6. METABOLISM OF PHOSPHOLIPIDS
271
the diglyceride and plasmal monoglyceride on a silicic acid column. It
is not known whether this is the means by which naturally occurring
plasmalogens are formed or how such a plasmal monoglyceride acceptor could be synthesized in nature.
F. BIOSYNTHESIS OF PHOSPHATIDYLINOSITOL AND POLYGLYCEROL
PHOSPHOLIPIDS
Recently it has been shown that cytidine coenzymes participate in
the in vitro synthesis of inositol phospholipids (21-23). The mechanism
of the synthesis of phosphatidylinositol is, however, markedly different
from that of lecithin or phosphatidylethanolamine in that the diglyceride
molecule is first converted into the compound cytidine diphosphate
diglyceride (Fig. 1). This is brought about by an enzyme found in
guinea pig liver and kidney. At present the source of the diglyceride
is not known, although it is possible that it is formed from phosphatidic
acid as in the biosynthesis of the other phosphoglycerides. This cytidine
diphosphate diglyceride in the presence of a liver enzyme can react
directly with free inositol to form phosphatidylinositol and free cytidine
monophosphate.
It is also likely that cytidine diphosphate diglyceride is an intermediary in the biosynthesis of polyglycerol phospholipids such as phosphatidylglycerol or cardiolipin. It is known, for example, that an enzymatically catalyzed reaction can occur between cytidine diphosphate
diglyceride and L-a-glycerophosphate (Kennedy, unpublished) which
produces a diglyceride phosphorylglycerophosphate.
G. DISTRIBUTION OF ENZYME SYSTEMS RESPONSIBLE FOR
PHOSPHPLIPID BIOSYNTHESIS
Table I shows the reported distribution in nature of the enzymes
that participate in phospholipid synthesis. It must be emphasized, however, that in the references quoted it was not the primary objective of
the work to ascertain the comparative distribution of such enzymes, and
clearly they must occur very much more widely if not universally in all
the living cells that synthesize phospholipids. However, it is known that
one of the synthetic enzymes, namely phosphorylcholine cytidyl transferase, is absent from many bacteria (7). This may explain why there
is generally an absence of lecithin in bacterial phospholipids.
In mammals, it is becoming increasingly obvious from isotopic studies
on slices of tissue from different organs that each tissue possesses the
requisite enzymes for the complete de novo synthesis of phospholipid
molecules. If such slices are incubated in the presence of labeled precursors of phospholipids, such as glycerol, orthophosphate, or acetate,
271
the diglyceride and plasmal monoglyceride on a silicic acid column. It
is not known whether this is the means by which naturally occurring
plasmalogens are formed or how such a plasmal monoglyceride acceptor could be synthesized in nature.
F. BIOSYNTHESIS OF PHOSPHATIDYLINOSITOL AND POLYGLYCEROL
PHOSPHOLIPIDS
Recently it has been shown that cytidine coenzymes participate in
the in vitro synthesis of inositol phospholipids (21-23). The mechanism
of the synthesis of phosphatidylinositol is, however, markedly different
from that of lecithin or phosphatidylethanolamine in that the diglyceride
molecule is first converted into the compound cytidine diphosphate
diglyceride (Fig. 1). This is brought about by an enzyme found in
guinea pig liver and kidney. At present the source of the diglyceride
is not known, although it is possible that it is formed from phosphatidic
acid as in the biosynthesis of the other phosphoglycerides. This cytidine
diphosphate diglyceride in the presence of a liver enzyme can react
directly with free inositol to form phosphatidylinositol and free cytidine
monophosphate.
It is also likely that cytidine diphosphate diglyceride is an intermediary in the biosynthesis of polyglycerol phospholipids such as phosphatidylglycerol or cardiolipin. It is known, for example, that an enzymatically catalyzed reaction can occur between cytidine diphosphate
diglyceride and L-a-glycerophosphate (Kennedy, unpublished) which
produces a diglyceride phosphorylglycerophosphate.
G. DISTRIBUTION OF ENZYME SYSTEMS RESPONSIBLE FOR
PHOSPHPLIPID BIOSYNTHESIS
Table I shows the reported distribution in nature of the enzymes
that participate in phospholipid synthesis. It must be emphasized, however, that in the references quoted it was not the primary objective of
the work to ascertain the comparative distribution of such enzymes, and
clearly they must occur very much more widely if not universally in all
the living cells that synthesize phospholipids. However, it is known that
one of the synthetic enzymes, namely phosphorylcholine cytidyl transferase, is absent from many bacteria (7). This may explain why there
is generally an absence of lecithin in bacterial phospholipids.
In mammals, it is becoming increasingly obvious from isotopic studies
on slices of tissue from different organs that each tissue possesses the
requisite enzymes for the complete de novo synthesis of phospholipid
molecules. If such slices are incubated in the presence of labeled precursors of phospholipids, such as glycerol, orthophosphate, or acetate,