Corrinoids from various ovine tissue samples (liver, blood, small intestinal fluid and faeces) were analysed using a combination of high-performance liquid chromatography (HPLC) and a
radioisotope dilution assay (
RIDA) to estimate the distribution of
corrinoids--the
cobalamins hydroxocobalamin (
OH-cbl),
methylcobalamin (me-cbl) and
5'-deoxyadenosylcobalamin (
ado-cbl), and
cobalamin analogues--in these tissues. Samples were taken from either
cobalt-deficient or
cobalt-replete ewes, and ruminant and pre-ruminant lambs. In liver,
ado-cbl predominated, followed by analogues,
OH-cbl and me-cbl. Supplementation with either
cobalt (ruminant) or
vitamin B12 injections (pre-ruminant) increased the amount of
ado-cbl and decreased analogues. In blood,
OH-cbl predominated, followed by
ado-cbl, analogues and me-cbl, respectively. In small intestinal fluid, the distribution from largest to smallest percentage was analogues,
ado-cbl,
OH-cbl and me-cbl. In faeces, analogues constituted the greatest proportion, followed by
OH-cbl,
ado-cbl and me-cbl, respectively. Owing to the small sample sizes only cautionary interpretations can be made. In contrast to humans, where me-cbl constitutes the highest proportion of
corrinoids in plasma and
ado-cbl in the liver, in sheep the amount of
ado-cbl was consistently higher than me-cbl in all tissues. This may be due to the higher metabolic need of sheep for
ado-cbl due to gluconeogenesis. Analogues and
OH-cbl were found in each tissue, contrary to previous postulations. The much higher amount of
vitamin B12 in small intestinal fluid compared with faeces indicates that a large proportion of the
vitamin is absorbed by the gastro-intestinal tract.