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Vitamin
D3 is normally made in animals by the deposition of 7-dehydrocholesterol
(a derivative of cholesterol commonly referred to as 7-D) in the skin.
Upon irradiation with ultraviolet light from the sun, this precursor
undergoes transformation to pre vitamin D (PRE). The pre isomer is
converted to cis-vitamin D (CIS) and adsorbed into the blood where
it is transported to the liver and metabolized to 25-hydroxyvitamin
D (25-OH). The 25-OH analog is circulated in the blood until abnormal
calcium levels trigger the need for a change in calcium blood concentration
(increase or decrease). The 25-OH analog is transported by the blood
to the kidney where it is further metabolized to 1,25-dihydroxyvitamin
D3 (1,25-OH2) which acts to facilitate the transport
of calcium into or out of bone as well as through the gut wall. The
process of the commercial manufacture of vitamin D3 mimics the manufacture
of the cholecalciferol in animals. The process starts with cholesterol,
which is obtained from wool grease (15% of wool grease; 30% of wool
wax alcohols). Wool grease and wool wax alcohols are obtained from
the cleaning of wool after shearing. The cholesterol is removed
from the wool grease and purified. The cholesterol undergoes a four
step process to make 7-dehydrocholesterol. This is the same
compound that is stored in the skin of animals. The
7-dehydrocholesterol is then irradiated with ultra violet light to
give pre vitamin D3. Some isomers of vitamin D are formed during irradiation
(tachysterol and lumisterol), resulting in a mixture of the pre vitamin
D, these isomers and 7-dehydrocholesterol (80%). The 7-D is removed
by crystallization from methanol; the methanol is removed leaving a
mixture of pre vitamin D, lumisterol and tachysterol. The mixture is
heated to convert the pre vitamin to cis vitamin D. This material is
in the form of a resin that melts at about room temperature and usually
has a potency of 25,000,000 to 30,000,000 International Units per gram.
This is compared to the pure cis vitamin D which can be isolated from
the resin and which has a potency of 40,000,000 International Units
per gram in its pure crystalline form.