12. QUINONES: STRUCTURE AND DISTRIBUTION
695
different tissues is consistent with its respiratory function, e.g., the Q 10
content of beef muscle (1.2 mg./kg. moist tissue) obtained from the
neck muscle of steers, is approximately 1% of that found in beef heart
tissue, which has a higher rate of respiration (288). High concentrations have been found in all mitochondrial sources examined, the
quinone being localized almost entirely in a lipoprotein fraction (292).
When mitochondria, or derived particles, are extracted with isooctane,
the coenzyme Q is removed and simultaneously succinic oxidase activity
is lost. This capacity can be restored by addition of coenzyme Q (281,
293). (Other lipid components are also extracted by isooctane, and it
may be necessary to add the Q-containing lipoprotein for complete
restoration of activity.) Coenzyme Q (either particle-bound or externally added) is rapidly reduced by substrates (e.g., succinate or reduced diphosphopyridine nucleotide), and the reduction product is
rapidly oxidized in the presence of mitochondrial enzymes, the oxidoreduction processes being affected in the usual way by conventional
inhibitors of the electron transport system (293). Thus coenzyme Q is
an active component of the electron transport particle, and accumulated
evidence (294, 295) indicates that it occupies a site in the succinic
chain between cytochromes b and c. Coenzyme Q activity is limited to
2-methyl-5,6-dimethoxybenzoquinones, which have an isoprenoid side
chain of at least ten carbon atoms at position 3 (296). Several other
quinones, including vitamins K and tocopherolquinone, failed to restore
succinic cytochrome c reductase and succinoxidase activity in mitochondria from which coenzyme Q 10 had been removed by solvent extraction, although plastoquinone shows some degree of coenzyme Q activity (76a, 294, 297).
By studying the oxidoreduction reactions of coenzyme Q under
steady-state conditions, Hatefi (298) has shown that it acts as an electron carrier in oxidative phosphorylation. Addition of inorganic phosphate stabilizes coenzyme Q in a reduced form (quinol phosphate), and
this effect is reversed by adenosine diphosphate (ADP). In the terminal
reaction ATP is formed by transfer of a high energy phosphate group
to ADP, and the quinone carrier becomes available again for a repetition
of the oxidative phosphorylation cycle.
Martius and Nitz-Litzow (301) postulated that vitamin K is also
concerned in oxidative phosphorylation, on the basis of experiments
with chick and rat liver mitochondria. This view has not found general
acceptance, and indeed there is no agreement on the existence of vitamin
K in animal mitochondria. According to Dam and co-workers (277) it
exists in beef liver mitochondria, but Lester and Crane (83) could find
no trace in beef heart lipids, and other workers have failed to detect
695
different tissues is consistent with its respiratory function, e.g., the Q 10
content of beef muscle (1.2 mg./kg. moist tissue) obtained from the
neck muscle of steers, is approximately 1% of that found in beef heart
tissue, which has a higher rate of respiration (288). High concentrations have been found in all mitochondrial sources examined, the
quinone being localized almost entirely in a lipoprotein fraction (292).
When mitochondria, or derived particles, are extracted with isooctane,
the coenzyme Q is removed and simultaneously succinic oxidase activity
is lost. This capacity can be restored by addition of coenzyme Q (281,
293). (Other lipid components are also extracted by isooctane, and it
may be necessary to add the Q-containing lipoprotein for complete
restoration of activity.) Coenzyme Q (either particle-bound or externally added) is rapidly reduced by substrates (e.g., succinate or reduced diphosphopyridine nucleotide), and the reduction product is
rapidly oxidized in the presence of mitochondrial enzymes, the oxidoreduction processes being affected in the usual way by conventional
inhibitors of the electron transport system (293). Thus coenzyme Q is
an active component of the electron transport particle, and accumulated
evidence (294, 295) indicates that it occupies a site in the succinic
chain between cytochromes b and c. Coenzyme Q activity is limited to
2-methyl-5,6-dimethoxybenzoquinones, which have an isoprenoid side
chain of at least ten carbon atoms at position 3 (296). Several other
quinones, including vitamins K and tocopherolquinone, failed to restore
succinic cytochrome c reductase and succinoxidase activity in mitochondria from which coenzyme Q 10 had been removed by solvent extraction, although plastoquinone shows some degree of coenzyme Q activity (76a, 294, 297).
By studying the oxidoreduction reactions of coenzyme Q under
steady-state conditions, Hatefi (298) has shown that it acts as an electron carrier in oxidative phosphorylation. Addition of inorganic phosphate stabilizes coenzyme Q in a reduced form (quinol phosphate), and
this effect is reversed by adenosine diphosphate (ADP). In the terminal
reaction ATP is formed by transfer of a high energy phosphate group
to ADP, and the quinone carrier becomes available again for a repetition
of the oxidative phosphorylation cycle.
Martius and Nitz-Litzow (301) postulated that vitamin K is also
concerned in oxidative phosphorylation, on the basis of experiments
with chick and rat liver mitochondria. This view has not found general
acceptance, and indeed there is no agreement on the existence of vitamin
K in animal mitochondria. According to Dam and co-workers (277) it
exists in beef liver mitochondria, but Lester and Crane (83) could find
no trace in beef heart lipids, and other workers have failed to detect