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analyzed for tetrapyrrole accumulation, while others were exposed to light. Oph
exhibited very potent porphyric insecticidal properties (Table 18.7 ). In the absence
of added ALA it induced the massive accumulation of Proto. In other words in
the absence of added ALA it behaved as an inducer of Proto accumulation. In the
presence of added ALA the inducing properties of Oph were obscured by the massive
enhancement of ALA conversion to Proto. To put it differently, Oph behaved as an
inducer-enhancer of Proto accumulation in T . ni (Rebeiz et al. 1990a ) The correlation
between photodynamic death and Proto accumulation was highly significant
(Table 18.7 ) .
18.3.2 Zn-Proto Accumulation in T. ni Larvae
Treated with ALA and Oph
It has been our experience that dicarboxylic and monocarboxylic tetrapyrroles of
plant, insect, and animal tissues are found in the hexane-extracted acetone fraction of
extracted tissues (Rebeiz 2002 ). Thus in the hexane-extracted acetone fraction of the
ALA + Oph-treated insects, in addition to fluorescence emission originating in the
Proto pool, another fluorescence emission band of smaller amplitude was observed.
It exhibited an emission maximum at 590 nm at room temperature and at 587 nm at
77 K in ether. Since the band exhibited fluorescence properties at room temperature
and at 77 K that were identical to those of Zn-Proto (Fig. 18.3 ), it was assigned to the
biosynthesis and accumulation of Zn-Proto in the treated larvae. The amount of
Zn-Proto formed following treatment with Oph is shown in (Table 18.7 ). It too
correlated positively with insect death. After the same period of dark-incubation, no
significant amounts of Proto or Zn-Proto were detected in control insects.
18.3.3 Proposal of a Dark-Death Hypothesis
The discovery of Zn-Proto accumulation suggested an explanation for insect
mortality observed during dark incubation. In addition to damage via singlet
oxygen, it is conceivable, that ALA + modulator-dependent larval death may also
be caused by the induction of a premature release of O 2 and ·OH radicals from the
active site of a damaged cytochrome c oxidase. Indeed cytochrome c oxidase is the
major consumer of O 2 in eukaryotic cells. Because of spin restrictions, O 2 cannot
accept four electrons at once. During cytochrome c -mediated electron transport it
therefore accepts electrons, one at a time. In the process, O 2 passes through a series
of partially-reduced intermediates including the highly reactive superoxide radical
(O 2 ) and the hydroxy radical ( · OH) (Halliwell 1984 ). In mitochondria, these highly
reactive oxygen radicals are kept tightly bound to the active site of cytochrome
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