A latrotoxin is a high-molecular mass neurotoxin

Neurotoxin

A neurotoxin is a toxin that acts specifically on nerve cells , usually by interacting with membrane proteins such as ion channels. Some sources are more general, and define the effect of neurotoxins as occurring at nerve tissue...

 found in the venom of spider

Spider

Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...

s of the genus Latrodectus (widow spiders). Latrotoxin are the main active components of the venom and are responsible for the symptoms of latrodectism

Latrodectism

Latrodectism is the clinical syndrome caused by the neurotoxic venom , that can be injected by the bite of any spider that is a member of the spider genus Latrodectus, in the family Theridiidae....

.

The following latrotoxins have been described: five insecticidal toxins, termed α, β, γ, δ and ε-latroinsectotoxins, one vertebrate-specific neurotoxin, alpha-latrotoxin

Alpha-latrotoxin

α-Latrotoxin can naturally be found in widow spiders of the genus Latrodectus. The most famous of those spiders are the black widows, Latrodectus mactans. The venom of widow spiders contains several protein toxins, called latrotoxins, which selectively target against either vertebrates, insects...

, and one toxin affecting crustaceans, α-latrocrustatoxin.

α-Latrotoxin

The best-studied latrotoxin is alpha-latrotoxin

Alpha-latrotoxin

α-Latrotoxin can naturally be found in widow spiders of the genus Latrodectus. The most famous of those spiders are the black widows, Latrodectus mactans. The venom of widow spiders contains several protein toxins, called latrotoxins, which selectively target against either vertebrates, insects...

, which acts presynaptically to release neurotransmitters (including acetylcholine

Acetylcholine

The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...

) from sensory and motor neurons, as well as on endocrine cells (to release insulin

Insulin

Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....

, for example). It is a ~130 kDa protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

which exists mainly in its dimerized or tetramerized forms.

Latroinsectotoxins

The natural prey of widow spiders are insects, and several insectotoxins are found in its venom. The latroinsectotoxins appear to have similar structures.

Other venom components

Apart from the high molecular weight latrotoxins described above, Latrodectus venom also contains low molecular weight proteins whose function has not been explored fully yet, but may be involved in facilitating membrane insertion of latrotoxins.