, is the methyl donor in the reactivation reaction. AdoMet is not wasted as a methyl donor in the catalytic cycle in which methionine is synthesized from homocysteine. The structures of the AdoMet binding site and the cobalamin-binding domains (previously determined) provide a starting point for understanding the methyl transfer reactions of methionine synthase.

RESULTS

We report the crystal structure of the 38 kDa C-terminal fragment of E.coli methionine synthase that comprises the AdoMet-binding site and is essential for reactivation. The structure, which includes residues 901-1227 of methionine synthase, is a C-shaped single domain whose central feature is a bent antiparallel betasheet. Database searches indicate that the observed polypeptide has no close relatives. AdoMet binds near the center of the inner surface of the domain and is held in place by both side chain and backbone interactions.

CONCLUSIONS

The conformation of bound AdoMet, and the interactions that determine its binding, differ from those found in other AdoMet-dependent enzymes. The sequence Arg-x-x-x-Gly-Tyr is critical for the binding of AdoMet to methionine synthase. The position of bound AdoMet suggests that large areas of the C-terminal and cobalamin-binding fragments must come in contact in order to transfer the methyl group of AdoMet to cobalamin. The catalytic and activation cycles may be turned off and on by alternating physical separation and approach of the reactants.

Demonstration that mammalian methionine synthases are predominantly cobalamin-loaded

Methionine synthase is an important cellular housekeeping enzyme and is dependent on the cofactor cobalamin, a derivative of vitamin B12, for activity. It functions in two major metabolic pathways including the tetrahydrofolate-dependent one-carbon cycle and the salvage pathway for methionine. Its dysfunction has several physiological ramifications and leads to the development of megaloblastic anemia. In addition, it is suspected to be involved in the pathogenesis of neural tube defects. An issue that is central in weighing therapeutic options for methionine synthase-related disorders is the extent to which the enzyme exists as apoenzyme in vivo and, thus, can be potentially responsive to vitamin B12 therapy. despite the importance of this issue, the extent of holo- versus apoenzyme in mammalian tissue is controversial and unresolved. To address this question, we have developed a convenient anaerobic assay that employs titanium citrate to deliver low potential electron equivalents. The reductive activation of this enzyme is essential under in vitro assay conditions. We find that both the human placental and porcine liver methionine synthases exist predominantly in the holoenzyme form (90-100%) in the crude homogenate. In addition, the activity of the pure enzyme measured in the titanium citrate assay is also independent of exogenous cofactor, revealing that the cobalamin is tightly bound to the active site.

Cobalamin-dependent methionine synthase: the structure of a methylcobalamin-binding fragment and implications for other B12-dependent enzymes

Cobalamin-dependent methionine synthase is a large enzyme composed of structurally and functionally distinct regions. Recent studies have begun to define the roles of several regions of the protein. In particular, the structure of a 27 kDa cobalamin-binding fragment of the enzyme from Escherichia coli has been determined by X-ray crystallography, and has revealed the motifs and interactions responsible for recognition of the cofactor. The amino acid sequences of several adenosylcobalamin-dependent enzymes, the methylmalonyl coenzyme A mutases and glutamate mutases, show homology with the cobalamin-binding region of methionine synthase and retain conserved residues that are determinants for the binding of the prosthetic group, suggesting that these mutases and methionine synthase share common three-dimensional structures.

Assignment of enzymatic function to specific protein regions of cobalamin-dependent methionine synthase from Escherichia coli

Cobalamin-dependent methionine synthase catalyzes methyl group transfer from methyltetrahydrofolate to homocysteine to form tetrahydrofolate and methionine, and the cobalamin prosthetic group serves as an intermediate methyl carrier. Enzyme possessing cobalamin in the cobalt(II) oxidation state is inactive, and this form is activated by one-electron reduction coupled to methylation by S-adenosylmethionine (AdoMet). The enzyme from Escherichia coli has been divided into separable fragments by limited proteolysis with trypsin, and the contribution of each of these fragments to substrate binding and catalysis has been evaluated. The 37.7-kDa carboxyl-terminal domain binds AdoMet, and this was demonstrated through covalent modification with radiolabeled AdoMet during ultraviolet irradiation. Following reductive activation with AdoMet, the enzyme was digested with trypsin and a 98.4-kDa amino-terminal fragment was isolated. It retained at least 70% of the activity of the intact enzyme and must therefore possess determinants sufficient for the binding of methyltetrahydrofolate and homocysteine, as well as residues required for catalysis. However, when the cobalamin was oxidized to the cob(II) alamin state, the 98.4-kDa fragment could not be reductively remethylated with AdoMet. A purified, 28-kDa domain within the 98.4-kDa fragment retained bound cobalamin and therefore must play a central role in catalysis, but the isolated 28-kDa domain retained no catalytic activity. Because AdoMet binds to a different domain of the protein than methyltetrahydrofolate and homocysteine, the enzyme probably uses conformational flexibility to allow the cobalamin access to the required methyl donor or acceptor at the appropriate time in catalysis.(ABSTRACT TRUNCATED AT 250 WORDS)

