Ethanol feeding of micropigs alters methionine metabolism and increases hepatocellular apoptosis and proliferation

Chronic alcoholism is associated with increased cancer risk that may be related to ethanol-induced alterations in methionine and deoxynucleotide metabolism. These metabolic relationships were studied in micropigs fed diets for 12 months that contained 40% ethanol or cornstarch control with adequate folate. Ethanol feeding altered methionine metabolism without changing mean terminal liver folate levels. After initial equilibration to diet, ethanol feeding significantly increased monthly serum homocysteine levels while reducing serum methionine levels over the time course of the experiment. After 12 months, hepatic methionine synthase activity and the ratio of S-adenosylmethionine (SAM) to S-adenosylhomocysteine (SAH) were significantly reduced in ethanol-fed animals, whereas the ratio of liver deoxyuridine triphosphate (dUTP) to deoxythymidine triphosphate (dTTP) was increased and correlated inversely with methionine synthase activity. These findings were associated with increased frequency of hepatocytes with apoptotic bodies and positivity for proliferating cell nuclear antigen (PCNA) in livers from ethanol-fed minipigs. These studies suggest that chronic ethanol feeding perturbs methionine metabolism by impairment of methionine synthase activity, resulting in deoxynucleoside triphosphate (dNTP) imbalance, increased apoptosis, and regenerative proliferation. These biochemical alterations may provide a promoting environment for carcinogenesis during long-term ethanol exposure.

High prevalence of cobalamin deficiency in the elderly

In summary, we have shown that there is a high prevalence of Cbl deficiency in the elderly. This observation is based on an increase in the number of low and low normal serum Cbl levels, an increase in elevated serum total homocysteine levels that correct with Cbl therapy, and an increase in elevated serum methylmalonic acid levels that also correct with Cbl therapy. These observations may be of great clinical importance since Cbl deficiency may be a common and treatable cause of vascular disease.

Effects of vitamin B12, folate, and vitamin B6 supplements in elderly people with normal serum vitamin concentrations

In a prospective, multicentre, double-blind controlled study, the effect of an intramuscular vitamin supplement containing 1 mg vitamin B12, 1.1 mg folate, and 5 mg vitamin B6 on serum concentrations of methylmalonic acid (MMA), homocysteine (HCYS), 2-methylcitric acid (2-MCA), and cystathionine (CYSTA) was compared with that of placebo in 175 elderly subjects living at home and 110 in hospital. Vitamin supplement and placebo were administered eight times over a 3-week period. Vitamin supplement but not placebo significantly reduced all four metabolite concentrations at the end of the study in both study groups. The maximum effects of treatment were usually seen within 5-12 days. Initially elevated metabolite concentrations returned to normal in a higher proportion of the vitamin than of the placebo group: 92% vs 20% for HYCS; 82% vs 20% for MMA; 62% vs 25% for 2-MCA; and 42% vs 25% for CYSTA. The response rate to vitamin supplements supports the notion that metabolic evidence of vitamin deficiency is common in the elderly, even in the presence of normal serum vitamin levels. Metabolite assays permit identification of elderly subjects who may benefit from vitamin supplements.

Simulating birth to investigate clinician-applied loads on newborns

This report describes the design, development and testing of a force-measuring birth stimulator. Designed to mimic the feel of an obstetric emergency, the simulator is named shoulder dystocia birth model (SDBM). The SDBM consists of a simply supported overhung beam, instrumented with two strain gauges and one commercially-available sensor, all of which are embedded in a maternal manikin that rests on a height-adjustable scissors jack. Rigidly affixed to the free end of the beam, a model of a newborn head protrudes from the manikin to allow physicians to simulate the loads they exert when delivering a newborn. A microcomputer-based data acquisition system monitors, analyses and processes the output from the gauges and the sensor in real time. The system measures axial force and vertical force to within 2% accuracy and the end-moment to within 5% accuracy. Testing with 15 University of Maryland clinicians simulating routine, difficult and SD deliveries reveals average peak force magnitudes of 68 N, 118 N and 172 N and average peak moment magnitudes of 138 N. cm, 384 N.cm and 621 N.cm, respectively. The standard deviations for these average peak values are large indicating wide variation in perceived loads required to deliver. Confirming clinical observation, average peak forces for some difficult and many shoulder dystocia deliveries exceed the force necessary to induce clavicle fracture at birth.

