Chemistry Reference
In-Depth Information
day
-1
) and also isoniazid intoxication (1 g PN g
-1
isoniazid). In some cases,
beneficial effects have been described for carpal tunnel syndrome (Aufiero et
al., 2004), pre-menstrual syndrome (Kashanian et al., 2007) and rheumatic
diseases, although the latter is still unclear (Chiang et al., 2003, 2005).
13.7.
Biotin (Vitamin B7)
The chemical structure of biotin (hexahydro-2-oxo-1H-thieno (3,4-
D
) imida-
zol-4-valeric acid) is shown in Figure 13.10. Although eight isomers exist,
only
D
-(+)-biotin is biologically active and occurs naturally. While in milk
(cow's as well as human), biotin occurs in the free form, most foods from
animal sources or cereals contain it in an enzyme-bound form named biocytin
(
"
-N-biotinyl-
L
-lysine).
Biotin is essential for many microorganisms, numerous animals and
humans but can be synthesized by the colonic microflora. Although recent
findings demonstrate an uptake of water-soluble vitamins (including biotin)
by colonocytes, it remains unclear whether the amount of this production is
sufficient to fulfil all physiological functions or may be only a kind of fine-
tuning of body homeostasis (Said and Mohammed, 2006). The loss of biotin
during processing or storage of food is generally small or negligible. There
was no loss of biotin in milk in the frozen state for some weeks, in dried milk
at room temperature for 1 year or after 2 h of sunlight or 10 Gy gamma
radiation. Even UHT sterilization caused no losses, while pasteurization and
in-container sterilization caused
<
10%. In evaporated, condensed dried
whole milk less than 15% loss could be detected (Biesalski and Back, 2002f).
13.7.1.
Functions of Biotin
Carboxylation and decarboxylation processes are the main reactions in
which biotin is involved. It is linked to the enzymes by an amide bond between
the amino group of a specific lysil residue in the active centre of the respective
O
ureido ring
HN
NH
Tetrahydrothiphene
ring
OH
Valeric acid side chain
C
S
O
Figure 13.10.
Structure of biotin.