A nonradioactive assay for N5-methyltetrahydrofolate-homocysteine methyltransferase (methionine synthase) based on o-phthaldialdehyde derivatization of methionine and fluorescence detection

The enzyme N5-methyltetrahydrofolate-homocysteine methyltransferase (methionine synthase, EC 2.1.1.13) catalyzes the conversion of homocysteine to methionine in the presence of a reducing system. N5-Methyltetrahydrofolate serves as a methyl donor in this reaction. An assay for the enzyme is described, which is based on methionine quantitation by o-phthaldialdehyde (OPA) derivatization and reversed-phase liquid chromatography. The enzymatic reaction is linear for at least 120 min under reducing conditions (125 mM 2-mercaptoethanol) and running the assay below an oil layer. This reducing system does not interfere with formation of the methionine-OPA adduct, which is separated from interfering compounds and an internal standard (norvaline) by a mobile phase adjusted to pH 5.0. The inclusion of internal standard increases the precision of the assay and corrects for the variable fluorescence yield due to occasional inaccurate pH adjustment before the derivatization step. Norvaline was suitable for this purpose because it elutes close to methionine and is not a natural amino acid present in biological extracts. This nonradioactive assay for methionine synthase was evaluated by comparison with a conventional method based on isolation of radioactive methionine by anion-exchange chromatography and by determination of enzyme activity in extract from cultured cells and liver.

Enzymatic synthesis and function of folylpolyglutamates

Derivatives of folic acid occur in nature predominantly as poly (gamma-glutamyl) derivatives containing 2-8 glutamate residues. The data regarding the function of these derivatives, and their biosynthesis by eucaryotic and procaryotic folylpolyglutamate synthetases, is reviewed. The most universal functions of folylpolyglutamates appear to be (a) as the actual cofactors in vivo for folate dependent enzymes, (b) as inhibitors of folate dependent enzymes for which they are not substrates, and (c) to increase retention of folates after they are transported into cells as monoglutamates. Folylpolyglutamates also have numerous specialized functions in specific organisms, e.g. as structural components of some coliphage, and as allosteric regulators in Neurospora crassa. A single enzyme appears responsible for synthesis of all polyglutamate derivatives, regardless of length. With the recent introduction of sensitive assays this folylpolyglutamate synthetase has begun to be characterized. Although procaryotic and eucaryotic synthetases have many dissimilar properties, both types catalyze the ATP-dependent addition of L-glutamate to the gamma-carboxyl of the glutamate present in the folate. Both types also require a monovalent cation and relatively high pH. The most significant differences between the two types are in their folate substrate specificity and the product lengths derived from various folates.

Effect of vitamin B12 deficiency on phosphatidylethanolamine methylation in rat liver

1. In vitamin B12 deficiency the activity of tetrahydropteroylglutamate methyltransferase (EC 2.1.1.13) is depressed and the synthesis of methionine is reduced. Because the methyl group of methionine is largely utilized for the methylation of phosphatidylethanolamine, we investigated the effects of vitamin B12 deficiency on phosphatidylcholine synthesis. 2. The incorporation of injected [14C]formaldehyde into liver phosphatidylcholine was reduced by approximately 50% in vitamin B12-deficient rats. Also the corresponding incorporation of 5-[14C]methyltetrahydrofolic acid tended to decrease. The findings are consistent with a lower conversion of these precursors to methionine. 3. The effect of the deficient methyl-group supply on phosphatidylcholine synthesis was also investigated by the injection of [14C]ethanolamine. The amount (%) of lipid-14C recovered in phosphatidylcholine was significantly reduced in vitamin B12 deficiency. 4. Chemical analysis of liver phospholipids showed that the vitamin B12-deficient rats had a higher proportion of phosphatidylethanolamine and a lower proportion of phosphatidylcholine, indicating that the impaired synthesis of phosphatidylcholine by methylation leads to changes in membrane phospholipid composition.

