Reactive oxygen species production in cardiac mitochondria after complex I inhibition: Modulation by substrate-dependent regulation of the NADH/NAD+ ratio
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Abstract
<p class="first" id="P1">Reactive oxygen species (ROS) production by isolated complex
I is steeply dependent
on the NADH/NAD
<sup>+</sup> ratio. We used alamethicin-permeabilized mitochondria to study the substrate-dependence
of matrix NADH and ROS production when complex I is inhibited by piericidin or rotenone.
When complex I was inhibited in the presence of malate/glutamate, membrane permeabilization
accelerated O
<sub>2</sub> consumption and ROS production due to a rapid increase in NADH generation
that was
not limited by matrix NAD(H) efflux. In the presence of inhibitor, both malate and
glutamate were required to generate a high enough NADH/NAD
<sup>+</sup> ratio to support ROS production through the coordinated activity of malate
dehydrogenase
(MDH) and aspartate aminotransferase (AST). With malate and glutamate present, the
rate of ROS production was closely related to local NADH generation, whereas in the
absence of substrates, ROS production was accelerated by increase in added [NADH].
With malate alone, oxaloacetate accumulation limited NADH production by MDH unless
glutamate was also added to promote oxaloacetate removal via AST. α-ketoglutarate
(KG) as well as AST inhibition also reversed NADH generation and inhibited ROS production.
If malate and glutamate were provided before rather than after piericidin or rotenone,
ROS generation was markedly reduced due to time-dependent efflux of CoA. CoA depletion
decreased KG oxidation by α-ketoglutarate dehydrogenase (KGDH), such that the resulting
increase in [KG] inhibited oxaloacetate removal by AST and NADH generation by MDH.
These findings were largely obscured in intact mitochondria due to robust H
<sub>2</sub>O
<sub>2</sub> scavenging and limited ability to control substrate concentrations in
the matrix.
We conclude that in mitochondria with inhibited complex I, malate/glutamate-stimulated
ROS generation depends strongly on oxaloacetate removal and on the ability of KGDH
to oxidize KG generated by AST.
</p>