PRINCIPLES OF TOXICOLOGY - Biology East Borneo

422 PROPERTIES AND EFFECTS OF NATURAL TOXINS AND VENOMSwhich is now naturalized in southern California, is similar and is also used for making decorativenecklaces. It contains ricin, a homologous protein with the same mechanism of action and potentiallethality. These toxins, like diphtheria toxin, are composed of two polypeptide chains: the A chain isthe active inhibitor of protein synthesis, while the B chain is needed to bind to the cell membrane andstimulate internalization of the toxin. The symptoms of poisoning by these two toxins develop ratherslowly during the first 24 h after ingestion, but if the victim has ingested several seeds, he or she maysuffer much during the ensuing couple days and then succumb to an awful death (Table 17.3). Thetoxins are embedded within the fibrous seed pit; if it is not broken up by chewing, the person may notreceive much toxin. Induced vomiting by ipecac syrup followed by gastric lavage is recommended assoon as possible during the first few hours after ingestion; otherwise, symptomatic treatment is all thatcan be done, since the toxins are internalized within the cell.As herbal medicines, mistletoe leaves and berries have been used to prepare orally administeredextracts 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 knowledgeof the contents of mistletoe, at present, there are no medical reports supporting the therapeutic use ofmistletoe extracts. Ingestion of mistletoe extracts is likely to be injurious to one’s health, due to thepresence of a toxin called viscumin whose action is similar to ricin and abrin, as well as smaller peptidetoxins called viscotoxins (Table 17.3), which depolarize muscle cell membranes and can causehypotension, bradycardia, and other problems.Plants Causing Contact DermatitisA wide variety of plants and animals are known to trigger inflammatory reactions. At the beginningof the twentieth century the Nobel-prize winning French physiologist Edward Richet initiated a studyFigure 17.5 Poison ivy, Toxicodendron radicans. Contact with this vine releases several chemically relatedcompounds called urushiols, which cause contact dermatitis on repeated contact. Virginia creeper, lower right, iscommonly mistaken for poison ivy. Its leaves and stems are harmless, although its berries are poisonous.