Uptake and metabolism of [3H]folate by normal and by vitamin B-12- and methionine-deficient rats

The uptake of an injected dose of [3H]folic acid and its metabolism to pteroylpoly-gamma-glutamate forms by the livers and kidneys of vitamin B-12- and methionine-deficient and -supplemented rats were investigated. The initial hepatic uptake of the labeled folate dose was the same in deficient and supplemented animals, demonstrating no involvement of vitamin B-12 or methionine in folate transport. At longer time periods, a decreased hepatic net uptake of labeled folate was observed in the deficient animals compared to supplemented animals, and this was directly correlated with the decreased ability of the deficient animals to synthesize pteroylpolyglutamates. The absolute rate of loss of labeled pteroylmonoglutamate from liver was the same in deficient and supplemented animals. These data are best explained by a modification of the 'methyl trap' hypothesis for the interrelationship of vitamin B-12 and folate metabolism. Vitamin B-12 deficiency can lead to lowered levels of 5-methyltetrahydrofolate:homocysteine methyltransferase, creating a functional folate deficiency by 'trapping' an increased proportion of folate as the methyl derivative. In addition, as methyltetrahydrofolate is a poor substrate for folylpoly-gamma-glutamate synthetase, there is a decreased synthesis of pteroylpolyglutamates, the forms of the vitamin that are preferentially retained by tissues. This results in decreased tissue folate levels under conditions of vitamin B-12 deficiency. Vitamin B-12 and methionine deficiency had no significant effect on the distribution of endogenous pteroylpolyglutamates in rat liver and kidney, although total endogenous folate in rat liver was reduced by about 60%. The distribution of labeled pteroylpolyglutamates in rat liver and kidney 48 h after the tracer dose of [3H]folate closely resembled the endogenous distribution in these tissues.

5-methyl-5,6,7,8-tetrahydropteroyl oligo-gamma-L-glutamates: synthesis and kinetic studies with methionine synthetase from bovine brain

The synthesis of 5-methyl-5,6,7,8-tetrahydropteroly tri-, pena-, and heptaglutamate has been accomplished by reductive methylation of the tetrahydropteroyl oligoglutamate with formaldehyde, followed by purification on DEAE-Sephadex. The corresponding [5-14-C]methyltetrahydropteroyl oligoglutamates were prepared from 14-CH-2-0, and tested as substrates for methionine synthetase (EC 2.1.1.13) ISOLATED FROM BOVINE BRAIN. In all cases, the polyglutamate conjugates were better substrates (lower Km, higher Vmax) than the corresponding monoglutamate forms. In addition, the nonradioactive methyltetrahydropteroyl oligoglutamates inhibited the methylation of homocysteine by methyltetrahydrofolate. This indicates that the monoglutamate and polyglutamates compete for the same enzyme, and established a role for the ubiquitous methyltetrahydropteroyl oligoglutamates in mammalian methionine biosynthesis.

Vitamin B 12

In spite of the considerable progress made in recent years toward the understanding of the chemistry and biological function of the cobalt-containing B(12) group of compounds, much of the information still is more descriptive than definitive in nature. In general terms, it is known that the free vitamin forms can function as methyl group carriers and that the 5'-deoxyadenosyl or coenzyme forms serve as hydrogen carriers; but the mechanism of these processes is not understood in detail. More systematic studies of the pure chemistry of these complex molecules containing a carbon-cobalt covalent bond are needed before the biochemist can interpret many of his observations on the enzyme-catalyzed reactions. Even in relatively simple solutions it is difficult to ascertain the state of oxidation of several of the vitamin forms, and these problems are compounded when the reactive thiol compounds and complex proteins of the biological systems also are present. For example, both vitamin B(12r) (the Co(2+) form) and corresponding analogs are known to disproportionate in solution to B(12s) (Co(1+)) and B(12a) (Co(3+)) under a variety of mild conditions (12, 57). This means that in the biological systems it is exceedingly difficult to ascertain the chemical nature of many B(12) intermediates and reaction products. The role of the protein moiety of the various B(12)-linked enzymes in the catalytic processes is little known as is, also, the mode of binding of the B(12) derivative to the protein. These types of questions perhaps can be answered eventually by the crystallographers, whose art is becoming increasingly sophisticated. Note added after preparation of manuscript. In contrast to the values given in Table 4 for the molecular weights of the two dissimilar protein moieties of glycerol dehydrase, a recent report (57a), gives a value of 188,000 for the molecular weight of a stable, catalytically inactive complex of 1 mole of hydroxocobalamin and 1 mole of the apoenzyme complex of glycerol dehydrase. The latter is presumed to contain one equivalent of each of the two dissimilar protein subunits. The original estimate of 240,000 as the molecular weight of the unstable sulfhydryl protein moiety (39) was undoubtedly made on partially aggregated material.