md2besoon
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Heat-Labile Enterotoxins. LT of ETEC are related antigenically and structurally to the cholera toxin (CT). They are called heat-labile because they can be inactivated at 65oC for 30 min. Several different types have been identified, LT1 is encoded by a plasmid whereas LT2 is antigenically distinct from LT1 and CT and is chromosomally encoded.
LT1 is similar to cholera-toxin in that it is composed of an A subunit (25-28 kDa) that has ADP-ribosylating activity which is bound to 5 B subunits (11-13 kDa) that also bind to the GM1 ganglioside on mammalian cells. After the B subunits bind to GM1, the A subunit enters the cell and irreversibly catalyzes the ADP-ribosylation of the regulatory G-proteins that regulates adenylate cyclase activity (transfer of ADP-ribose from NAD+ to G-protein that has bound GTP), the ADP-ribosylation prevents the hydrolysis of the bound GTP molecule which maintains the G-protein in an activation conformation, continuously activating adenylate cyclase. The increased levels of cAMP that occur results in an decreased absorption of Na+ by the tips of the intestinal villi and an increased secretion of Cl- by the crypt areas between the villi. The net result in absorption of water into the intestine which manifests itself as diarrhea.
Heat-Stable Enterotoxins. The Heat-Stable enterotoxins of E. coli are designated ST (stable toxins). Compared to LTs, the STs are a family of small molecular weight polypeptides of from 1.5 to 4.0 kDa. Their small size provides them with their heat stability (little tertiary structure to denature by heat) as well as their non-antigenicity. That is, they do not induce serum antibodies after infection. Several different STs are demonstrated: STA/ST1 are mainly isolated from humans and other animals whereas STB/ST2 are usually isolated from swine. STA is known to stimulate guanylate cyclase but STB has not been shown to stimulate either adenylate or guanylate cyclase - it's mechanism seems to involves transport of bicarbonate rather than Cl- ions
from----> http://www.okstate.edu/ag/fapc/fsw/ecoli/ecolipm....
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| Bela
Forum Guru
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Dang man  You actually took all that time to type this up? I appreciate it. Actually I was just looking for a simple answer like:
Heat Labile :arrow: ETEC :arrow: Activates cAMP :arrow: causes release of Cloride and water in the intestine :arrow: diarrhea (same as Cholera like toxin)
Heat Stable :arrow: EHEC :arrow: Activates cGMP (same as Shiga like toxin)
However, I think I read somewhere that ETEC has both heat stable and heat labile toxins but I am not sure.
Thnx again 
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