The B-group (part B)

2015

A/Prof Rozanne Kruger

151.232 ~ Nutrition and metabolism151.232 ~ Nutrition and metabolism151.232 ~ Nutrition and metabolism151.232 ~ Nutrition and metabolism

B-group vitamins learning objectives

Copyright 2005 Wadsworth Group, a division of Thomson Learning

To be able to describe their absorption, transport and

storage

To be able to describe important dietary sources of each vitamin

To be able to discuss the metabolic functions of these vitamins and the consequences of suboptimal and deficient intakes

To know how to assess them in vivo

Relevant pages in Thompson: chapters 8-12

Vitamin B12 (cobalamin)First recognised for its role in curing pernicious anaemia, which occurred in elderly patients and quickly lead to death.

Vitamin B12 was first isolated from liver in 1948 and the intrinsic factor required for its absorption (a glycoprotein secreted in the stomach) was identified in 1966.

Vitamin B12 (cobalamin) Closely related to folate

Each depends on the other for activation When folate gives up its methyl group it

activates vit B12 Regeneration of AA methionine (methyl group

donor) and

Homocysteine

Cystathionine

Cysteine

THF

SAM

5 methyl THF

B12

Methionine

Folate, B12 and homocysteine metabolism

5, 10 methyleneTHF

MS

MS =methionine synthase

DNA

Vitamin B12 (cobalamin) Closely related to folate

Each depends on the other for activation When folate gives up its methyl group it

activates vit B12 Regeneration of AA methionine (methyl group

donor), and synthesis of DNA+RNA depend both on vit

B12 and Folate Without folate, however, B12 still maintains the

sheath that covers nerve fibers and promotes their normal growth

Bone cell activity and metabolism also depend on vit B12

Structure of Vitamin B12 Active Vitamin B12 consists of a porphyrin

ring attached to ribose and phosphoric acid.

The porphyrin ring has a cobalt atom in the centre.

The cobalt atom may have either a methyl group or a 5-deoxyadenosyl group bound in active forms of the vitamin.

The synthetic form has a cyanide group.

Active forms and functions of Vitamin B12

Methylcobalamin: Methionine synthase

(See the methylation cycle)

Deoxyadenosylcobalamin: Methylmalonyl CoA mutase

(Converts methylmalonyl CoA, derived from some amino acids and propionate, to succinate to enter the citric acid cycle.)

Dietary sources

Second hand Fermentation products from yeast and bacteria (most vegetarians deficient).

Third hand Vitamin B12 is made by microorganisms and assimilated into the food chain through incorporation by herbivorous animals, which are then eaten (or their products, e.g. milk, are eaten) by omnivorous animals.

Absorption of Vitamin B12 Released from food by action of acid and

pepsin in stomach. Bound by glycoproteins, R binders. R binders broken down in duodenum, then

intrinsic factor (IF) (glycoprotein secreted by parietal cells in stomach) binds B12.

(IF-B12) complex absorbed by ileum.

Limitation on absorption Vit B12 + IF travels to end of small

intestine where specific receptors recognize the complex (only bound, not alone).

Uptake is limited by the capacity of the ileal uptake system, which is saturated by amounts of about 1.5-2.0 g of B12 in a meal.

During absorption B12 is bound to its transport protein, transcobalamin II (TCII).

Conservation of B12 Little B12 is excreted in urine as it is so well

bound to transport proteins, TC I, II and III in plasma.

About 1 mg daily is secreted in bile, but is almost all reabsorbed associated with IF enterohepatic circulation

Lack of gastric secretion, and hence lack of IF, causes loss of this B12 as well as that from dietary malabsorption this leads to pernicious anaemia - fatal if not treated.

Deficiency of Vitamin B12 Megaloblastic anaemia morphologically

indistinguishable from that seen in folate deficiency B12 deficiency causes a build up of cellular 5-methyl

H4PteGlu and reduction of available 5-methylene H4PteGlu. Trapping of folate in this form results in lack of folate

coenzymes for thymidylate and purine synthesis -affects DNA synthesis.

