PHOTODYNAMICS OF PORPHYRIC INSECTICIDES

The discovery of porphyric insecticides was a direct fallout of the di scovery and development of photodynamic herbicides. Tetrapyrrole-depen dent photodynamic herbicides are compounds that force green plants to accumulate undesirable amounts of metabolic intermediates of the chlor ophyll and heme metabolic pathways, namely, tetrapyrroles. In light, t he accumulated tetrapyrroles photosensitize the formation of singlet o xygen that kills treated plants by oxidation of their cellular membran es. Demonstration of the potential for tetrapyrrole accumulation in in sects was achieved by spraying T, ni larvae with delta-aminolevulinic acid (ALA) and 2,2-dipyridyl (Dpy). Treated larvae were placed overnig ht in darkness at 28 degrees C in order to allow for tetrapyrrole accu mulation. Extraction of treated, dark-incubated larvae with ammoniacal acetone, followed by spectrofluorometric examination of the larval ex tract, revealed the accumulation of massive amounts of protoporphyrin IX (Proto). A high degree of correlation was observed between Proto ac cumulation in darkness and larval death in the light. A few hours afte r exposure to light, the larvae became sluggish and flaccid due to los s of body fluids. Death was accompanied by extensive desiccation. Beca use control of insects by ingestion is as viable an option as control by spraying, and offers certain advantages under household conditions, studies were conducted to determine whether combinations of ALA and p orphyric insecticide modulators would be effective if ingested with th e food. The effect of ALA and 1,10-phenanthroline (Oph) were determine d by incorporating them into the diet of T. ni larvae. After exposure to light, following 17 h of dark incubation, larvae underwent violent convulsions and vomiting and died within 20 to 40 s. Tetrapyrrole anal ysis of the treated larvae immediately after dark incubation revealed significant amounts of Proto and Zn-Proto accumulation. Correlation be tween tetrapyrrole accumulation and larval death was significant. Simi lar results were obtained when ALA and Dpy were administered to the la rvae with the diet. The above results indicated that in addition to co ntact via spraying, porphyric insecticides had the potential to be ver y potent when ingested. For a more thorough understanding of the mode of action of porphyric insecticides, the phenomenology of tissue, cell ular, and subcellular sites of tetrapyrrole accumulation in representa tive insect species was investigated. In T. ni larvae, on a unit prote in basis, about 59% of the accumulated Proto was observed in the hemol ymph, 35% in the gut, and 6% in the integument. Further understanding of the response of insect organs and tissues to porphyric insecticide treatment was obtained by investigating the response of isolated organ s and tissues to incubation with ALA + Dpy or ALA + Oph in adult Blatt ella germanica (German cockroach), adult Anthonomus grandis (cotton bo il weevil), fifth instar larvae of Heliothus ten (corn earworm), and f ifth instar larvae of T. ni (cabbage looper). In T. ni, and H. tea, si gnificant Proto accumulation was observed in incubated midgut and fat bodies. Proto accumulation occurred when tissues were incubated with D py, ALA + Dpy, Oph, and ALA + Oph (2). No response to treatment with A LA alone was observed. In cockroaches, more of the Proto appeared to a ccumulate in the male and female guts than in their abdomen. As in T. ni and H. tea, the response was elicited by each of the treatments tha t included Dpy or Oph. Cotton bell weevil abdomens appeared to be less responsive than the abdomens of the other three species. To determine whether Proto accumulation resulted in photodynamic damage of incubat ed tissues, T. ni midguts were incubated in darkness either in buffer, with ALA, or with Oph + ALA. Oxygen consumption of the tissue was mon itored before and after exposure to 2-h of illumination. A 30% decreas e in O-2 consumption was observed in midguts treated with Oph or with ALA + Oph after 2 h in the light. The decrease in oxygen consumption o bserved in isolated T. ni midguts was shown to be caused by photodynam ic damage to mitochondrial enzymes. Finally, structure-function photod ynamic insecticidal studies led to the identification of 36 compounds belonging to 10 different chemical families that were effective (>70% mortality) against at least one insect species. Of the 36 modulators, 10 exhibited potent activity toward cockroaches.