2. STEROLS: STRUCTURE AND DISTRIBUTION
139
terol. The search for other components is complicated by the fact, observed both in the United States and in Japan (139), that sterol mixtures undergo fluctuations in composition, the significance of which is
not yet understood.
Mollusk sterols and in particular certain bivalve sterols are among
the richest natural sources of A
5 '
7 -sterols (I), or provitamins D. This
interesting fact was discovered in connection with a systematic search
for natural precursors of vitamin D which upon irradiation would afford
a product of the antirachitic activity of cod liver oil rather than of
irradiated ergosterol. The characteristic ultraviolet spectrum of the
A
57 -sterols greatly facilitates their detection and quantitative determination. The sterols from certain mussels, such as Modiolus demissus, were
found to be sufficiently rich in provitamins D to warrant their commercial exploitation for the manufacture of vitamin D preparations
(145).
The isolation of a pure, identifiable A
5 '
7 -sterol from bivalves has
been a rather vexing problem. The general complexity of the bivalve
sterols appears to be reflected in the number of A
57 -sterols present in
the sterol mixtures. Mixtures of A
5 '
7 -sterols are particularly difficult to
separate chromatographically. Even systematic studies on commercial
mussel sterols, once available in very large quantities, have not led to
the isolation of a structurally identifiable A
5>7
-sterol. It could only be
suggested on the basis of rather tenuous qualitative and quantitative
evidence that the mussel provitamins consist of about equal parts of
7-dehydrocholesterol (I, R = A) and a mixture containing ergosterol
(I, R = F), the unknown A
5 '
7 '
22 -cholesta-trienol (II, R = C), and provitamin D x (146). In contrast, other investigators obtained none of
these sterols from the mussel Modiolus demissus, but instead a compound of the order C 29 , provisionally called provitamin D m (147).
In Table VI the known A
5 '
7 -sterols are compared with those isolated
from mollusks. As mentioned in the introductory chapter, the data for
7-dehydrositosterol and 7-dehydroclionasterol must be regarded as provisional. Similarities of certain data suggest the identity of certain
mollusk sterols and known 7-dehydrosterols. Thus, the identity of the
Littorina provitamin D with 7-dehydroclionasterol has been suspected
(148). Corbisterol, which Matsumoto and Toyama have isolated from
Japanese bivalves, appears to be a uniform compound. It is a triunsaturated compound and it is believed to be identical with 7-dehydrostigmasterol (I, R = K) (149-152). This formula, however is not in
accord with the antirachitic activity of irradiated corbisterol. In the
rat test its activity is the same as that of irradiated ergosterol (I, R = F)
and 7-dehydrocholesterol (I, R = A), but significantly higher than that
of the irradiation product of 7-dehydrostigmasterol (I, R = K) sus-
139
terol. The search for other components is complicated by the fact, observed both in the United States and in Japan (139), that sterol mixtures undergo fluctuations in composition, the significance of which is
not yet understood.
Mollusk sterols and in particular certain bivalve sterols are among
the richest natural sources of A
5 '
7 -sterols (I), or provitamins D. This
interesting fact was discovered in connection with a systematic search
for natural precursors of vitamin D which upon irradiation would afford
a product of the antirachitic activity of cod liver oil rather than of
irradiated ergosterol. The characteristic ultraviolet spectrum of the
A
57 -sterols greatly facilitates their detection and quantitative determination. The sterols from certain mussels, such as Modiolus demissus, were
found to be sufficiently rich in provitamins D to warrant their commercial exploitation for the manufacture of vitamin D preparations
(145).
The isolation of a pure, identifiable A
5 '
7 -sterol from bivalves has
been a rather vexing problem. The general complexity of the bivalve
sterols appears to be reflected in the number of A
57 -sterols present in
the sterol mixtures. Mixtures of A
5 '
7 -sterols are particularly difficult to
separate chromatographically. Even systematic studies on commercial
mussel sterols, once available in very large quantities, have not led to
the isolation of a structurally identifiable A
5>7
-sterol. It could only be
suggested on the basis of rather tenuous qualitative and quantitative
evidence that the mussel provitamins consist of about equal parts of
7-dehydrocholesterol (I, R = A) and a mixture containing ergosterol
(I, R = F), the unknown A
5 '
7 '
22 -cholesta-trienol (II, R = C), and provitamin D x (146). In contrast, other investigators obtained none of
these sterols from the mussel Modiolus demissus, but instead a compound of the order C 29 , provisionally called provitamin D m (147).
In Table VI the known A
5 '
7 -sterols are compared with those isolated
from mollusks. As mentioned in the introductory chapter, the data for
7-dehydrositosterol and 7-dehydroclionasterol must be regarded as provisional. Similarities of certain data suggest the identity of certain
mollusk sterols and known 7-dehydrosterols. Thus, the identity of the
Littorina provitamin D with 7-dehydroclionasterol has been suspected
(148). Corbisterol, which Matsumoto and Toyama have isolated from
Japanese bivalves, appears to be a uniform compound. It is a triunsaturated compound and it is believed to be identical with 7-dehydrostigmasterol (I, R = K) (149-152). This formula, however is not in
accord with the antirachitic activity of irradiated corbisterol. In the
rat test its activity is the same as that of irradiated ergosterol (I, R = F)
and 7-dehydrocholesterol (I, R = A), but significantly higher than that
of the irradiation product of 7-dehydrostigmasterol (I, R = K) sus-