Comparing clinician-applied loads for routine, difficult, and shoulder dystocia deliveries

OBJECTIVE

Our goal was to examine and compare clinician-applied loads during simulated vaginal delivery.

STUDY DESIGN

We developed a birthing model and a microcomputer data acquisition system and used them to measure clinician-applied extraction forces, moment, and rates for three perceived categories of delivery. In 39 experiments, clinicians simulated delivery of the fetal shoulders during vaginal delivery for routine, difficult, and shoulder dystocia deliveries.

RESULTS

Clinicians averaged 84 N combined force and 473 N-cm neck-bending moment for routine deliveries, 122 N and 697 N-cm for difficult deliveries, and 163 N and 700 N-cm for shoulder dystocia deliveries (p < 0.002). No force or moment parameter was associated with clinician gender or experience. Force levels exceeding 100 N are reached for many clinicians (74% and 82%) for difficult and shoulder dystocia deliveries and for some clinicians (31%) for routine deliveries (p < 0.0001).

CONCLUSION

We conclude that simulating shoulder dystocia in the laboratory may be useful in measuring extraction forces and neck-bending moment and that birthing models can be used to train clinicians in force, moment, and rate perception.

Plasma cysteine concentrations in infants with respiratory distress

Because factors that predispose infants to persistent pulmonary hypertension of the newborn (PPHN) may cause oxidant stress, which in turn may increase demands for cysteine and glutathione, we investigated the availability of cysteine and its precursors in PPHN and related disorders. Plasma concentrations of four sulfur-containing and two non-sulfur-containing amino acids were measured by gas chromatography-mass spectrometry in blood from infants with PPHN, both those managed conventionally (PPHN group) and those treated with extracorporeal membrane oxygenation, as well as from infants with hyaline membrane disease. Concentrations also were measured in umbilical venous cord blood samples from a healthy control population, in venous plasma from infants receiving only intravenously administered glucose-containing solutions because they had noncardiopulmonary illnesses ("fasted" group), and from otherwise healthy, orally fed infants ("fed" group). The plasma total cyst(e)ine concentration was markedly lower in the three groups (PPHN, PPHN and extracorpeal membrane oxygenation, and hyaline membrane disease) receiving an elevated inspired oxygen concentration (0.6 to 1.0) than in fasted or fed control infants. In contrast, levels of plasma methionine, the other major sulfur amino acid, were low in the three groups receiving an elevated inspired oxygen concentration, as well as in fasted infants. Glycine and serine, two non-sulfur-containing amino acids, had a pattern similar to that of plasma methionine. Thus infants with PPHN and hyaline membrane disease have low plasma total cyst(e)ine levels, an effect that does not appear to result primarily from nutritional deprivation. We speculate that the role of cysteine in bioactivation of nitric oxide and as a precursor of glutathione may be relevant to the pathogenesis and evolution of PPHN and respiratory distress syndrome. Further studies are needed to determine whether increased demands for cysteine exist in these disorders.

Prevalence of cobalamin deficiency in the Framingham elderly population

To determine whether the increased prevalence of low serum cobalamin concentrations in elderly people represents true deficiency, serum concentrations of cobalamin and folate and of metabolites that are sensitive indicators of cobalamin deficiency were measured in 548 surviving members of the original Framingham Study cohort. Serum cobalamin concentrations < 258 pmol/L were found in 222 subjects (40.5%) compared with 17.9% of younger control subjects (P < 0.001). Serum methylmalonic acid and total homocysteine concentrations were markedly elevated in association with cobalamin values < 258 pmol/L in 11.3% and 5.7%, respectively, of the cohort. Both metabolites were increased in 3.8% of the cohort, associated with significantly lower erythrocyte counts and higher mean cell volumes. Serum metabolites correlated best with serum cobalamin values, even when subnormal determinations were excluded. The prevalence of cobalamin deficiency was > or = 12% in a large sample of free-living elderly Americans. Many elderly people with "normal" serum vitamin concentrations are metabolically deficient in cobalamin or folate.

