Absorption of Vitamin B12 by the Guinea‐Pig I. SUBCELLULAR LOCALIZATION OF VITAMIN B12 IN THE ILEAL ENTEROCYTE DURING ABSORPTION*

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Abstract
Summary.: Guinea‐pig intestinal mucosa was fractionated into brush borders, nuclei, mitochondria, lysosomes, microsomes and a soluble fraction; the mitochondria were subfractionated into inner and outer membranes. Vitamin B12, assayed micro‐biologically using Euglena gracilis, was found to be predominantly localized to the inner mitochondrial membranes. In the fasted animal, the concentration of vitamin B12 was similar in the jejunum and ileum.Approximately 25% of 10 ng of [58Co]cyanocobalamin given orally was retained after 7 days, when measured by a whole‐body counting technique.Fasted guinea‐pigs were fed 8–11 ng of [57Co]cyanocobalamin and the distribution of radioactivity throughout the gastro‐intestinal tract was studied. Although radioactivity was present in the ileal mucosa within 1 hr of feeding, no significant hepatic activity was present until 3 hr, confirming the presence of a mucosal delay in vitamin B12 absorption, a characteristic finding of intrinsic factor mediated vitamin B12 absorption. Seventy per cent of the vitamin B12 was found in the colon at 3 and 4 hr indicating that approximately 30% of the dose was absorbed.The ileal mucosa showed a highly significant rise in mitochondrial specific activity of labelled vitamin B12 during the first 2 hr post‐feeding. This subsequently decreased coincidentally with the rise in hepatic activity. It is suggested that the mitochondria play a hitherto unsuspected role in vitamin B12 absorption. No significant lysosomal radioactivity occurred during the absorptive period; this observation does not support the suggestion that vitamin B12 is absorbed by pinocytosis.

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