PRINCIPLES OF TOXICOLOGY

422 PROPERTIES AND EFFECTS OF NATURAL TOXINS AND VENOMS
which is now naturalized in southern California, is similar and is also used for making decorative
necklaces. It contains ricin, a homologous protein with the same mechanism of action and potential
lethality. These toxins, like diphtheria toxin, are composed of two polypeptide chains: the A chain is
the active inhibitor of protein synthesis, while the B chain is needed to bind to the cell membrane and
stimulate internalization of the toxin. The symptoms of poisoning by these two toxins develop rather
slowly during the first 24 h after ingestion, but if the victim has ingested several seeds, he or she may
suffer much during the ensuing couple days and then succumb to an awful death (Table 17.3). The
toxins are embedded within the fibrous seed pit; if it is not broken up by chewing, the person may not
receive much toxin. Induced vomiting by ipecac syrup followed by gastric lavage is recommended as
soon as possible during the first few hours after ingestion; otherwise, symptomatic treatment is all that
can be done, since the toxins are internalized within the cell.
As herbal medicines, mistletoe leaves and berries have been used to prepare orally administered
extracts and teas for the treatment of a variety of conditions including high blood pressure, tachycardia,
insomnia, depression, sterility, ulcers, and cancer, to name only a few. While a few of these conditions,
such as hypertension and tachycardia, might ostensibly be ameliorated, based upon present knowledge
of the contents of mistletoe, at present, there are no medical reports supporting the therapeutic use of
mistletoe extracts. Ingestion of mistletoe extracts is likely to be injurious to one’s health, due to the
presence of a toxin called viscumin whose action is similar to ricin and abrin, as well as smaller peptide
toxins called viscotoxins (Table 17.3), which depolarize muscle cell membranes and can cause
hypotension, bradycardia, and other problems.
Plants Causing Contact Dermatitis
A wide variety of plants and animals are known to trigger inflammatory reactions. At the beginning
of the twentieth century the Nobel-prize winning French physiologist Edward Richet initiated a study
Figure 17.5 Poison ivy, Toxicodendron radicans. Contact with this vine releases several chemically related
compounds called urushiols, which cause contact dermatitis on repeated contact. Virginia creeper, lower right, is
commonly mistaken for poison ivy. Its leaves and stems are harmless, although its berries are poisonous.