Β. C 27 Sterols
61
Three C 26 sterols, including 1, have been reported from the tunicate
Halocynthia roretzi by Alcaide et al. (1971). The two isomers appear to have
their side-chain unsaturation at C-23—one is a cholestane, the other a
A
7 -cholestene.
B. C 27 Sterols
If one recalls the structural parameters of the known marine sterols, which
were mentioned above, one is not surprised that—except for the remarkable
crustecdysone, marthasterone, and scymnol—only five conventional C 27
sterols have so far been recorded. In addition to cholesterol, which is still the
most important of all sterols, we have a A
7 -analog of cholesterol, two compounds that differ from cholesterol by an additional double bond in the side
chain, and one A
7 -compound with an additional side-chain double bond.
Discussion of the strikingly different, oxygenated steroids will follow the
account of the five conventional sterols.
1. CHOLESTEROL (CHOLEST-5-EN-3£-OL)
Cholesterol (2) is hardly unique to marine organisms ; nevertheless it should
be included here, for it is doubtless the most commonly occurring sterol even
HO-^^
2
among marine invertebrates. Furthermore, for many years cholesterol was
believed to be the animal sterol; this tenet was shown to be erroneous by
Tsuda et al. (1957a, 1958a) who first demonstrated the occurrence of cholesterol in a plant—the red alga Rhodoglossum pulcherum. With improved separation techniques cholesterol has been shown to occur widely in marine
organisms, sometimes as the predominant constituent, at times as a minor or
even a trace constituent. The presence or absence of a given sterol in an
organism is no longer a burning question. Of far greater interest are the
questions of why a given organism will synthesize its characteristic complement of sterols and how this biosynthetic control is achieved.
61
Three C 26 sterols, including 1, have been reported from the tunicate
Halocynthia roretzi by Alcaide et al. (1971). The two isomers appear to have
their side-chain unsaturation at C-23—one is a cholestane, the other a
A
7 -cholestene.
B. C 27 Sterols
If one recalls the structural parameters of the known marine sterols, which
were mentioned above, one is not surprised that—except for the remarkable
crustecdysone, marthasterone, and scymnol—only five conventional C 27
sterols have so far been recorded. In addition to cholesterol, which is still the
most important of all sterols, we have a A
7 -analog of cholesterol, two compounds that differ from cholesterol by an additional double bond in the side
chain, and one A
7 -compound with an additional side-chain double bond.
Discussion of the strikingly different, oxygenated steroids will follow the
account of the five conventional sterols.
1. CHOLESTEROL (CHOLEST-5-EN-3£-OL)
Cholesterol (2) is hardly unique to marine organisms ; nevertheless it should
be included here, for it is doubtless the most commonly occurring sterol even
HO-^^
2
among marine invertebrates. Furthermore, for many years cholesterol was
believed to be the animal sterol; this tenet was shown to be erroneous by
Tsuda et al. (1957a, 1958a) who first demonstrated the occurrence of cholesterol in a plant—the red alga Rhodoglossum pulcherum. With improved separation techniques cholesterol has been shown to occur widely in marine
organisms, sometimes as the predominant constituent, at times as a minor or
even a trace constituent. The presence or absence of a given sterol in an
organism is no longer a burning question. Of far greater interest are the
questions of why a given organism will synthesize its characteristic complement of sterols and how this biosynthetic control is achieved.