Page 74 - Essential Haematology

60 / Chapter 5 Macrocytic anaemias

Table 5.2 Vitamin B 12 and folate: nutritional aspects.
Vitamin B 12 Folate

Normal daily dietary intake 7 – 30 μ g 200 – 250 μ g

Main foods Animal produce only Most, especially liver, greens and
yeast
Cooking Little effect Easily destroyed

Minimal adult daily 1 – 2 μ g 100 – 150 μ g
requirement
Body stores 2 – 3 mg (suffi cient for 2 – 4 years) 10 – 12 mg (suffi cient for 4 months)
Absorption
Site Ileum Duodenum and jejunum
Mechanism Intrinsic factor Conversion to
methyltetrahydrofolate

Limit 2 – 3 μ g/day 50 – 80% of dietary content
Enterohepatic circulation 5 – 10 μ g/day 90 μ g/day

Transport in plasma Most bound to haptocorrin; TC Weakly bound to albumin
essential for cell uptake
Major intracellular Methyl - and Reduced polyglutamate
physiological forms deoxyadenosylcobalamin derivatives
Usual therapeutic form Hydroxocobalamin Folic (pteroylglutamic) acid

TC, transcobalamin; haptocorrin = transcobalamin 1.

distal ileum where B 12 is absorbed and IF destroyed tive diseases where granulocyte production is greatly
(Fig. 5.2 ). increased, the haptocorrin and B 12 levels in serum
both rise considerably. B 12 bound to haptocorrin
does not transfer to marrow; it appears to be func-
Transport: the t ranscobalamins
‘
tionally dead ’ . Closely related glycoproteins to
Vitamin B 12 is absorbed into portal blood where it plasma haptocorrin are present in gastric juice, milk
becomes attached to the plasma - binding protein and other body fl uids.
transcobalamin (TC, previously called transcobala-
min II) which delivers B 12 to bone marrow and
other tissues. Although TC is the essential plasma Biochemical f unction
protein for transferring B 12 into the cells of the Vitamin B 12 is a coenzyme for two biochemical
body, the amount of B 12 on TC is normally very reactions in the body: first, as methyl B 12 it is a

low ( < 50 ng/L). TC defi ciency causes megaloblastic cofactor for methionine synthase, the enzyme
anaemia because of failure of B 12 to enter marrow responsible for methylation of homocysteine to
(and other cells) from plasma but the serum B 12 methionine using methyl tetrahydrofolate (methyl

level in TC deficiency is normal. This is because THF) as methyl donor (Fig. 5.3 a); and, secondly,

most B 12 in plasma is bound to another transport as deoxyadenosyl B 12 (ado B 12 ) it assists in conver-
protein, haptocorrin (previously called transcobala- sion of methylmalonyl coenzyme A (CoA) to suc-

min I). This is a glycoprotein largely synthesized by cinyl CoA (Fig. 5.3 b). Assays of homocysteine
granulocytes and macrophages. In myeloprolifera- and methylmalonic acid in plasma are used in