2. STEROLS: STRUCTURE AND DISTRIBUTION
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in conjunction with a A
5 -sterol ring system (II) and among the methyl
sterols (VI and VII). Unsaturation has also been found between C-24
and the group attached to it (D and I). The C-24-methylene sterols
have long been suspected to be present among the minor yeast sterols
(III, V, R = D). 24-Methylenecholesterol (II, R = D) enjoys a lather
wide distribution among marine invertebrates. The presence of this
terminal methylene group may readily be recognized by its characteristic bands at 11.31 μ in the infrared spectrum. The 24-ethylidene
side chain (I) has so far been found only in conjunction with the Δ
5 -
ring system as in the algal sterols. The 24-methylene (D) and 24ethylidene (I) side chains may well occupy pivotal positions. Their
biochemical hydrogenation establishes asymmetry at C-24 and could
thus lead into the «-series (E, F and J, K) or ß-series (G, H and L, M)
of 24-substituted sterols (8).
There are some indications that sterols with di-unsaturated side
chains such as O and P may also be encountered in animal tissues. Side
chain O, a conjugated system quite prone to undergo air-oxidation has
already been found among the minor yeast sterols (I, R = O) (19). The
other, P, is suspected to occur in a highly unsaturated fraction of certain mollusk sterols (20). The methyl-substituted C-20 is also asymmetric, and the natural occurrence of two sets of isomers epimeric at
this point has for some time been suspected (21). The vast majority of
known sterols possess at this point the same configuration as cholesterol
(II, R = A). Rather recently, however, it has been found that sargasterol, the principal sterol of Sargassum seaweed, is the C-20-epimer of
fucosterol (II, R = N) (22). It is probable that additional members
of this 20-iso series will be encountered among the many, as yet unseparated, mixtures of animal sterols now in the hands of investigators.
C. TYPES OF NATURAL STEROLS
The following paragraphs list in their appropriate groups the sterols
to be encountered in animals. Several sterols have been included which
as yet have been isolated only from plants but the occurrence of which
in animals is suspected for reasons of analogy. With but few exceptions
the listing is restricted to sterols the structures of which have been well
established either by methods of degradation or synthesis. Because of
the difficulties encountered in obtaining truly pure sterol samples, the
reported physical properties of the sterols and their acetates are subject to changes as purer samples become available. Ultimately only
synthetic materials will be used as reference samples. As yet, however,
the syntheses of the sterols most difficult to obtain pure, 24-alkyl
cholesterols of known configuration, have not been accomplished. The
111
in conjunction with a A
5 -sterol ring system (II) and among the methyl
sterols (VI and VII). Unsaturation has also been found between C-24
and the group attached to it (D and I). The C-24-methylene sterols
have long been suspected to be present among the minor yeast sterols
(III, V, R = D). 24-Methylenecholesterol (II, R = D) enjoys a lather
wide distribution among marine invertebrates. The presence of this
terminal methylene group may readily be recognized by its characteristic bands at 11.31 μ in the infrared spectrum. The 24-ethylidene
side chain (I) has so far been found only in conjunction with the Δ
5 -
ring system as in the algal sterols. The 24-methylene (D) and 24ethylidene (I) side chains may well occupy pivotal positions. Their
biochemical hydrogenation establishes asymmetry at C-24 and could
thus lead into the «-series (E, F and J, K) or ß-series (G, H and L, M)
of 24-substituted sterols (8).
There are some indications that sterols with di-unsaturated side
chains such as O and P may also be encountered in animal tissues. Side
chain O, a conjugated system quite prone to undergo air-oxidation has
already been found among the minor yeast sterols (I, R = O) (19). The
other, P, is suspected to occur in a highly unsaturated fraction of certain mollusk sterols (20). The methyl-substituted C-20 is also asymmetric, and the natural occurrence of two sets of isomers epimeric at
this point has for some time been suspected (21). The vast majority of
known sterols possess at this point the same configuration as cholesterol
(II, R = A). Rather recently, however, it has been found that sargasterol, the principal sterol of Sargassum seaweed, is the C-20-epimer of
fucosterol (II, R = N) (22). It is probable that additional members
of this 20-iso series will be encountered among the many, as yet unseparated, mixtures of animal sterols now in the hands of investigators.
C. TYPES OF NATURAL STEROLS
The following paragraphs list in their appropriate groups the sterols
to be encountered in animals. Several sterols have been included which
as yet have been isolated only from plants but the occurrence of which
in animals is suspected for reasons of analogy. With but few exceptions
the listing is restricted to sterols the structures of which have been well
established either by methods of degradation or synthesis. Because of
the difficulties encountered in obtaining truly pure sterol samples, the
reported physical properties of the sterols and their acetates are subject to changes as purer samples become available. Ultimately only
synthetic materials will be used as reference samples. As yet, however,
the syntheses of the sterols most difficult to obtain pure, 24-alkyl
cholesterols of known configuration, have not been accomplished. The