Sensitivity of serum methylmalonic acid and total homocysteine determinations for diagnosing cobalamin and folate deficiencies

PURPOSE

Patients with cobalamin (vitamin B12) deficiency usually lack many of the classic features of severe megaloblastic anemia; because of the low diagnostic specificity of decreased serum cobalamin levels, demonstrating the deficiency unequivocally is often difficult. We examined the sensitivity of measuring serum concentrations of methylmalonic acid and total homocysteine for diagnosing patients with clear-cut cobalamin deficiency and compared the results with those of patients with clear-cut folate deficiency.

PATIENTS AND METHODS

Serum metabolites were measured for all patients seen from 1982 to 1989 at two university hospitals who met the criteria for cobalamin and folate deficiency states and for such patients seen from 1968 to 1981 from whom stored sera were available. In all, 406 patients had 434 episodes of cobalamin deficiency and 119 patients had 123 episodes of folate deficiency. Criteria for deficiency states included serum vitamin levels, hematologic and neurologic findings, and responses to therapy. Responses were documented in 97% of cobalamin-deficient patients and 76% of folate-deficient patients. Metabolite levels were measured by modified techniques using capillary-gas chromatography and mass spectrometry.

RESULTS

Most of the cobalamin-deficient patients had underlying pernicious anemia; two thirds were blacks or Latinos. Hematocrits were normal in 28% and mean cell volumes in 17%. Of the 434 episodes of cobalamin deficiency, 98.4% of serum methylmalonic acid levels and 95.9% of serum homocysteine levels were elevated (greater than 3 standard deviations above the mean in normal subjects). Only one patient had normal levels of both metabolites. Serum homocysteine levels were increased in 91% of the 123 episodes of folate deficiency. Methylmalonic acid was elevated in 12.2% of the folate-deficient patients; in all but one, the elevation was attributable to renal insufficiency or hypovolemia.

CONCLUSIONS

For the cobalamin-deficient patients, measuring serum metabolite concentrations proved to be a highly sensitive test of deficiency. We conclude that normal levels of both methylmalonic acid and total homocysteine rule out clinically significant cobalamin deficiency with virtual certainty.

Metabolic abnormalities in cobalamin (vitamin B12) and folate deficiency

Mammalian cells contain two Cbl-dependent enzymes, L-methylmalonyl-CoA mutase and methionine synthase. The former requires adenosyl-Cbl and catalyzes the conversion of L-methylmalonyl-CoA to succinyl-CoA. The latter requires CH3-Cbl and catalyzes the conversion of 5-CH3-tetrahydrofolate and homocysteine to tetrahydrofolate and methionine, respectively. Biochemical abnormalities related to a decrease in the activity of methionine synthase are thought to be responsible for the indistinguishable hematologic abnormalities seen in both Cbl and folate deficiency. The biochemical basis for the neuropsychiatric abnormalities seen in Cbl deficiency, but not in folate deficiency, is not known although hypotheses have been proposed that implicate one or the other of the two Cbl-dependent enzymes. Recent studies have shown that levels of serum methylmalonic acid, 2-methylcitric acids I and II, total homocysteine, and cystathionine are elevated in most patients with Cbl deficiency and that total homocysteine, cystathionine, N,N-dimethylglycine, and N-methylglycine are elevated in most patients with folate deficiency. Analysis of these metabolic abnormalities in various patient groups fails to support hypotheses that either L-methylmalonyl-CoA mutase or methionine synthase alone are responsible for the neuropsychiatric abnormalities. We suggest that they may result from a third, unknown mammalian Cbl-dependent enzyme or from a combined deficiency of both Cbl-dependent enzymes together with an unknown genetic or environmental factor.

