| dc.description.abstract | This study was intended to investigate the effects of four
biologically active benzoquinones namely: maesanin, maesaquinone,
juglone and embelin on mitochondria isolated from rat liver and insect
flight muscle.
These benzoquinones occur naturally in the plant family
myrsinaceae. Five species of myrsinaceae have been identified in
Kenya and are widely distributed. Several studies have shown that
most of the benzoquinones have pharmacological potential and can
also protect stored grains against insect pests. There is, however, no
report relating the interaction of benzoquinones with insect energy
metabolism.
Three insect species were used in this study i.e. Schistocerca
gregaria (Lipid and carbohydrate metab oltsmg), Periplaneta americana
(carbohydrate me1.abolising) and Glos~ina morsitans (amino acid
metabolising) .
The effects of benzoquinones on energy metabolism was
determined by measuring the rate of oxygen consumption, oxidative
phosphorylation and ATPase activity.
Mitochondria from the insect thoracic muscle and rat liver
mitochondria were both prepared by differential centrifugation of the
homogenates from the respective tissues.
The rate of oxygen consumption was recorded
polarographically by an oxygen electrode at 250C in a closed
magnetically stirred chamber. ATPase activity was measured
enzymatically by pyridine nucleotide coupled assays using Perkin
Elmer spectrophotometer.
In the absence of benzoquinones (controls) the respiratory
control ratios were found to be above 3 for the four types of
mitochondria studied, which was an indication that the mitochondria
were well coupled and could effectively phosphorylate ADP to ATP.
When either maesaquinone or embelin were added in the
reaction chamber after the substrate, the-rate of oxygen consumption
was increased. Subsequent additions ...of ADP and CCCP after the e- ,'"
benzoquinones increased the rate of oxygen consumption in rat liver
and Glossina mitochondria. These observations indicated that the
rrutochondrta were partially uncoupled. Embelin had similar effect on
Schistocerca mitochondria.
However, when either ADP or CCCP were added after the
benzoquinones (maesaquinone and embelin), there was no further
stimulation in the rate of oxygen consumption by Periplaneta
mitochondria. A similar effect was observed with maesaquinone on
Schistocerca mitochondria. From these observations, it was
suggested that the mitochondria were completely uncoupled.
Juglone was found to completely uncouple the mitochondria
from rat liver and flight muscle of Schistocerca and Periplaneta
while it inhibited oxygen consumption by mitochondria isolated
from Glossina.
The uncoupling (partial or complete) was depicted by low
respiratory control ratios (about 1.00) and ADP/0 ratios.
When the effect of maesanin was "investigated, it was found
that addition of this benzoquinone after-the substrates (malate plus
glutamate or succinate) inhibited both state 4 and state 3 rates of
oxygen consumption. This effect was observed on the four types of
mitochondria used. From this observation it was suggested that
maesanin possibly inhibited the electron flow along the electron
transport chain.
Further experiments with the known inhibitors of electron
transport chain i.e. rotenone for Site I, antimycin for Site II and
KCN for Site III indicated that maesanin inhibited the electron
transport chain before cytochrome C. This was found to be true for
the three types of insects and rat liver mitochondria.
The ATPase activity was studied on both directions i.e. toward
ATP formation (ATPase phosphorylating) and toward ATP hydrolysis
(ATPase dephosphorylating).
Effects of benzoquinones on ATPase (phosphorylating)
indicated that the enzyme activity was inhibited by maesanin in all
the four types of mitochondria. On the other hand, maesaquinone
and embelin were found to stimulate the activity of this enzyme
though in Glossina the stimulation was not significant. Juglone also
had a similar effect in all the four cases though it lowered the
activity in Glossina mitochondria. "
The ATPase activity in the dtreetton of ATP hydrolysis of
mitochondria from the four sources was stimulated by
maesaquinone and embelin except that in the Glossina
mitochondria the stimulation was not significant. However, juglone
inhibited the activity and maesanin had rio significant effect. These
observations indicate that these benzoquinones interfere with
energy generation.
The fact that benzoquinones either uncouples the mitochondria
or inhibits the electron transport chain and interferes with ATPase
activity suggest that they interrupt energy metabolism in the
mitochondria of the three types of insects and rat liver. It is yet to be
investigated whether in vivo the mechanism of toxicity is the same in
insect and rat. | en |
