Venoms contain many components that have been recognized. They contain proteins, lipids, steroids, aminopolysaccharides, amines, quinines, neurotransmitters, and other compounds, and are capable of causing many effects. Elapid venom is the least complex, while pit vipers have the most complex venoms. Elapid venoms have higher concentrations of esterases, such as acetylcholinesterase, while viper venoms have higher concentrations of endopeptidases. This difference is important because it helps understand why elapid venom exerts effects on the nervous system while viper venom is mainly a somatic toxin.
To better understand the diverse effects of venom, let us examine several of the common components found in snake venoms. Proteolytic enzymes are trypsin like and account for much of the digestive reactions of snake venoms. These enzymes break the peptide bonds between amino acids and denature proteins. Arginine ester hyrdolases break carbon-oxygen bonds that are not neurotransmitter related (neurotransmitter esterases will be explained in greater detail later), but rather cause breakup of certain proteins where arginine residues are frequent. Collagenase degrades collagen, which is a major component of connective tissue, skin and flexible vascular tissue. This enzyme is found in crotalid and viperid venoms and this explains the necrosis often seen following viper bites. Phospholipases A and B degrade lipids to free fatty acids and can cause damage to the cell membrane causing lysis and apoptosis. Phosphodiesterases break the phosphate bonds that provide the backbone for nucleic acids, thus rendering DNA and RNA useless in the effected cell, eventually causing apoptosis. Acetylcholinesterase is a neurotransmitter esterase that breaks the acetate ester bond found in acetylcholine. The main site of action is in the synapse, although some vesicle-contained acetylcholine may be degraded as well. The end result of this action is an inability to enervate smooth muscle and the inability to relax striated muscle resulting in spasmodic paralysis and sometimes a concurrent drop in blood pressure and difficulty breathing. Some venoms also contain highly competitive antagonists that prevent acetylcholine from binding at the postsynaptic membrane receptors, also causing neurotoxic symptoms and often apnea and asphyxiation will result. DNase and Rnase are enzymes that degrade DNA and RNA respectively. NAD Nucleotidase degrades nicotinamide, which is an important part of the cellular metabolism machinery. Cellular respiration is interrupted and cell death may ensue. L-Amino acid oxidase is found in all known snake venoms. Procoagulants cause blood coagulation to occur; conversely Anticoagulants prevent blood from clotting. Both chemicals may be found in the same venom, which is perplexing since they may antagonize each other. Anticoagulant action is more frequent and will cause bleeding at the site of envenomation as well as internal bleeding and tissue edema.
There is a fantastic analysis of venom biochemistry by Anthony Tu. The text is well written, but requires a strong knowledge of chemistry and cell biology. Venoms: Chemistry and Molecular Biology is the name of the text, and it is excellent for students.
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