Serum betaine, N,N-dimethylglycine and N-methylglycine levels in patients with cobalamin and folate deficiency and related inborn errors of metabolism

Homocysteine and 5-CH3-tetrahydrofolate (5-CH3-THF) are converted to methionine and THF by the CH3-cobalamin (CH3-Cbl)-dependent enzyme methionine synthase. Serum homocysteine levels are elevated in more than 95% of patients with Cbl or folate deficiency and in patients with inborn errors involving the synthesis of 5-CH3-THF or CH3-Cbl. Homocysteine and betaine are converted to methionine and N,N-dimethylglycine by betaine-homocysteine methyltransferase. It requires neither Cbl nor folate, although N,N-dimethylglycine is converted to N-methylglycine and then to glycine in reactions that both involve the formation of 5,10-CH2-THF from THF. Large amounts of betaine are often given orally to patients with inborn errors, even though little is known about its metabolism in normal subjects or these patients. Thus we developed new gas chromatographic-mass spectrometric assays for serum betaine, N,N-dimethylglycine, and N-methylglycine. In 60 blood donors, we found ranges for normal serum of 17.6 to 73.3, 1.42 to 5.27, and 0.60 to 2.67 mumol/L for the three metabolites, respectively, which were normal in the majority of 50 patients with Cbl deficiency, none of whom had increased levels of N-methylglycine. In 25 patients with folate deficiency, serum betaine level was normal in most, but 76% and 60% had elevations of N,N-dimethylglycine and N-methylglycine levels that ranged as high as 343 and 43.2 mumol/L, respectively. All of seven patients on betaine therapy for inborn errors had high values for betaine (167 to 3,900 mumol/L), N,N-dimethylglycine (15.1 to 250 mumol/L), and N-methylglycine (2.93 to 49.3 mumol/L). Serum total homocysteine levels remained very high at 47.2 to 156 mumol/L (normal, 5.4 to 16.2). In patients with cbl C and cbl D mutations, methionine levels remained low or low-normal at 8.3 to 15.6 mumol/L (normal, 13.3 to 42.7) despite betaine treatment. We conclude that (1) betaine levels are maintained in most patients with Cbl and folate deficiency; (2) levels of N,N-dimethylglycine and N-methylglycine are increased in most patients with folate deficiency; and (3) betaine therapy is relatively ineffective in patients with defective synthesis of CH3-Cbl.

Metabolic evidence that deficiencies of vitamin B-12 (cobalamin), folate, and vitamin B-6 occur commonly in elderly people

Measurements of the serum concentrations of the metabolites homocysteine, cystathionine, methylmalonic acid, and 2-methylcitric acid, which accumulates when vitamin B-12-, folate-, and vitamin B-6-dependent enzymatic reactions are impaired, should provide a better indication of intracellular deficiency of these vitamins. We measured the serum concentration of these vitamins and the four metabolites in 99 healthy young people, 64 healthy elderly subjects, and 286 elderly hospitalized patients. A low serum vitamin B-12 concentration was found in 6% and 5%, low folate in 5% and 19%, and low vitamin B-6 in 9% and 51%, and one or more metabolites were elevated in 63% and 83% of healthy elderly subjects and elderly hospitalized patients, respectively. These results strongly suggest that the prevalence of tissue deficiencies of vitamin B-12, folate, and vitamin B-6 as demonstrated by the elevated metabolite concentrations is substantially higher than that estimated by measuring concentrations of the vitamins.

