Radioactively-labelled cholecalciferol: [1a-3H] cholecalciferol
(2 Ci/mmol) and [1,2-3H] cholecalciferol (276.9 mCi/mmol) were
kindly prepared by P.A. Bell and B. Pelc by the method of Callow
et al. [11]. Determination of the distribution of 3H on carbon-1 of
[1a-3H] cholecalciferol has shown that 85% is in the 1a-position
and 15% is in the 1b-position [12]. Therefore, in the enzymemediated
1a-hydroxylation of this tritium-labelled molecule,
about 85% of the 3H is lost as 3H2O. [4-14C] Cholecalciferol
(32.3 mCi/mmol) and 25-hydroxy[26,27-3H] cholecalciferol
(11 Ci/mmol) were obtained from The Radiochemical Centre,
Amersham, U.K. [4-14C, 1a-3H] 25-hydroxycholecalciferol was
prepared by standard Sephadex LH20 chromatography, from lipid
extracts of the blood of pigs which had been injected intravenously
with either [1a-3H] cholecalciferol or [4-14C] cholecalciferol