Hymenoptera Venoms

Hymenopteran venoms are rich biochemical mixtures of compounds that paralyze prey, induce pain in large predators, or act as toxicants. Probably fewer than a hundred of these venom compounds have been identified, while many more remain to be discovered and characterized. There are three general categories of venom compounds: (1) small, nonproteinaceous molecules with molecular weights less than 300; (2) peptides with molecular weights 1,500-4,000; and (3) larger proteins and enzymes with molecular weights over 10,000. Compounds of the first category include histamines, serotonin, and various catecholamines that induce itching, immediate pain, redness, and changes in capillary permeability. The second category includes peptides such as hemolysins, which destroy red blood cells and cause pain; neurotoxins; and other pain-inducing compounds, such as kinins. The third category (larger proteins and enzymes) generally does not cause pain, but aids in the spread and activity of other venom components. A common example is hyaluronidase, which facilitates the spread of toxic components through the tissues. Exceptions are phospholipases that are toxic, disrupt the cell membranes, and cause release of pain-inducing agents (Schmidt, 1986a, 1986c, 1992).

Venoms of solitary wasps are designed to cause paralysis in insects, spiders, and other arthropods on which they prey. These venoms directly affect the nervous system and cause a general decline in the rate of metabolic processes. The purpose of these venoms is not to cause the death of the prey, but to incapacitate it as food for the larvae. Common components of the venoms of various solitary wasps are histamines, polyamines, and substances such as bradykinins that cause smooth muscles to contract. Some of their venoms also contain large amounts of acetylcholine, as in the case of the sphecid wasp Philan-thus triangulum. The venoms of solitary wasps generally produce only momentary, slight pain in humans.

Ant Venoms

Ant venoms serve a variety of functions, including defense, prey capture, aggregation, trail marking, alarm, and repelling intruders. Only the components of ant defensive venoms which are toxic to vertebrate animals are discussed here. The toxins normally are injected via the sting; however, some ants lack a sting and spray formic acid at their attackers (e.g., many formicine ants). Formic acid is a very effective deterrent, especially if sprayed into the eyes or applied directly into wounds made with the mandibles of the ant.

Venoms of the majority of stinging ants and other aculeate Hymenoptera are predominately composed of proteinaceous mixtures of simple organic compounds. Fire ant venoms, however, largely consist of alkaloids (95%), with only a small proteinaceous component (0.1—1%). The alkaloids cause most of the local sting reactions, whereas the proteins contain active allergenic antigens. The alkaloids are methyl-n-alkylpiperidines, called solenopsins, and a piperideine. The alkaloids are cytotoxic, hemolytic, fungicidal, insecticidal, and bactericidal. The characteristic dermal necrosis that becomes evident at the sting site is due to these alkaloids.

Protein-rich ant venoms are found in most subfamilies of ants, including the Ponerinae, Myrmeciinae, Pseudomyrmecinae, Ecitoninae, and some of the Myr-micinae (Table I). These venoms have not been well investigated because of difficulty in obtaining sufficient quantities of pure venom for analysis. The only studies available are those of primitive ants in the genus Myrmecia (Myrmeciinae) and the highly evolved Myrmica and Pogonomyrmex (Myrmicinae). Harvester ants (Pogono-myrmex) have a proteinaceous venom with high amounts of phospholipase A and B, hyaluronidase, a potent hemolysin called barbatolysin, and histamines (Schmidt, 1986b). Most ant venoms contain only small amounts of these materials. Other enzymes that have been identified in ant venoms include acid phosphatase, alkaline phosphatase, phosphodiesterase, lipase, esterase, and a nonspecific protease. The primary function of these compounds is to cause pain, either directly or through tissue destruction.

Vespid Venoms

Like those of most stinging social hymenopterans, vespid venoms are designed to cause pain. Their venom typically produces immediate pain, local swelling, and erythema caused by an increase in the permeability of blood vessels at the sting site. The pain often continues for several hours, whereas itching at the sting site may persist for days. Vespid venoms also cause the contraction of smooth muscles, reduced blood pressure, and the release of histamine and other biogenic amines. Hemolysis induced by lytic peptides and phospholipases may cause kidney damage. There is usually additional damage to surrounding tissues from the products of histolysis. Vespid venoms contain biologically active amines such as serotonin, histamines, tyramine, and catecholamines, all of which tend to produce pain. Acetylcholine has been reported to occur in the venoms of some Vespa species. However, the primary pain-causing substances are kinins. In addition, the venoms contain mast-cell degranulating peptides called mastoparans that cause the release of histamines.

Venoms also contain enzymes that can act as specific allergens and, in some species, neurotoxic compounds. The immediate pain caused by a vespid sting is principally due to serotonin and kinins. Venoms of some vespids contain alarm pheromones that function to alert nestmates to an intruder and focus the defense.

Honey Bee Venom

The venom of honey bees is a complex mixture of proteins, peptides, and small organic molecules (Schmidt, 1992). The most dangerous components for humans are phospholipases and hyaluronidase. Individuals can become sensitized to these materials and subsequentiy even die from a serious allergic reaction. Bee venom contains large quantities of a potent membrane-disrupting material called melittin, which makes membranes extremely susceptible to attack by phospholipases. Melittin also causes pain, increases capillary blood flow and cell permeability, triggers lysis of red blood cells, and enhances the spread of toxins. The effects of melittin, phos-pholipase, and a mast-cell degranulating peptide cause the release of histamine and serotonin from red blood cells and mast cells. While the components of honey bee venom that cause pain are very different from those in vespid venom, the end results are very similar. Some components of honey bee venom regulate and/or decrease inflammatory responses in some individuals. This perhaps explains why bee venom therapy has been useful in the treatment of certain forms of arthritis. Another component of honey bee venom, a neurotoxin called apamin, seems to cause more effects in insects than in humans.

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