Total homocysteine in plasma or serum: methods and clinical applications

Total homocysteine is defined as the sum of all homocysteine species in plasma/serum, including free and protein-bound forms. In the present review, we compare and evaluate several techniques for the determination of total homocysteine. Because these assays include the conversion of all forms into a single species by reduction, the redistribution between free and protein-bound homocysteine through disulfide interchange does not affect the results, and total homocysteine can be measured in stored samples. Total homocysteine in whole blood increases at room temperature because of a continuous production and release of homocysteine from blood cells, but artificial increase is low if the blood sample is centrifuged within 1 h of collection or placed on ice. Different methods correlate well, and values between 5 and 15 mumol/L in fasting subjects are considered normal. Total homocysteine in serum/plasma is increased markedly in patients with cobalamin or folate deficiency, and decreases only when they are treated with the deficient vitamin. Total homocysteine is therefore of value for the diagnosis and follow-up of these deficiency states and may compensate for weaknesses of the traditional laboratory tests. In addition, total homocysteine is an independent risk factor for premature cardiovascular diseases. These disorders justify introduction of the total homocysteine assay in the routine clinical chemistry laboratory.

Total homocysteine in plasma or serum: methods and clinical applications

Total homocysteine is defined as the sum of all homocysteine species in plasma/serum, including free and protein-bound forms. In the present review, we compare and evaluate several techniques for the determination of total homocysteine. Because these assays include the conversion of all forms into a single species by reduction, the redistribution between free and protein-bound homocysteine through disulfide interchange does not affect the results, and total homocysteine can be measured in stored samples. Total homocysteine in whole blood increases at room temperature because of a continuous production and release of homocysteine from blood cells, but artificial increase is low if the blood sample is centrifuged within 1 h of collection or placed on ice. Different methods correlate well, and values between 5 and 15 mumol/L in fasting subjects are considered normal. Total homocysteine in serum/plasma is increased markedly in patients with cobalamin or folate deficiency, and decreases only when they are treated with the deficient vitamin. Total homocysteine is therefore of value for the diagnosis and follow-up of these deficiency states and may compensate for weaknesses of the traditional laboratory tests. In addition, total homocysteine is an independent risk factor for premature cardiovascular diseases. These disorders justify introduction of the total homocysteine assay in the routine clinical chemistry laboratory.

Elevation of 2-methylcitric acid I and II levels in serum, urine, and cerebrospinal fluid of patients with cobalamin deficiency

Citrate synthase catalyzes the condensation of acetyl-coenzyme A (CoA) and oxaloacetic acid to form citric acid. The enzyme also catalyzes the condensation of propionyl-CoA and oxaloacetic acid with a maximal reaction velocity (Vmax) approximately 10(-4) times that of acetyl-CoA to form 2-methylcitric acid, which contains two asymmetric carbon atoms and exists as two pairs of related enantiomers designated as 2-methylcitric acid I and II. Cobalamin (Cbl) deficiency can lead to increases in intracellular levels of propionyl-CoA. To assess the magnitude of increased synthesis of 2-methylcitric acid in Cbl deficiency, we developed a new capillary gas chromatographic-mass spectrometric assay and measured 2-methylcitric acid levels in serum and cerebrospinal fluid (CSF) of normal subjects and patients with clinically confirmed Cbl deficiency. The normal range for 2-methylcitric acid level was 60 to 228 nmol/L for serum in 50 normal blood donors and 323 to 1,070 nmol/L for CSF in 19 normal subjects. In 50 patients with clinically confirmed Cbl deficiency, values for 2-methylcitric acid in serum ranged from 93 to 13,500 nmol/L; 44 (88%) had values above the normal range. In five patients with clinically confirmed Cbl deficiency, levels of the sum of 2-methylcitric acid I and II ranged from 1,370 to 16,300 nmol/L in CSF, and all five (100%) patients had levels above the normal range. We conclude that levels of 2-methylcitric acid are elevated in serum and CSF of most patients with Cbl deficiency.