Neuropathy sub-acute combined degeneration of the spinal chord. Creeping paralysis. Irreversible in final stage. Not seen in folate deficiency (with sufficient folate, the

symptoms may develop without evidence of anaemia masking (cures the blood symptoms)

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Vitamin B12 Deficiency of folate or B12 produces (due to

folate lack) pernicious anemia

Copyright 2005 Wadsworth Group, a division of Thomson Learning

Macrocytic and microcytic anemia

Macrocytic anemia: Red blood cells enlarged. Folate and vitamin B12 deficiencies lead to

megaloblastic anemia where red blood cells are enlarged due to abnormal DNA synthesis (unripe / immature rbc).

Microcytic anemia: Red blood cells small. Vitamin B6 or iron deficiency lead to reduced haeme

synthesis and hence small red cells (mature rbc).

Causes of Deficiency Inadequate dietary intake (rare)

Vitamin B12 is in foods of animal origin, hence vegetarians and vegans are at risk.

Intestinal malabsorption (most common) Pernicious anemia, parietal cells, destroyed by

autoimmune response, do not secrete IF or HCL.

Loss of HCL producing cells from disease e.g. Helicobacter pylori / stomach resection

Causes of deficiency continued Pancreatic insufficiency

Destruction of R-binders by pancreatic enzymes is essential for absorption.

Drug Interactions Alcohol, colchicine, neomycin and p-

aminosalicylate cause malabsorption. Drugs such as cimetidine, ranitidine,

omeprazole may decrease absorption by lowering gastric secretions.

Assessment methods Measurement of Transcobalamin 1 in the

plasma Competitive binding to IF in radiometric assays In B12 deficiency methylmalonyl CoA

accumulates is not converted to succinyl CoA, and is broken down to MMA for excretion; hence MMA increases when mutase activity is low; requires a mass spectrometer for the assay

Homocysteine elevation also increased in folate and B6 deficiency.

Recommended daily allowancesRDI for B12 - 2.4 g/day.

Pregnancy 2.6 g/dayLactation 2.6 g/day

Therapeutic Uses People having a diet very low in animal products

should use oral vitamin B12 supplement. Those with malabsorption syndromes (atrophic

gastritis), e.g. pernicious anaemia, are usually treated with intra-muscular injections of 100-1000 g of B12 every 1-2 months.

Very high doses do not appear to be toxic, but serve no purpose except in treating malabsorption syndromes.

Folate (Folic Acid)Pteroylglutamic acid (PGA)

Lucy Wills 1933 - type of macrocytic anemia that occurred in poor pregnant women in India was similar to the pernicious anemia that occurred in elderly subjects.

The elderly would die if not treated, but pregnant women recovered after giving birth.

A similar anemia induced in rats and monkeys by a diet deficient in proteins and vegetables could be cured by MARMITE (and so could her patients).

Leafy dark green Leafy dark green Leafy dark green Leafy dark green

vegetables (such as vegetables (such as vegetables (such as vegetables (such as

spinach and broccoli), spinach and broccoli), spinach and broccoli), spinach and broccoli),

legumes (such as black legumes (such as black legumes (such as black legumes (such as black

beans, kidney beans, beans, kidney beans, beans, kidney beans, beans, kidney beans,

and blackand blackand blackand black----eyed peas), eyed peas), eyed peas), eyed peas),

liver, andliver, andliver, andliver, and

some fruits (notably some fruits (notably some fruits (notably some fruits (notably

citrus fruits and juices) citrus fruits and juices) citrus fruits and juices) citrus fruits and juices)

are naturally rich in are naturally rich in are naturally rich in are naturally rich in

folate.folate.folate.folate.

NB USA has mandatory f.a. fortification of all flour products

Folate

Generic term for compounds with common vitamin activity synthetic folic acid

(pteroyl monoglutamic acid) natural food folates

(pteroyl polyglutamates)

Structure of folic acid

Folic acid and THF structures

Absorption Polyglutamate in food hydrolysed by brush

border enzymes monoglutamate form before uptake by intestinal cells by a saturatable carrier mediated process.

Half of natural folates are 5-methyltetrahydrofolate, the rest are converted to this form in intestinal cells after uptake.

5-methyltetrahydrofolate monoglutamate is therefore the main form in plasma.

