Agriculture Reference
In-Depth Information
(DHF) and then to tetrahydrofolate (THF) by a NADPH-dependent reductase. THF
is then converted to either 5-methyl THF or 10-formyl THF, and these compounds
enter the portal blood plasma. Hepatic uptake of folic acid is carrier mediated, and
THF, 5-methyl THF, 10-formyl THF, and 5-formyl THF may be found in the liver.
In systemic blood plasma, about two thirds of the folacin compounds are bound to
protein, whereas about one third are free. THF derivatives function as coenzymes by
accepting one-carbon groups from amino acid metabolism and interact with vitamin
B
6
and vitamin B
12
in conversion of homocysteine to cystathionine. They also are
involved in synthesis of purines and pyrimidines and are thus essential for cell divi-
sion (particularly important for production of red cells).
Good food sources of folacin include green vegetables (such as spinach, broc-
coli, asparagus, and turnip greens), legumes, citrus fruits, liver, and fortified breads
and cereals. Recommended dietary allowances range from 150 µg per day for 1- to
3-year-old children to 600 µg per day for pregnant women.
v
i t a m i n
b
12
(C
o b a L a m if n
)
Cobalamin
is a generic term for compounds having a corrin nucleus with a cen-
tral cobalt atom to which 5,6-dimethylbenzimidazole and a cyanide, hydroxyl,
water, nitrite, 5′-deoxyadenosyl, or methyl group is attached. Their respective
names are cyanocobalamin, hydroxocobalamin, aquocobalamin, nitritocobalamin,
5′-deoxyadenosylcobalamin (adenosylcobalamin), and methylcobalamin. Only the
last two are active as coenzymes. Methylcobalamin is required for the conversion
of homocysteine to methionine. The methyl group for this conversion is acquired
from 5-methyl tetrahydrofolate. Adenosylcobalamin is required for the conver-
sion of l-methylmalonyl CoA to succinyl CoA in the Krebs cycle (l-methylmalonyl
CoA is made from d-methylmalonyl CoA, which is generated from propionyl CoA
in a biotin-dependent reaction). Digestion and absorption of cobalamin involves a
cobalamin-binding R protein in saliva and gastric juice that may bind to the vita-
min before release from food proteins. Cobalamin bound to R protein enters the
duodenum, where pancreatic proteases release the cobalamin. Then, intrinsic factor
(IF), a glycoprotein produced by parietal cells in the stomach, binds the cobalamin
and travels to the ileum, where cobalamin receptors (cubilins) form a cubilin-IF-
cobalamin complex that may be absorbed by receptor-mediated endocytosis (trans-
port inward by a cell vesicle that surrounds the complex). However, if cobalamin
concentrations are high, passive diffusion may account for most of the absorption.
Following absorption and transport to the liver, enterohepatic circulation is signifi-
cant, and cobalamin secreted in bile can be reabsorbed in the ileum. Cobalamin in
the plasma is bound to one of three transcobalamins: TCI, TCII, or TCIII. Uptake of
cobalamin by tissues appears to involve receptors for TCII, and the TCII-cobalamin
complex is taken into cells by endocytosis, followed by lysosomal degradation of
TCII and release of cobalamin within the cytosol (cell fluid).
Food sources of cobalamin are animal products. However, microbes are the ulti-
mate source since the tissues of higher plants or animals are incapable of synthesizing
cobalamin. Meat, meat products, poultry, fish, shellfish, and eggs are good sources
and contain predominantly adenosyl- and hydroxocobalamin. Milk, cheese, and
