Nutritional Megaloblastic Anemias

DR. NABIL BASHIR HLS, 2018

Definition: Macrocytic Anemia

MCV>100fL Impaired DNA formation due to lack of: B12 or folate in ultimately active form use of antimetabolite drugs

Macrocytosis also caused by Liver disease with inadequate cholesterol esterification Alcohol abuse independent of folate (MCV 100-105) Myelodysplasia Post-splenectomy HIV drugs Dilantin

Vitamin B12: Cobalamin

• Meat and dairy products only

• Minimum daily requirement 6-9 mcg/d

• Total body store 2-5 mg (half in liver)

• Helps to synthesize thymine, thus deficiency leads to problems with DNA replication

B12: Cobalamin absorption

• Initially bound to protein in diet, liberated by acid and pepsin, then binds to R factors in saliva and gastric acids

• Freed from R factors by pancreatic proteases then binds to Intrinsic Factor secreted by gastric parietal cells

• Absorbed together (Cbl + IF) in ileum

• Released from IF in ileal cell then exocytosed bound to trans-Cbl II

• Cbl bound to transcobalamin II binds to cell surface receptors and is endocytosed

Function of vitamin B12

1) deoxyadenosylcobalamin form is a co-enzyme methylmalonyl CoA

mutase.

2) Coenzyme in a reaction involved in methionine

Metabolism.

H4folate is converted to N5-methyl-H4folate in a number of different

reactions as it accepts methyl groups. The methyl group can only be

removed and the H4folate regenerated by the above reaction.

(See folic acid)

Important for DNA synthesis,

nervous tissue and fat metabolism

in the liver

an intermediate of the citric

acid cycle, porphyrin synthesis

(Heme synthesis)

Actions of Cobalamin & Folate

Causes of B12 Deficiency: Pernicious Anemia

• Autoantibody to Intrinsic Factor . – 2 types of anti-IF antibody

• Blocks attachment of Cbl to IF

• Blocks attachment of Cbl-IF complex to ileal receptors

• Chronic atrophic gastritis – Autoantibody against parietal cells (H-K-ATPase)

– Increased risk of gastric cancer (carcinoid and intestinal- type)

Helicobacter pylori ?

Folate

• Animal products (liver), yeast and leafy vegetables

• Normal requirement 400mcg/day

• Pregnancy/Lactation: 500-800mcg/day

• Body stores: 5-10mg

Folic Acid

• Folic acid is a conjugated molecule consisting of a pteridine ring structure linked to para-aminobenzoic acid (PABA) that forms pteroic acid.

• Folic acid itself is then generated through the conjugation of glutamic acid residues to pteroic acid.

positions 7 and 8 carry hydrogens in dihydrofolate (DHF)

positions 5–8 carry hydrogens in tetrahydrofolate (THF)

•

Function of Folate

• The function of THF derivatives is to carry and transfer various forms of one carbon units during biosynthetic reactions. The one carbon units are either methyl, methylene, methenyl, formyl or formimino groups.

• These one carbon transfer reactions are required in the biosynthesis of serine, methionine, glycine, choline and the purine nucleotides and dTMP

Folate Metabolism

• Binds to folate receptor, becomes polyglutamated intracellularly

• Many drugs (trimethoprim, methotrexate, pyrimethamine) inhibit dihydrofolate reductase

• folic acid exists in a polyglutamate form. • Intestinal mucosal cells remove some of the

glutamate residues through the action of the lysosomal enzyme, conjugase.

• The removal of glutamate residues makes folate less negatively charged (from the polyglutamic acids) and therefore more capable of passing through the basal lamenal membrane of the epithelial cells of the intestine and into the bloodstream.

Causes of Folate Deficiency

• Malnutrition: Destroyed by heat during cooking • Alcoholism (decreased in 2-4 days): impairs

enterohepatic cycle and inhibits absorption • Increased requirement in hemolytic anemia, pregnancy,

skin disease • celiac sprue • Drugs

– Trimethoprim, Methotrexate, Primethamine (inhib DHFR)

Lab testing for diagnosis

Serum B12

Serum Folate MMA Homocysteine

Normal >300 >4 70-270 5-14

Deficiency <200 <2

Confirm B12 200-300 High High

Confirm folate 2-4 Normal High

Intrisic factor antibody assay can be falsely positive if pt has recently

received a B12 shot with B12 >800, thus important to add-on.

N5-methyl-tetrahydrofolate in relation to vitamin B12 biochemistry

N5-Methyl-tetrahydrofolate + Homocysteine

↓

Tetrahydrofolate + Methionine

This is the only reaction regenerating tetrahydrofolate from N5-methyl-tetrahydrofolate. Without it all the tetrahydrofolate will eventually end up trapped as N5-methyl-tetrahydrofolate.

Synthesis of Deoxythymidylate (dTMP), N5,N10-methylene tetrahydrofolate transfers a methyl group to deoxyuridylate (dUMP) to form dTMP: Essential for DNA synthesis.

•N5,N10-methylene tetrahydrofolate is absolutely essential for

DNA synthesis in cells that are dividing rapidly such as red

blood cell producing bone marrow cells, hair follicles,

intestinal mucosa cells and cancer cells (rapidly dividing cells

need to replicate their DNA often).

•Methotrexate (analogue of folic acid) binds to folate reductase

1000 times more tightly than folate.This inhibits the conversion

of folate and dihydrofolate into active tetrahydrofolate

•Methotrexate is used to treat leukemia. It works by inhibiting

the production of tetrahydrofolate which in turn limits the

amount of N5,N10-methylene tetrahydrofolate available for

dTMP synthesis

.

Activation of folic acid Folic acid is not the active form of the vitamin. It needs to be reduced to

tetrahydrofolate (H4folate).

Megaloblastic anaemia and folic acid deficiency

Pernicious anaemia due to primary deficiency of vitamin B12 giving

secondary deficiency of folic acid because all the folate ends up trapped

as N5-methyl-tetrahydrofolate

Folic acid deficiency reduces the capacity of the body to make

dTMP which affects the rapidly dividing bone marrow cells associated

with red blood cell production.