Production, absorption, distribution and excretion of vitamin B 12 in sheep.

I. The efficiency of production and utilization of vitamin BIZ was studied with sheep given a cobalt-deficient diet with and without supplementary Co (I mg/d). Vitamin B,, to lignin ratios in rumen contents were used to estimate minimum rates of production and these were related to faecal and urinary excretion. Tissue distribution and excretion of vitamin Blz were studied with [58Co]cyanocobalamin and ~'-deoxyadenosyl[60Co]cobalamin. 2. Labelled Co was rapidly sequestered by particulate material in the rumen and was largely excreted in the faeces. Most of the vitamin Blz in whole rumen contents was contained in micro-organisms, but was released on incubation at pH 2. Added cyanocobalamin was partly degraded in the rumen. 3. The vitamin B,, to lignin ratio in rumen contents began to decline 1-3 d after cessation of a daily Co drench. Estimated ruminal production of vitamin B,, on full feed was not less than 400-700 pg/d with supplementary Co and 50-1 10 pg/d from the Co ( O O I O . ~ ~ pg/g dry weight) in the basal diet. Production of vitamin B,, appeared to be limited by food intake with or without additional Co. 4. At full feed the efficiency of production of vitamin Blz from Co in the basal diet was about 13 yo while that from added Co was about 3 yo. Part of the vitamin B,, produced in the rumen was degraded before reaching the faeces and about 5 % was absorbed. The minimum total requirements of sheep for vitamin B,, are assessed at about I I ,ug/d. 5. Injected 5'-deoxyadenosylcobalamin was better retained than injected cyanocobalamin, faecal excretion exceeded urinary excretion with both. Labelled cobalamin was selectively retained by liver (particularly by the mitochondria), kidneys and the walls of parts of the alimentary tract. Vitamin B,, was secreted into the duodenum and reabsorbed in the ileum, but little secretion occurred above the duodenum and little absorption below the small intestine.