Folate

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Elongation of the glutamate chain The methylated monoglutamate form of

tetrahydrofolate can have further glutamate residues added in an unusual - linkage to form 5-Methyl tetrahydrofolylpoly- -glutamate

h4PteGlu(n) The polyglutamate forms are generally the most

effective substrates for folate-dependent enzymes, showing increased affinities, and are also retained better in cells.

Uptake into cells 5-methyltetrahydrofolate monoglutamate enters

cells by diffusion, and is retained by conversion to polyglumate forms

after demethylation via the vitamin B12 dependent methionine synthase.

If polyglutamate forms are already present at high levels, little of the monoglutamate form is demethylated and retained.

Homocysteine

Cystathionine

Cysteine

THF

SAM

5 methyl THF

B12

Methionine

Folate and homocysteine metabolism

5, 10 methyleneTHF

MS

MS =methionine synthase

DNA

Cellular folate retention is therefore regulated by:

Cellular folate concentration Plasma folate concentration Vitamin B12 status

Biosynthetic reactions using 1-carbon units.

One-carbon units are transferred by folates in biosynthetic reactions, e.g.: Pyrimidine synthesis Purine biosynthesis Conversion of homocysteine to methionine

Methionine acts as a source of methyl groups in a wide range of methylation reactions.

Part of coenzymes THF (tetrahydrofolate) and DHF (dihydrofolate) used in DNA synthesis and therefore important in new cell formation

Catabolism and Excretion Breakdown products are pteridines and p-

aminobenzoylglutamate These are further catabolised before

excretion. Very little intact folate is excreted, so

catabolism is the main factor determining folate requirement.

Folate deficiencyDeficiency leads to: Megaloblastic anemia (large red cells). Neural tube defects (NTDs) during fetal

developments e.g. spinal bifida Possibly to elevated risk of cancer and

heart disease and to mental abnormalities.

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Figure 15-6Page 512

Spina Bifida

Spina Bifida Normal Spine

Vertebra

Meninges

Spinal cord

Spinal fluid

Spine Spine

Mega = large Macrocytic

Folate deficiency Other deficiency symptoms

Smooth, red tongue Mental confusion, weakness, fatigue, irritability, headache

Causes of deficiency

Low dietary intake Reduced intestinal absorption (sprue and

coeliac disease) Pregnancy due to increased folate catabolism Anticonvulsant drugs; some lipid lowering drugs High alcohol intake leading to decreased

absorption, increased catabolism, often accompanied by poor diet.

Assessment of folate status Most widely used method:

Measurement of folate in plasma or serum. Better method:

Measurement of red cell folate (more steady). Another method:

Elevation of plasma homocysteine; occurs with even marginal deficiency, but also found

with B12 deficiency need to exclude this.

Dietary Sources Folate is widely distributed in foods. Best natural dietary source is liver. Difficult to achieve high intakes needed to

prevent NTDs during pregnancy. Fresh vegetables good (folate in foliage),

but need a lot more than is included in most diets.

RequirementsRDI 400 g/day.

Pregnancy 600 g/dayLactation - 500 g/day

Bioavailability ranges from 50%(food) -100% (supplements on empty stomach)Supplements 1.7x more available than food)

Mandatory fortification in USA To reduce the incidence of NTD in USA Mandatory f.a. fortification from 1 Jan 1998

140g/100g cereal product Dietary Folate Equivalents (DFE) introduced to

account for differences in bioavailability of natural and synthetic forms 1 units DFE synthetic f.a.= 1.7 units natural folate Units of folate are reported as g DFE

100g food + 100g supplement = 270 DFE (dietary folate equivalents)

Therapeutic uses, possible toxicity Deficiency usually treated with supplements of

about 5 mg per day for days or weeks. Supplementation in a high-risk pregnancy can

be at 4mg a day. Normal pregnancy 600 g/day is recommended,

and supplements should be taken peri-conceptionally if possible [800 g/day 4 weeks prior to conception & 1st 3 months of pregnancy]

BUT there may be risks with high intakes of folate.

Possible toxicity High folate intake, particularly with the

synthetic form, folic acid, used in supplements, may: Mask accompanying B12 deficiency, due to

correction of anemia, without correcting neurological symptoms

De-stablise epilepsy. Accelerate growth of some tumours.