To elucidate the effect of aging on the vitamin B
12 ( B
12) transport in enterocytes, young and old (3-4 months and over 1.5 years, respectively) female rats were studied. Two units of rat intrinsic factor (IF), saturated with
57Co-labeled cyanocobalamin were orally administered, and the amount of B
12 absorbed into each subcellular fraction of enterocytes was assayed. Concentration of endogenous B
12 in each subcellular fraction was also studied. Absorption of radioactive B
12 in the lysosomal fraction was maximum between 2 and 4 hr in each age group. In the older rats, the amount absorbed was lower than in the young rats. The older rats showed a significantly lower value of endogenous B
12 in the mitochondria. It has already been reported by us that there exist two B
12 binders in enterocytes: lysosomal and microsomal binders. The concentrations of lysosomal and microsomal binders as well as B
12 uptake in the mitochondria were significantly lower in the older rats than in the young rats. These data might help to explain the lower B
12 absorption in the lysosomal fraction and lower B
12 contents in the mitochondrial fraction.
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