Elevation of serum cystathionine levels in patients with cobalamin and folate deficiency

Homocysteine can be methylated to form methionine by the cobalamin- (Cbl) and folate-dependent enzyme, methionine synthase; serum levels of total homocysteine are elevated in greater than 95% of patients with either Cbl or folate deficiency. Homocysteine can also condense with serine to form cystathionine in a pyridoxal phosphate-dependent reaction catalyzed by cystathionine beta-synthase. Cystathionine is subsequently cleaved to cysteine and alpha-ketobutyrate by the pyridoxal phosphate-dependent enzyme gamma-cystathionase. To assess levels of cystathionine in Cbl and folate deficiency, we developed a new capillary gas chromatographic-mass spectrometric assay and measured cystathionine in the serum of normal subjects and patients with clinically confirmed deficiencies of these vitamins. The normal range for serum cystathionine was 65 to 301 nmol/L (median = 126 nmol/L) for 50 normal blood donors. In 30 patients with clinically confirmed Cbl deficiency, values for cystathionine ranged from 208 nmol/L to 2,920 nmol/L (median = 816 nmol/L) and 26 (87%) had levels above the normal range. In 20 patients with clinically confirmed folate deficiency, cystathionine concentrations ranged from 138 nmol/L to 4,150 nmol/L (median = 1,560 nmol/L) and 19 (95%) had values above the normal range. Five homozygotes for cystathionine beta-synthase deficiency had high values for serum-total homocysteine and low or low-normal values for serum cystathionine that ranged from 30 nmol/L to 114 nmol/L even though they were on treatment with pyridoxine and had partially responded. One patient with a defect in the synthesis of 5-CH3-tetrahydrofolate and five patients with defects in the synthesis of CH3-Cbl had high values for serum-total homocysteine and high values for cystathionine that ranged from 311 nmol/L to 1,500 nmol/L even though they were on treatment with folic acid and Cbl, respectively, and had partially responded. We conclude that levels of cystathionine are evaluated in the serum of most patients with Cbl and folate deficiency and that they are useful in the differential diagnosis of an elevated serum-total homocysteine level.

Identification and perturbation of mutant human fibroblasts based on measurements of methylmalonic acid and total homocysteine in the culture media

Human and mammalian cells contain two cobalamin-dependent enzymes. Methylmalonyl-CoA mutase isomerizes L-methylmalonyl-CoA to succinyl-CoA in the propionyl-CoA pathway while methionine synthase catalyzes the transfer of the methyl group of 5-methyltetrahydrofolate to homocysteine to form methionine. Decreased activity of mutase leads to an increased methylmalonic acid in the serum of humans while decreased activity of methionine synthase leads to increased homocysteine in the serum of humans. In current studies of cultured fibroblasts, methylmalonic acid levels were specifically increased in media of normal fibroblasts exposed to propionate or in fibroblasts with mutations involving mutase. Homocysteine levels were specifically increased in media of normal fibroblasts exposed to reduced folate concentrations or in fibroblasts involving mutations of methionine synthase. In addition, exposure of normal cells to inhibitory cobalamin analogues resulted in an increase of both methylmalonic acid and homocysteine in the media. This method of analysis appears to be both specific and sensitive for reduced activity of these two enzymes in tissue culture.

High prevalence of cobalamin deficiency in elderly outpatients

OBJECTIVE

To measure the prevalence of cobalamin (vitamin B12) deficiency in geriatric outpatients as documented by both low serum cobalamin levels and elevations of serum methylmalonic acid and homocysteine and to determine the response to cobalamin treatment.

DESIGN

Prospective study screening elderly subjects for cobalamin deficiency using radiodilution cobalamin assays as well as stable isotope dilution gas chromatography-mass spectrometry methylmalonic acid and homocysteine assays. In patients with serum cobalamin levels < or = 300 pg/mL, the response to cobalamin treatment in the group with levels of methylmalonic acid and/or homocysteine > 3 standard deviations (SD) above the mean for normals was compared with that of those without such elevations.

SETTING

Outpatient geriatric clinics at the VA Medical Center and University Health Sciences Center, Denver, CO.

