Chemistry Reference
In-Depth Information
product
which,
however,
is
used
for
therapeutic
purposes
as
also
is
hydroxycobalamin.
Only microorganisms are able to synthesize vitamin B12. Thus, some
animal species have a sufficient supply from their intestinal microorganisms.
In humans, however, the synthesizing organisms are localized in the colonic
part of the intestine which is too distal from the small intestine (ileum), where
vitamin B12 must be taken up. Consequently, humans obtain B12 exclusively
from their diet and only animal-derived foods contain sufficient amounts of
vitamin B12 (Tables 13.11 and 13.12). Another prerequisite for the uptake of
vitamin B12 is the intrinsic factor which is secreted by gastric parietal cells
and facilitates ileal uptake of cobalamin (Said and Kumar, 1999).
While storage has only minor effects on cobalamin concentration in milk
(
30-40% loss in sterilized milk after 90 days at room temperature) and
radiation also has small effects, heat destruction plays a major role. Losses in
cow's milk caused by heat treatment are sterilization: 20-100%; evaporated
milk:
50%; boiling: 20%; pasteurization:
<
10%; UHT: 5-10%. In cheese, an
overall loss of 10-50% can be assumed, although there are differences between
cheese types, e.g. in Gruy`re cobalamin concentration even increases due to
vitamin B12-synthesizing microorganisms (Biesalski and Back, 2002 g).
13.9.1.
Functions of Cobalamin
Adenosylcobalamin (in the cytosol) and methylcobalamin (in the mito-
chondria) are the co-enzyme forms of cobalamin. In humans, these co-
enzymes are involved in three metabolic reactions (for details see relevant
textbooks on biochemistry):
1. Leucine 2,3-amino-mutase reversibly changes
-leucine to
-leucine
(3-aminocaproic acid), thus starting the degradation of this amino
acid.
2. Methionine synthetase (N5-methyltetrahydrofolate homocysteine
methyltransferase) reaction. Methionine synthetase needs methyl-
cobalamin as a co-factor for the re-methylation of homocysteine
to methionine. During this process, the methyl group from 5-
methyltetrahydrofolate is transferred to homocysteine resulting
in methionine and tetrahydrofolate. The latter is converted to
N5,10-methylenetetrahydrofolate,
a
co-factor
of
thymidylate
synthetase, finally ending up in DNA synthesis.
3. Adenosylcobalamin is needed by methylmalonyl-CoA-mutase for
the isomerization of methylmalonyl-CoA to succinyl-CoA during
the degradation of propionic acid, thus offering the entrance to the
citric acid cycle.
