|MadSci Network: Biochemistry|
Botox is the common name for botulinum neurotoxin A, a toxic protein that is produced by the bacterium clostridium botulinum. It is a neurotoxin that causes muscle relaxation at low doses and muscle paralysis at higher doses. To explain how it works, let me briefly describe the process by which neurons transmit signals from one to another. Within a neuron, a nerve impulse is transmitted electrically. However, between two neurons, the signal is transmitted by chemicals that are released into the gap between the neurons. When these chemicals (in general, called neurotransmitters) are not in use, they are stored within synaptic vesicles – small spheres within the nerve cell. The vesicles are surrounded by a thin membrane. When a nerve impulse arrives, the vesicles move toward the outer membrane of the cell and make contact with it. The membrane of the vesicle merges with the membrane of the cell. For this fusion to occur, certain proteins must be present on the surface of the vesicle. When the fusion occurs, the contents of the vesicle, the neurotransmitters, spill outside of the cell. The neurotransmitters are then sensed by the adjacent neuron, and an electrical signal is generated and passed on. Botox is a protein that functions as an enzyme (specifically, a zinc endopeptidase) that cleaves another protein called synaptosomal associated protein or SNAP-25. SNAP-25 is located on the surface the membranes of the synaptic vesicles that contain neurotransmitters. This is one of the proteins that is necessary for the fusion of the vesicle with the cell membrane. When botox is present, it cleaves SNAP-25 causing it to be non- functional. The vesicles can’t fuse with the cell membrane, the neurotransmitters are not released, and a nerve impulse cannot be transmitted from one neuron to another. When this occurs in muscle tissue the end result is paralysis or at least a barrier to easy function, which manifests as relaxation. So to answer your question, if the article you read stated that “Botox injections inhibited the production of an enzyme that causes muscle contraction” this does not appear to be accurate. A simple one-sentence description of how botox works might be better stated this way: Botox is an enzyme which indirectly inhibits the release of neurotransmitters that are necessary for muscle contraction. [Moderator's note : I believe that the primary reference for the work Michael described is : Blasi J, Chapman ER, Link E, Binz T, Yamasaki S, De Camilli P, Sudhof TC, Niemann H, Jahn R. (1993) Botulinum neurotoxin A selectively cleaves the synaptic protein SNAP-25. Nature. 365:160-3. ]
Try the links in the MadSci Library for more information on Biochemistry.