Swayback disease, a
neurodegenerative disorder of lambs, and
Menkes disease, the human equivalent, are caused by a deficiency of dietary
copper. Reports of low enzymic activity suggest that several
copper-containing
enzymes, including
cytochrome-c oxidase (COX), may influence the progress of these diseases. To investigate its role in the development of
neurodegenerative disorders, in particular
swayback disease, we isolated COX from the brains and livers of
swayback-diseased lambs. Comparative
sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) combined with densitometric analysis revealed that whereas the structure of COX from the liver of diseased animals was normal, the corresponding brain
enzyme was subunits II-, III-, and IV-deficient; the deficiency was 55, 30, and 65% respectively. The activities of liver and brain COX from normal and diseased lambs were compared by polarographic assay at low ionic strength. Whereas the
enzyme from normal brains and both forms of the liver
enzyme yielded characteristic biphasic Eadie-Hofstee plots, the brain
enzyme from diseased animals displayed a single phase with a K(m) of 4.7 +/- 2.4 x 10(-6) M: the K(m) values of COX from the normal brain were 12 +/- 2.5 x 10(-6) and 5.5 +/- 0.5 x 10(-7) M. We conclude that the altered
enzyme structure accounts for the uncharacteristic kinetics and low activity we have observed for the isolated brain
enzyme. We also conclude that the altered
enzyme structure partly accounts for the low
oxidase activity and decreased
ATP synthesis that has been widely reported for brain tissue from
swayback-diseased animals. We postulate that the subunit deficiency probably results from incomplete crosslinking between the subunits and the membrane, and predict that similar structural and kinetic factors may also account for low COX activity in
Menkes disease.