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1 Medical Research Service,
Previous studies
have characterized different aspects of the cellular/membrane mechanism
and regulation of the intestinal uptake process of the water-soluble
vitamin biotin. Little, however, is known about the molecular
mechanisms of the uptake process. In this study, we have identified a
cDNA from rat small intestine that appears to be involved in biotin
transport. The open reading frame of this cloned cDNA consisted of
1,905 bases and was identical to that identified for the vitamin
transporter in placental tissue. Significant heterogeneity, however,
was found in the 5' untranslated region of this clone, with three
distinct variants (II, III, IV) being identified in the small
intestine; the placental variant (variant I), however, was not present
in the small gut. Variant II was found to be the predominant form
expressed in the rat small and large intestines. Functional identity of
the cloned intestinal cDNA was confirmed by stable expression in COS-7
cells, which showed a four- to fivefold increase in biotin uptake in
transfected COS-7 cells compared with controls. The induced biotin
uptake in transfected COS-7 cells was found to be
1)
Na+ dependent,
2) saturable as a function of
concentration with an apparent
Km of 8.77 µM
and a Vmax of
779.7 pmol · mg
protein
1 · 3 min1, and
3) inhibited by unlabeled biotin and
pantothenic acid and their structural analogs. The distribution of
complementary mRNA transcripts of the cloned cDNA along the vertical
and longitudinal axes of the intestinal tract was also determined.
Results of this study describe the molecular characteristics of the
intestinal biotin absorption process and report the identification of a
cDNA that encodes a Na+-dependent
biotin uptake carrier that appears to exist in the form of multiple variants.
water-soluble vitamin; molecular transport mechanism; 5' untranslated region; reverse transcriptase-polymerase chain reaction
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