5. DISTRIBUTION OF PHOSPHOLIPIDS
237
Analogs of sphingomyelin have not been isolated and probably do not
occur.
In many cases it is difficult to evaluate the quantitative data given
for sphingomyelin. Analyses have been carried out by a variety of
methods, but almost all of these are subject to errors, especially when
applied to tissues in which the phospholipids have not been fully
characterized. The analyses included here should be accepted with
the reservation that no obvious reason for discounting them is presently
known. Further, although the variety of fatty acids found in sphingomyelins is more limited than that found in the glycerophosphatides,
there is still a wide range of sphingomyelins, only a few of which have
been identified.
IV. The Distribution of Phospholipids in Animal Tissues
A. BRAIN AND OTHER NERVOUS TISSUES
Brain, of all tissues which have been studied, has the greatest phospholipid content. From 25 to 30% of the dry weight of brain consists of
phospholipids as compared to half this proportion in liver. Other parts
of the nervous system, such as spinal cord or peripheral nerve, usually
have as high concentrations of phospholipid. The diversity of phospholipids in brain is also greater than in any other animal tissue. Sphingomyelin (18, 22, 25, 26), phosphatidylcholine (27), phosphatidylethanolamine (28), and phosphatidylserine (28, 29) have all been isolated and
characterized. Of the plasmalogens, that containing ethanolamine predominates (30, 31), but both a serine plasmalogen (11) and, probably,
an inositol plasmalogen are present (12). Phosphatidylinositol (32, 33)
is also present, and another phosphoinositide, the diphosphoinositide
isolated by Folch (34, 35), is probably unique to brain. This latter compound contains inositol, glycerol, fatty acid, and phosphorus in a ratio
of 1:1:1:2 and yields inositol diphosphate on acid hydrolysis. Most
of these lipids have been isolated from or demonstrated in mammalian
brains, and whether the complete range is found in all species is not
certain. No simple method for estimating all the individual phospholipids
of the brain is available.
A number of studies of the comparative aspects of phospholipids in
brain and other nervous tissue have been carried out (22, 36-38). In
general, only total phospholipid, lecithin, cephalin, and sphingomyelin
have been determined. Data for a wide range of vertebrates have been
summarized in Table I. A much more limited selection of data for vertebrate spinal cord and for invertebrate nervous tissue is given in
Tables II and III, respectively. It should be understood that the method
237
Analogs of sphingomyelin have not been isolated and probably do not
occur.
In many cases it is difficult to evaluate the quantitative data given
for sphingomyelin. Analyses have been carried out by a variety of
methods, but almost all of these are subject to errors, especially when
applied to tissues in which the phospholipids have not been fully
characterized. The analyses included here should be accepted with
the reservation that no obvious reason for discounting them is presently
known. Further, although the variety of fatty acids found in sphingomyelins is more limited than that found in the glycerophosphatides,
there is still a wide range of sphingomyelins, only a few of which have
been identified.
IV. The Distribution of Phospholipids in Animal Tissues
A. BRAIN AND OTHER NERVOUS TISSUES
Brain, of all tissues which have been studied, has the greatest phospholipid content. From 25 to 30% of the dry weight of brain consists of
phospholipids as compared to half this proportion in liver. Other parts
of the nervous system, such as spinal cord or peripheral nerve, usually
have as high concentrations of phospholipid. The diversity of phospholipids in brain is also greater than in any other animal tissue. Sphingomyelin (18, 22, 25, 26), phosphatidylcholine (27), phosphatidylethanolamine (28), and phosphatidylserine (28, 29) have all been isolated and
characterized. Of the plasmalogens, that containing ethanolamine predominates (30, 31), but both a serine plasmalogen (11) and, probably,
an inositol plasmalogen are present (12). Phosphatidylinositol (32, 33)
is also present, and another phosphoinositide, the diphosphoinositide
isolated by Folch (34, 35), is probably unique to brain. This latter compound contains inositol, glycerol, fatty acid, and phosphorus in a ratio
of 1:1:1:2 and yields inositol diphosphate on acid hydrolysis. Most
of these lipids have been isolated from or demonstrated in mammalian
brains, and whether the complete range is found in all species is not
certain. No simple method for estimating all the individual phospholipids
of the brain is available.
A number of studies of the comparative aspects of phospholipids in
brain and other nervous tissue have been carried out (22, 36-38). In
general, only total phospholipid, lecithin, cephalin, and sphingomyelin
have been determined. Data for a wide range of vertebrates have been
summarized in Table I. A much more limited selection of data for vertebrate spinal cord and for invertebrate nervous tissue is given in
Tables II and III, respectively. It should be understood that the method