DrVirgo
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2- 36 y/o M presents/w sudden onset of massive diarrhea. Cultures of his stool grow a curved Gr(-) rod that secretes an enterotoxin consisting of a toxic A subunit & a binding B subunit. This enterotoxin causes diarrhea by:
A- Inhibiting the conversion of Gi-GDP to Gi-GTP
B- Inhibiting the conversion of Gs-GTP to Gs-GDP
C- Stimulating the conversion of Gi-GDP to Gi-GTP
D- Stimulating the conversion of Gs-GDP to Gs-GTP
E- Stimulating the conversion of Gs-GTP to Gs-GDP
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| DrVirgo
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Please explain your answer.
Thanks. 
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| Natasa
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 It is activating Gs-GDP to become Gs-GTP(active form) which in turn activates adenylate cyclase and increase cAMP levels and there you have profuse diarrhea. So the answer is B because it is constantly activated(no conversion from GTP to GDP)
Edited by Natasa on 02/07/06 - 06:15 AM
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| reet
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WHY not D? 
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| DrVirgo
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You are right Natasha. Its B.
Reet I think not D because it works on Gs and not Gi.
Am i right natasha?
Anyone else have any other ideas?
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| Natasa
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RIGHT DR.VIRGO.
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| DrVirgo
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Thanks Natasha.
by the way, nice pic... where is it from?
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| DrVirgo
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Reet, to explain:
This is cholera toxin mechanism of action:
It's LT part does ADP ribosylation of GTP binding protein which turns on AC, which inturn converts ATP to cAMP and causes fluid release and watery diarrhea.
Cholera toxin works by activating Gs
(Pertussis toxin has the same effect but works by inhibiting Gi; it will also increase cAMP)
-Again, if someone else could confirim the above I would appreciate it...
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| Ashley84
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As u have already said, cholera toxin Adp-ribosylates Gs subunit and causes increase in cAMP level because of costitutive activation of Ac.
The increase level of cAMp causes release of Cl- anion and also secretion of Na and water to maintain osmotic balance and elettrolitic neutrality.
The result is watery diarrhea
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| vanshita
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it works on Gs but in option D also Gs
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| reet
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vanishta,, i m also confused on this part... may be something is hidden in COVERSION thing,
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| tikka
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its hard to understand .. .
can any one pl explain
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| msyamp
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hi tikka
normally whathappens is the adenylcycylase i the enzyme that converts ATP to cyclicAMP.
this enzyme is activated by Gs-GTP complex.that forms due to some stimulus.and as soon as Gs-GTP complex forms it is inactivated by Gi protein nto a Gs and GDP.
now what cholera toxin does is it makes Gs-GTP-ADPribose complex that stimulates adenylcyclase but does not get inactivated by Gi. so there is no inactivation of the enzyme leading to high cAMP which causes increased Na+ K+ and Cl- secretion leading to watery diarhhea.
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| robin082006
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good picture Natasa
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| DrVirgo
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good explanation msyamp.
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| Andrew000
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Its really confusing why not the answer will be D.........it is also Gs..........not Gi as someone said above.
Please make clear ur explation.
It will be a great help........& Don't explain the mechanism of cholera toxin that i understand well.
Just like to Know the point that wil help me to select B Not D.
Thx
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| Slacker
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Yeah, why not D. I understand the mechanism and according to the mechanism the answer should be D i think??? Unless i'm not getting what the question is asking...
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