12. QUINONES: STRUCTURE AND DISTRIBUTION
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by certain insects as a means of defense, and one or two simple naphthoquinones (e.g., juglone, plumbagin) known to be poisonous to animals
and plants. However, toxicity is not restricted to simple structures, for
a number of more complex quinones (e.g., skyrin) occurring in plant
pathogens are harmful to various organisms.
The only example of 1,6-addition to the conjugated system (c, above)
to be considered is that of hydrogen. Reversible reduction is virtually
the only reaction common to all the naturally occurring quinones. It
has long been thought that quinones might participate in cellular
respiration, and the coexistence of a quinone and its corresponding
quinol, e.g., in cultures of Aspergillus fumigatus Fresenius, supports this
view. Although this may be true in a few instances, it is probably not
the case in general. However, it has recently been established that coenzyme Q (ubiquinone) is an active member of the respiratory chain in
mammalian mitochondria, and there is now evidence that plastoquinone
plays a vital part in photosynthetic phosphorylation. The new chloroplast pigment plastoquinone may also be involved in electron transport
in photosynthesis. These important functions, dependent upon the property of reversible oxidation-reduction, are probably confined to a comparatively few, widely distributed, lipid-soluble, quinones. In all other
cases, definite evidence concerning the function, if any, of natural
quinones is lacking, and in view of the considerable variations both in
their structure and properties, and in their location within living tissues,
it is unwise to make generalizations about their behavior.
II. Distribution of Quinones in Nature
A. THE PLANT KINGDOM
1. Flowering Plants
a. Angiospermae. The largest group of natural quinones occurs in
flowering plants. Nearly half of them are found in one family, the
Rubiaceae, and the rest are dispersed among some thirty to forty
families only, so that our present knowledge of the distribution of
quinones in angiosperms is very limited.* The few systematic surveys of
plant quinones which have been recorded are all concerned with anthraquinones, and the information available comes largely from investigations on plants of economic, or potentially economic importance, mainly
* Only quinones that have been isolated and identified are considered. No
attention is paid to the numerous claims to have detected quinones, based solely
on color reactions, as these are not always reliable and in any case do not
appreciably increase the number of quinone-forming families.
635
by certain insects as a means of defense, and one or two simple naphthoquinones (e.g., juglone, plumbagin) known to be poisonous to animals
and plants. However, toxicity is not restricted to simple structures, for
a number of more complex quinones (e.g., skyrin) occurring in plant
pathogens are harmful to various organisms.
The only example of 1,6-addition to the conjugated system (c, above)
to be considered is that of hydrogen. Reversible reduction is virtually
the only reaction common to all the naturally occurring quinones. It
has long been thought that quinones might participate in cellular
respiration, and the coexistence of a quinone and its corresponding
quinol, e.g., in cultures of Aspergillus fumigatus Fresenius, supports this
view. Although this may be true in a few instances, it is probably not
the case in general. However, it has recently been established that coenzyme Q (ubiquinone) is an active member of the respiratory chain in
mammalian mitochondria, and there is now evidence that plastoquinone
plays a vital part in photosynthetic phosphorylation. The new chloroplast pigment plastoquinone may also be involved in electron transport
in photosynthesis. These important functions, dependent upon the property of reversible oxidation-reduction, are probably confined to a comparatively few, widely distributed, lipid-soluble, quinones. In all other
cases, definite evidence concerning the function, if any, of natural
quinones is lacking, and in view of the considerable variations both in
their structure and properties, and in their location within living tissues,
it is unwise to make generalizations about their behavior.
II. Distribution of Quinones in Nature
A. THE PLANT KINGDOM
1. Flowering Plants
a. Angiospermae. The largest group of natural quinones occurs in
flowering plants. Nearly half of them are found in one family, the
Rubiaceae, and the rest are dispersed among some thirty to forty
families only, so that our present knowledge of the distribution of
quinones in angiosperms is very limited.* The few systematic surveys of
plant quinones which have been recorded are all concerned with anthraquinones, and the information available comes largely from investigations on plants of economic, or potentially economic importance, mainly
* Only quinones that have been isolated and identified are considered. No
attention is paid to the numerous claims to have detected quinones, based solely
on color reactions, as these are not always reliable and in any case do not
appreciably increase the number of quinone-forming families.