12. QUINONES: STRUCTURE AND DISTRIBUTION
647
obtained from the leaves of Lawsonia alba Lam. (Lythraceae) (44),
and the root of Dyer's bugloss (Alkanna tinctoria Tausch.) (Boraginaceae) (45) contains alkannin (LII) in the form of its angelic ester.
Onosma echioides Linn. (Boraginaceae) is said to contain 19.4% of
alkannin in the root bark (46).
Vitamin K lt The best known naphthoquinone produced by higher
plants is vitamin K l5 2-methyl-3-phytyl-l,4-naphthoquinone (LIV),
which was isolated from alfalfa leaves (Medicago sativa Linn.) as a
light-yellow oil by Dam and Karrer and their colleagues (49) and by
Doisy and co-workers (50), who determined its structure. Several syntheses have been described (51). After his recognition that the hemorrhagic condition produced in chicks kept on an ether-extracted diet was
due to a new fat-soluble factor, Dam, and others, surveyed many possible sources of this new vitamin (51). They found that an antihemorrhagic factor was widely distributed in green plants: chestnut leaves,
cabbage, spinach, and nettles proved to be rich sources, but the active
principle was not identified. (Plastoquinone and ubiquinone are probably present in all the plants that were examined, but these quinones
do not have vitamin K activity.) The findings of Dam, and others, were
based on bioassay procedures that do not distinguish between 2-methyl3-phytyl-l,4-naphthoquinone and other active compounds, and it is unfortunate that these bioassays have not been followed by chemical investigation, except in the case of alfalfa. From many compounds tested
it has been demonstrated that vitamin K activity is limited to 2-methyl1,4-naphthoquinones (or naphthalene precursors easily oxidized thereto)
unsubstituted in the benzene ring and alkylated at position 3, a C 20
side chain giving optimal activity (52). Hence the active compounds
occurring in green leaves may comprise a family of isoprenologs related to 2-methyl-3-phytyl-l,4-naphthoquinone, including perhaps
2-methyl-l,4-naphthoquinone itself.
Both vitamin Κ τ and chlorophyll possess a phytyl side chain, and
some early experiments (53) with germinating seedlings suggested that
the biosynthesis of vitamin Ki ran parallel with that of chlorophyll.
The apparent localization of vitamin K* in chloroplasts led Dam (54)
to suggest, about twenty years ago, that it might be involved in photosynthesis, and a tentative mechanism was proposed later by Wessels
(55). Bishop (56) has found recently that the photochemical activity
of isolated chloroplasts can be reversibly inhibited by extraction with
light petroleum. Although the activity of extracted chloroplasts can be
* Hereinafter the term vitamin K refers to an unidentified compound(s) having
vitamin K activity which may or may not be identical with one of the known K
vitamins.
647
obtained from the leaves of Lawsonia alba Lam. (Lythraceae) (44),
and the root of Dyer's bugloss (Alkanna tinctoria Tausch.) (Boraginaceae) (45) contains alkannin (LII) in the form of its angelic ester.
Onosma echioides Linn. (Boraginaceae) is said to contain 19.4% of
alkannin in the root bark (46).
Vitamin K lt The best known naphthoquinone produced by higher
plants is vitamin K l5 2-methyl-3-phytyl-l,4-naphthoquinone (LIV),
which was isolated from alfalfa leaves (Medicago sativa Linn.) as a
light-yellow oil by Dam and Karrer and their colleagues (49) and by
Doisy and co-workers (50), who determined its structure. Several syntheses have been described (51). After his recognition that the hemorrhagic condition produced in chicks kept on an ether-extracted diet was
due to a new fat-soluble factor, Dam, and others, surveyed many possible sources of this new vitamin (51). They found that an antihemorrhagic factor was widely distributed in green plants: chestnut leaves,
cabbage, spinach, and nettles proved to be rich sources, but the active
principle was not identified. (Plastoquinone and ubiquinone are probably present in all the plants that were examined, but these quinones
do not have vitamin K activity.) The findings of Dam, and others, were
based on bioassay procedures that do not distinguish between 2-methyl3-phytyl-l,4-naphthoquinone and other active compounds, and it is unfortunate that these bioassays have not been followed by chemical investigation, except in the case of alfalfa. From many compounds tested
it has been demonstrated that vitamin K activity is limited to 2-methyl1,4-naphthoquinones (or naphthalene precursors easily oxidized thereto)
unsubstituted in the benzene ring and alkylated at position 3, a C 20
side chain giving optimal activity (52). Hence the active compounds
occurring in green leaves may comprise a family of isoprenologs related to 2-methyl-3-phytyl-l,4-naphthoquinone, including perhaps
2-methyl-l,4-naphthoquinone itself.
Both vitamin Κ τ and chlorophyll possess a phytyl side chain, and
some early experiments (53) with germinating seedlings suggested that
the biosynthesis of vitamin Ki ran parallel with that of chlorophyll.
The apparent localization of vitamin K* in chloroplasts led Dam (54)
to suggest, about twenty years ago, that it might be involved in photosynthesis, and a tentative mechanism was proposed later by Wessels
(55). Bishop (56) has found recently that the photochemical activity
of isolated chloroplasts can be reversibly inhibited by extraction with
light petroleum. Although the activity of extracted chloroplasts can be
* Hereinafter the term vitamin K refers to an unidentified compound(s) having
vitamin K activity which may or may not be identical with one of the known K
vitamins.