Abstract
Compared to other groups of naturally occurring toxins, the pyrrolizidine alkaloids (PAs) are understudied. There is a relative paucity of information concerning the health consequences of exposure to humans, but the scope and nature of this problem should not be underestimated. About 3% of the flowering plants of the world are estimated to contain toxic PAs. The distribution of the alkaloids has been reviewed. 1–3 Some 160 esterified alkaloids are known, and the number is increasing yearly. 1 These alkaloids are scattered in 332 species and distributed through 63 genera and 13 families. 4 A complicating factor is that often these alkaloids are present as, or coexistent with, their corresponding N-oxides. With certain exceptions, these neutral substances are almost as toxic as their eponymous alkaloids. 5 , 6 In 1968, only 105 total alkaloids, esters and nonesters, were listed in the monograph of Bull et al. 7 As this century winds down, PAs are probably the most significant group of natural products in terms of their adverse effects on humans and their economic activities.