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Absorption of Vitamin B 12 by the Guinea‐Pig I. SUBCELLULAR LOCALIZATION OF VITAMIN B 12 IN THE ILEAL ENTEROCYTE DURING ABSORPTION *
Author(s) -
Peters T. J.,
Hoffbrand A. V.
Publication year - 1970
Publication title -
british journal of haematology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.907
H-Index - 186
eISSN - 1365-2141
pISSN - 0007-1048
DOI - 10.1111/j.1365-2141.1970.tb01634.x
Subject(s) - vitamin , enterocyte , intrinsic factor , guinea pig , ileum , medicine , brush border , jejunum , microsome , absorption (acoustics) , intestinal mucosa , endocrinology , chemistry , biology , mitochondrion , small intestine , biochemistry , vitamin b12 , membrane , enzyme , vesicle , physics , acoustics
S ummary . 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 B 12 , 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 B 12 was similar in the jejunum and ileum. Approximately 25% of 10 ng of [ 58 Co]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 [ 57 Co]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 B 12 absorption, a characteristic finding of intrinsic factor mediated vitamin B 12 absorption. Seventy per cent of the vitamin B 12 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 B 12 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 B 12 absorption. No significant lysosomal radioactivity occurred during the absorptive period; this observation does not support the suggestion that vitamin B 12 is absorbed by pinocytosis.