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R. M. C. DAWSON
it is found that, provided suitable metabolic conditions pertain, the
individual phosphoHpids isolated from the slice become radioactive.
This applies even to such organs as the thyroid, pituitary, or pancreas
glands (24-26). The transport of freshly synthesized intact phospholipid
molecules from tissue to tissue via the plasma is thought to be small.
If, for example, labeled acetate or phosphate is injected into animals in
which the synthesis of the plasma phosphoHpids has been prevented by
hepatectomy, the incorporation of isotope into the phosphoHpids of
other organs is more or less normal (26a,b). We are left, therefore, with
a picture of each cellular tissue acting as an independent synthetic unit
for the new phosphoHpids it requires for growth and replacement.
The liver appears to be the sole site of synthesis of plasma phosphoHpids in the fasting dog and rat. This is shown by removing the liver
from these animals and injecting a labeled phospholipid precursor, e.g.,
phosphate,, when it is found that negligible incorporation into the
plasma phosphoHpids occurs compared with that in the normal animal
(26aJo, 27). In the nonfasting rat however some plasma phospholipid
can be synthesized in the intestine, especially when the animal is digesting a high fat-content meal. In the fasting rabbit (28) and chicken
(29) the liver is again the major site for plasma phospholipid synthesis,
but some plasma phosphoHpids can be formed in extrahepatic tissues.
Although there is no evidence that significant amounts of plasma phospholipid can be formed by the bone marrow (30), recent work indicates that in man some of the phosphoHpids in the plasma ß-lipoprotein can be synthesized in the blood cells (31), especially the leukocytes.
II. Enzyme Systems That Degrade PhosphoHpids
Enzymes that degrade phosphoHpids have been studied and characterized for much longer than those responsible for their synthesis.
The most extensively studied substrate has been lecithin, hence in the
older literature the enzymes are usually called lecithinases. More recently it has been realized that these enzymes attack other substrates
than lecithin, and the term phospholipase has been widely accepted.
The phospholipases are usually classified from the ester bond of lecithin
which they hydrolyze. A difficulty arises in that sharp controversy still
exists as to whether the a or ß fatty acid is removed from lecithin by
phospholipase A (32). Furthermore, it is now known that under certain circumstances, phospholipase B can remove both fatty acids (33).
Consequently, for the purpose of this article, phospholipase A is defined
as the enzyme that removes a single fatty acid from lecithin, forming
lysolecithin; phospholipase B, as the enzyme that hydrolyzes the fatty
R. M. C. DAWSON
it is found that, provided suitable metabolic conditions pertain, the
individual phosphoHpids isolated from the slice become radioactive.
This applies even to such organs as the thyroid, pituitary, or pancreas
glands (24-26). The transport of freshly synthesized intact phospholipid
molecules from tissue to tissue via the plasma is thought to be small.
If, for example, labeled acetate or phosphate is injected into animals in
which the synthesis of the plasma phosphoHpids has been prevented by
hepatectomy, the incorporation of isotope into the phosphoHpids of
other organs is more or less normal (26a,b). We are left, therefore, with
a picture of each cellular tissue acting as an independent synthetic unit
for the new phosphoHpids it requires for growth and replacement.
The liver appears to be the sole site of synthesis of plasma phosphoHpids in the fasting dog and rat. This is shown by removing the liver
from these animals and injecting a labeled phospholipid precursor, e.g.,
phosphate,, when it is found that negligible incorporation into the
plasma phosphoHpids occurs compared with that in the normal animal
(26aJo, 27). In the nonfasting rat however some plasma phospholipid
can be synthesized in the intestine, especially when the animal is digesting a high fat-content meal. In the fasting rabbit (28) and chicken
(29) the liver is again the major site for plasma phospholipid synthesis,
but some plasma phosphoHpids can be formed in extrahepatic tissues.
Although there is no evidence that significant amounts of plasma phospholipid can be formed by the bone marrow (30), recent work indicates that in man some of the phosphoHpids in the plasma ß-lipoprotein can be synthesized in the blood cells (31), especially the leukocytes.
II. Enzyme Systems That Degrade PhosphoHpids
Enzymes that degrade phosphoHpids have been studied and characterized for much longer than those responsible for their synthesis.
The most extensively studied substrate has been lecithin, hence in the
older literature the enzymes are usually called lecithinases. More recently it has been realized that these enzymes attack other substrates
than lecithin, and the term phospholipase has been widely accepted.
The phospholipases are usually classified from the ester bond of lecithin
which they hydrolyze. A difficulty arises in that sharp controversy still
exists as to whether the a or ß fatty acid is removed from lecithin by
phospholipase A (32). Furthermore, it is now known that under certain circumstances, phospholipase B can remove both fatty acids (33).
Consequently, for the purpose of this article, phospholipase A is defined
as the enzyme that removes a single fatty acid from lecithin, forming
lysolecithin; phospholipase B, as the enzyme that hydrolyzes the fatty