PATIENTS

One-hundred and fifty-two consecutive outpatients, ages 65 to 99, were screened. Twenty-nine subjects with serum cobalamin levels < or = 300 pg/mL were prospectively evaluated and treated with cobalamin.

MAIN OUTCOME MEASURES

Cobalamin, methylmalonic acid, homocysteine, complete blood counts, neurologic examination, and neuropsychological testing.

RESULTS

The prevalence of cobalamin deficiency as defined by a serum cobalamin level < or = 300 pg/mL and levels of serum methylmalonic acid and/or homocysteine elevated to > 3 SD was 14.5% of the screened outpatients. A similar proportion of patients with low normal serum cobalamin levels (between 201 and 300 pg/mL) demonstrated elevated metabolites > 3 SD (56%) compared with patients with low serum cobalamin levels (< or = 200 pg/mL) (62%). Cobalamin therapy caused a marked fall or complete correction of the elevated methylmalonic acid and homocysteine levels in each patient who was treated prospectively. Results for complete blood count, lactate dehydrogenase, bilirubin, baseline neurologic score, and baseline neuropsychologic scores did not differ in the group of patients with elevated metabolites compared with those with normal metabolites. The mean red cell volume fell significantly in the patients with elevated metabolites after 6 months of cobalamin treatment. One patient with elevated metabolites had marked improvement in his neurologic abnormalities after 6 months of cobalamin treatment.

CONCLUSION

There was a high (14.5%) prevalence of cobalamin deficiency as demonstrated by elevations in serum methylmalonic acid and homocysteine in addition to low or low normal serum cobalamin levels in elderly outpatients. The serum cobalamin level was insensitive for screening since similar numbers of patients with low normal serum cobalamin levels of 201-300 pg/mL compared with patients with low cobalamin levels (< or = 200 pg/mL) had markedly elevated metabolites which fell with cobalamin treatment. Additional studies will be required to define the full clinical benefit from treatment with Cbl in elderly subjects.

Analysis of the nucleoside moiety of cobalamin and cobalamin analogues using gas chromatography-mass spectrometry

Existing techniques for identification of cobalamin and cobalamin analogues generally use the intact molecule during characterization with somewhat ambiguous results. In this study a method is described for the identification of the nucleoside in the lower axial ligand of cobalamin and a variety of naturally occurring cobalamin analogues that differ from cobalamin in the base that is present in the nucleoside. Cobalamin and cobalamin analogues were isolated from biological samples by affinity chromatography using R-protein-Sepharose columns. The nucleosides of the lower axial ligand were then hydrolyzed and isolated by column chromatography using a mixed bed column. Nucleosides were oxidized with periodate and reduced with borohydride. After reisolation, the t-butyldimethylsilyl derivatives were prepared and analyzed using gas chromatography/mass spectrometry with selected ion monitoring. A stable isotope internal standard of cobalamin was biosynthetically produced and used to quantitate cobalamin in rabbit kidney. Cobalamin analogues were also shown to be present in rabbit kidney, but they contain the 5,6-dimethylbenzimidazole nucleoside (alpha-ribazole) in the lower axial ligand, indicating that these analogues differ from cobalamin in the corrin ring region of the molecule.

Mechanism of conversion of human apo- to holomethionine synthase by various forms of cobalamin

