Yes, it seems like it. Adenosine is the combination of adenine, and
ribose, and in phosphorylated form a very important molecule in
biochemistry (it's part of DNA, RNA and as the triphosphorylated form,
ATP).
I did some search for papers, and found a review on nucleoside
transporters (Nucleoside and nucleobase transport systems of mammalian
cells, DA Griffith and SM Jarvis; BIOCHIMICA ET BIOPHYSICA
ACTA-REVIEWS ON BIOMEMBRANES 1286 (3): 153-181 OCT 29 1996), which
summarizes that there are two types of transporters, ones that spend
energy to actively take up adenosine, called concentrative nucleotide
transport; and ones that allow equilibration of adenosine inside and
outside of the cell. These are "just" selective pores.
The two papers below make the following points:
1. such equilibrative transporters allow adenosine to pass the
blood-brain barrier in mice, and probably in humans
2. in the intestinal epithelium, providing the boundary between food,
and our body, on the "food-side" adenosine is actively taken up, while
on the back of the cells, the "body-side" adenosine is allowed to
diffuse out.
So, yes, we do actively take up food.
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Title: Functional characterization of adenosine transport across the BBB in mice
Author(s): Murakami H, Ohkura A, Takanaga H, Matsuo H, Koyabu N, Naito
M, Tsuruo T, Ohtani H, Sawada Y
Source: INTERNATIONAL JOURNAL OF PHARMACEUTICS 290 (1-2): 37-44 FEB 16 2005
Document Type: Article
Language: English
Abstract: We investigated transport characteristics of adenosine
across the blood-brain barrier (BBB) in mice. Uptake clearance across
the BBB was measured by using an in situ mouse brain perfusion
technique and cultured mouse brain capillary endothelial cell line
(MBEC4 cells). Nucleoside transporter was cloned by RT-PCR and
expressed on Xenopus laevis oocyte. Both in situ and in vitro studies
revealed that the adenosine uptake is concentration-dependent,
Na+-independent and S-(p-nitrobenzyl)-6-thioinosine (NBMPR)-sensitive.
The K-1 values of in situ and in vitro studies were 31.7 +/- 13.8 muM
and 11.9 +/- 2.84 muM, respectively. A good correlation was found for
the inhibitory effects of nucleoside analogs to adenosine uptake
between in situ and in vitro studies. RT-PCR revealed the expression
of RNA of mouse equilibrative nucleoside transporter (mENT1) in mouse
brain capillary and MBEC4 cells. In mENT1 expressed on X. laevis
oocyte, K-1 value of adenosine transport was 6.9 +/- 2.7 muM (and
comparable to those in situ and in vitro studies). In conclusion, we
characterized the adenosine transport across the BBB in mice by using
in situ brain perfusion technique and MBEC4 cells and found that these
transports share common characteristics with mENT1-mediated transport.
Transport of adenosine across the BBB in mice may be attributable to
mENT1.
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Title: Nucleoside transporters in absorptive epithelia
Author(s): Casado FJ, Lostao MP, Aymerich I, Larrayoz IM, Duflot S,
Rodriguez-Mulero S, Pastor-Anglada M
Source: JOURNAL OF PHYSIOLOGY AND BIOCHEMISTRY 58 (4): 207-216 DEC 2002
Document Type: Review
Language: English
Abstract: There are two families of nucleoside transporters,
concentrative (termed CNTs) and equilibrative (called ENTs). The
members of both families mediate the transmembrane transport of
natural nucleosides and some drugs whose structure is based on
nucleosides. CNT transporters show a high affinity for their natural
substrates (with Km values in the low micromolar range) and are
substrate selective. In contrast, ENT transporters show lower affinity
and are more permissive regarding the substrates they accept. Both
types of transporters are tightly regulated in all cell types studied
so far, both by endocrine and growth factors and by substrate
availability. The degree of cell differentiation and the proliferation
status of a cell also affect the pattern of expressed transporters.
Although the presence of both types of transporters in the cells of
absortive epithelia suggested the possibility of a transepithelial
flux of nucleosides, their exact localization in the different plasma
membrane domains of epithelial cells had not been demonstrated until
recently. Concentrative transporters are found in the apical membrane
while equlibrative transporters are located in the basolateral
membrane, thus strengthening the hypothesis of a transepithelial flux
of nucleosides. |
