Abstract
The factors affecting acetaldehyde oxidation were studied by direct measurement of redox changes of NADH and cytochrome c in isolated rat liver mitochondria which contain the natural complement of mitochondrial substrates and co-factors, etc. Thus, this system affords a quantitative model for mitochondrial acetaldehyde metabolism simulating in vivo conditions. The activity of acetaldehyde dehydrogenase, as measured by the turnover number of cytochrome c, k3, depends upon the substrate concentration in a complex way. It reaches a maximum below 0.033 mM acetaldehyde and decreases abruptly at higher acetaldehyde concentration, interpreted here to be due to substrate inhibition. When mitochondria undergo hypotonic swelling, the maximal value of k3 is lowered by a factor of 15 and the substrate inhibition largely disappears. When mitochondria are stripped of the outer membrane and are suspended in pyrophosphate buffer, similar effects are obtained. It is concluded that acetaldehyde oxidation in mitochondria is dependent upon the state of mitochondria and the substrate concentration, and that the mitochondrial metabolism of acetaldehyde cannot be accurately predicted from in vitro data of solubilized enzyme.
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