The heavy chain contains domains with several functions; it has the domain responsible for binding specifically to presynaptic nerve terminals, as well as the domain responsible for mediating translocation of the light chain into the cell cytoplasm as the vacuole acidifies. The light chain is a M27-family zinc metalloprotease and is the active part of the toxin. It is translocated into the host cell cytoplasm where it cleaves the host protein SNAP-25, a member of the SNARE protein family, which is responsible for fusion. The cleaved SNAP-25 cannot mediate fusion of vesicles with the host cell membrane, thus preventing the release of the neurotransmitter acetylcholine from axon endings. This blockage is slowly reversed as the toxin loses activity and the SNARE proteins are slowly regenerated by the affected cell.The seven toxin serotypes (A–G) are traditionally separated by their antigenicity. They have different tertiary structures and sequence differences. While the different toxin types all target members of the SNARE family, different toxin types target different SNARE family members. The A, B, and E serotypes cause human botulism, with the activities of types A and B enduring longest in vivo (from several weeks to months). Existing toxin types can recombine to create "hybrid" (mosaic, chimeric) types. Examples include BoNT/CD, BoNT/DC, and BoNT/FA, with the first letter indicating the light chain type and the latter indicating the heavy chain type.
Pharmacology
Mechanism of action
It is a synthetic prostaglandin analog (or more specifically, an analog of prostaglandin F2α) that works by increasing the outflow of aqueous fluid from the eyes.Like other analogs of prostaglandin F2α such as tafluprost and latanoprost, travoprost is an ester prodrug of the free acid, which acts as an agonist at the prostaglandin F receptor, increasing outflow of aqueous fluid from the eye and thus lowering intraocular pressure. Pharmacokinetics
Travoprost is absorbed through the cornea, where it is hydrolysed to the free travoprost acid. Highest concentrations of the acid in the eye are reached one to two hours after application, and its half-life in the aqueous fluid is 1.5 hours. Once it reaches the bloodstream, it is quickly metabolised, so that concentrations in the system do not exceed 25 pg/ml (compared to 20 ng/ml in the eye, which is higher by nearly a factor of 1000).Metabolites are formed by beta oxidation of the acidic chain (compare Tafluprost#Pharmacokinetics), oxidation of the OH-group in the other side chain, and reduction of the double bond next to this OH-group. Travoprost acid and its metabolites are mainly excreted via the kidney with a terminal half-life of 45 minutes. References
External links
"Travoprost". Drug Information Portal. U.S. National Library of Medicine.