Platyphora boucardi leaf-beetles sequester tertiary
pyrrolizidine alkaloids of the
lycopsamine type acquired from their host-plant Prestonia portobellensis (Apocynaceae) and synthesize their own
alkaloids from exogenous
retronecine and aliphatic 2-hydroxy
acids. Tracer studies with [14C]
rinderine and its N-
oxide revealed that P. boucardi sequesters both alkaloidal forms with the same efficiency, but accumulates exclusively tertiary
alkaloids. There is no substantial
alkaloid accumulation in the body outside the defensive glands. Feeding studies with [2H][14C]
rinderine confirmed that P. boucardi specifically epimerizes
rinderine to its stereoisomers
intermedine and
lycopsamine. Feeding studies with [2H][14C]
retronecine proved the ability of P. boucardi to synthesize O7- and O9-(2-hydroxyisovaleryl)-retronecine and O7-lactyl-O9-(2-hydroxyisovaleryl)-retronecine. Both,
alkaloids of the
lycopsamine type and self-synthesized
retronecine esters accumulate in the defensive secretions at concentrations up to 38 mM and 33 mM, respectively. The different biochemical strategies to maintain pro-toxic
pyrrolizidine alkaloids and to prevent self-
poisoning, developed by specialized insects, are compared. There are two major findings: (1) the chemical defense mediated by plant acquired
pyrrolizidine alkaloids in the taxonomically related palaearctic Oreina and neotropical Platyphora leaf beetles have been evolved independently, since the biochemical mechanisms of storing and maintaining the
alkaloids is completely different in the two genera; (2) unexpected parallels exist between taxonomically unrelated Coleoptera and Lepidoptera in their ability to synthesize the same
retronecine esters and to catalyze the same site-specific epimerizations of the
lycopsamine stereoisomers.