Methionine synthase catalyzes the conversion of N5-methyltetrahydrofolate and homocysteine to tetrahydrofolate and methionine. Methylcobalamin (Me-Cbl) is tightly bound to methionine synthase and is required for enzymatic activity. When added to crude tissue homogenates, Me-Cbl stimulates methionine synthase but similar stimulation is observed with hydroxocobalamin, cyanocobalamin (CN-Cbl), and adenosyl-Cbl, although the mechanisms involved are unknown. We prepared human apomethionine synthase and studied its activation in the presence of [14C]CN-Cbl and [14CH3]Me-Cbl with concentrations of 2-mercaptoethanol ranging from 0.15 to 100 mM. We observed that the removal of the labeled upper axial ligands from CN-Cbl and Me-Cbl both paralleled the activation of human apomethionine synthase. Spectral studies employing CN-Cbl and Me-Cbl showed that both forms of Cbl must be converted to Cob(II)alamin before they can bind to human apomethionine synthase and convert it to its activated holoenzyme form. Studies with 14 different Cbl analogues with alterations in various portions of the corrin ring and the nucleotide showed that all of the analogues were able to fully activate human methionine synthase when they were reduced with 2-mercaptoethanol. Full activation occurred at lower concentrations of many of the Cbl analogues than occurred with Cbl itself. We conclude that Me-Cbl and other forms of Cob(III)alamin do not bind to human apomethionine synthase and that all must first be reduced to Cob(II)alamin before such binding can occur. The fact that human methionine synthase shows little absolute specificity for alterations in various portions of the Cbl molecule suggests that the potent inhibition of mammalian methionine synthase activity observed in vivo with various Cbl analogues is due to inhibition of intracellular Cbl transport or to inhibition of the enzymatic formation of Cob(II)alamin rather than to direct inhibition of mammalian methionine synthase itself.

Cerebrospinal fluid methylmalonic acid levels in normal subjects and patients with cobalamin deficiency

We measured methylmalonic acid, which accumulates in the blood and tissues of patients with cobalamin deficiency, in the CSF of 65 patients using capillary-gas chromatography and mass spectrometry. In 58 control patients, methylmalonic acid concentrations were always higher in CSF than in serum (mean CSF: serum ratio, 2.65; range, 1.17 to 7.78). In contrast, in six patients with elevated serum methylmalonic acid levels due to renal failure, CSF concentrations were normal in five and the CSF: serum ratio was less than one in four. In three patients with neuropsychiatric syndromes due to cobalamin deficiency and one patient with a normal serum cobalamin level who was an abuser of nitrous oxide, CSF concentrations were markedly increased (mean level, 600 times that of controls), out of proportion to those in the serum (mean CSF: serum ratio, 8.38; range, 3.5 to 13.5). The potential usefulness of CSF metabolite levels in the diagnosis of cobalamin deficiency is undetermined.

Inhibition of cobalamin-dependent enzymes by cobalamin analogues in rats

To determine which parts of the cobalamin (cbl) molecule are required for enzyme activity and which parts, if altered, might inhibit cbl-dependent enzyme activity, we synthesized 16 cbl analogues and administered them to nutritionally normal rats. The cbl analogues, with either modifications of the propionamide side chains of the A-, B-, and C-rings, the acetamide side chain of the B-ring, or the nucleotide moiety, were administered to rats by continuous 14-d subcutaneous infusion. Infusion of cbl-stimulated, cbl-dependent activity. Changes in any part of the cbl molecule always abolished stimulation and, in some cases, caused potent inhibition of both cbl-dependent enzymes. The most inhibitory analogues, OH-cbl[c-lactam], a B-ring analogue, and OH-cbl[e-dimethylamide] and OH-cbl[e-methylamide], two C-ring analogues, decreased mean liver holo-L-methylmalonyl-coenzyme A mutase activity to 65% of control values and increased serum methylmalonic acid concentrations to as high as 3,200% of the control values. Liver methionine synthetase activity was decreased to approximately 20% of the control and mean serum total homocysteine concentrations were increased to 340% of control. A similar level of inhibition was demonstrated in rats who were exposed to 28 d of inhaled nitrous oxide or a prolonged period of dietary cbl deficiency. The inhibitory cbl analogues, nitrous oxide, and diet deficiency all depleted liver cbl. The naturally occurring cbl analogues with modifications of the nucleotide moiety had no effects. We conclude that all parts of the cbl molecule are necessary for in vivo cbl-dependent enzyme activity and that modifications of the side chains of the B and C rings are associated with potent in vivo inhibition of cbl-dependent enzyme activity.