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 B 12 1) deoxyadenosylcobalamin form is a co-enzyme methylmalonyl CoA mutase.
2) Coenzyme in a reaction involved in methionine Metabolism. H 4 folate is converted to N 5 -methyl-h 4 folate in a number of different reactions as it accepts methyl groups. The methyl group can only be removed and the H 4 folate 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.
N 5 -methyl-tetrahydrofolate in relation to vitamin B 12 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
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Activation of folic acid Folic acid is not the active form of the vitamin. It needs to be reduced to tetrahydrofolate (H 4 folate).
Megaloblastic anaemia and folic acid deficiency 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. Pernicious anaemia due to primary deficiency of vitamin B 12 giving secondary deficiency of folic acid because all the folate ends up trapped as N 5 -methyl-tetrahydrofolate