632
R. H. THOMSON
ments. In contrast to the carotenoids, individual quinones are seldom
widely dispersed, and, until recently, the occurrence of the same
quinone in both the animal and plant kingdoms was unknown, although
closely related quinones are found in both. The exceptions to this are the
ubiquinones or coenzymes Q—a family of quinones that appears to be
very widely distributed, notably in mammals, including man—and possibly also plastoquinone and vitamin Ki, which may exist generally in
green leaves. It must be emphasized however, that there are large gaps
in our knowledge of the distribution of quinones, so that, as yet, structure can be correlated with taxonomy only to a limited extent. Fortunately this situation is steadily improving as the isolation of new
quinones from natural sources continues.
The chemistry of the naturally occurring quinones is well established
(1); there are comparatively few whose structures are not known completely, and the great majority have been synthesized in the laboratory.
In contrast, their biochemistry is not well understood. Considerable
progress has been made in the study of their biogenesis, but otherwise
we have little knowledge of their metabolism or function, with the
exception again of the coenzymes Q and vitamins K, which have recently
been subjected to intensive investigation. In many cases the quinone
pigments appear to be merely metabolic by-products. Occasionally they
are formed in surprisingly large amounts, notably by fungi, yet mutants
may thrive when the production of quinones is completely blocked.
Much quinone biogenesis is probably a deviation from fatty acid
metabolism, a metabolic shunt which comes into play when the enzyme
systems become saturated with respect to their substrates. As many of
the pigments so formed have antibiotic properties, their existence is no
doubt advantageous to the organisms that produce them.
In general, the quinones are yellow to red, or brown, in color, but
where present as salts (of, e.g., hydroxyquinones) the range may extend
to purple, blue, or green. (Black polymeric quinones are not considered
in this chapter.) Regarded simply as coloring matters they are of little
importance; many occur in bark or roots or are formed by microorganisms, and only those few present in the higher fungi and lichens or in
the spines and tests of sea urchins, make a significant contribution to
natural coloring. Occasionally a quinone and the corresponding quinol
occur together; sometimes only the colorless quinol has been found.
The naturally occurring quinones can be divided chemically into
the following groups: benzoquinones, 40; naphthoquinones, 40; anthraquinones, 70; and miscellaneous quinones, 25, which include tetracenequinones, phenanthrenequinones, and extended quinones. [In extended
quinones the two carbonyl groups are located in different rings, e.g.
R. H. THOMSON
ments. In contrast to the carotenoids, individual quinones are seldom
widely dispersed, and, until recently, the occurrence of the same
quinone in both the animal and plant kingdoms was unknown, although
closely related quinones are found in both. The exceptions to this are the
ubiquinones or coenzymes Q—a family of quinones that appears to be
very widely distributed, notably in mammals, including man—and possibly also plastoquinone and vitamin Ki, which may exist generally in
green leaves. It must be emphasized however, that there are large gaps
in our knowledge of the distribution of quinones, so that, as yet, structure can be correlated with taxonomy only to a limited extent. Fortunately this situation is steadily improving as the isolation of new
quinones from natural sources continues.
The chemistry of the naturally occurring quinones is well established
(1); there are comparatively few whose structures are not known completely, and the great majority have been synthesized in the laboratory.
In contrast, their biochemistry is not well understood. Considerable
progress has been made in the study of their biogenesis, but otherwise
we have little knowledge of their metabolism or function, with the
exception again of the coenzymes Q and vitamins K, which have recently
been subjected to intensive investigation. In many cases the quinone
pigments appear to be merely metabolic by-products. Occasionally they
are formed in surprisingly large amounts, notably by fungi, yet mutants
may thrive when the production of quinones is completely blocked.
Much quinone biogenesis is probably a deviation from fatty acid
metabolism, a metabolic shunt which comes into play when the enzyme
systems become saturated with respect to their substrates. As many of
the pigments so formed have antibiotic properties, their existence is no
doubt advantageous to the organisms that produce them.
In general, the quinones are yellow to red, or brown, in color, but
where present as salts (of, e.g., hydroxyquinones) the range may extend
to purple, blue, or green. (Black polymeric quinones are not considered
in this chapter.) Regarded simply as coloring matters they are of little
importance; many occur in bark or roots or are formed by microorganisms, and only those few present in the higher fungi and lichens or in
the spines and tests of sea urchins, make a significant contribution to
natural coloring. Occasionally a quinone and the corresponding quinol
occur together; sometimes only the colorless quinol has been found.
The naturally occurring quinones can be divided chemically into
the following groups: benzoquinones, 40; naphthoquinones, 40; anthraquinones, 70; and miscellaneous quinones, 25, which include tetracenequinones, phenanthrenequinones, and extended quinones. [In extended
quinones the two carbonyl groups are located in different rings, e.g.