6. METABOLISM OF PHOSPHOLIPIDS
281
enzyme and its substrate may be controlled by electrostatic forces analogous to those of the Penicillium notatum phospholipase B-lecithin interaction (33, 60).
The pH optimum of the enzyme lies between 4.8 and 6 and seems
to depend less on the source than on the mode of determination of
activity. In general its resistance to heat appears to be less than that of
phospholipases A and C (94-96).
Two of the richest sources of phospholipase D are the Savoy cabbage
and brussels sprouts, and in these it appears to be selectively located
in the inner leaves (93). Smaller activities have also been described in
the turnip, potato, beet root, and pea plants (93) as well as in rutabago,
spinach, green stringbean, sweet corn, and onion (96). In cereals it is
found in wheat, barley, and oats (100), and it has also been described
in the latex of Hevea brasiliensis (101).
F. OTHER ENZYME SYSTEMS THAT DEGRADE PHOSPHOLIPIDS
An enzyme has been detected in ox pancreas (61) and rat liver (102)
which catalyzes the hydrolysis of monophosphoinositide to phosphorylinositol and a diglyceride (monophosphoinositidase C). Impure preparations of this enzyme require Ca
2+ ions for full activity, and they are
inactive toward lecithin or phosphatidylserine, although lysolecithin is
attacked. Diphosphoinositide, the complex inositol-containing lipid of
the nervous system, can be broken down rapidly by homogenates from
guinea pig brain, liver, kidney, and spleen, yielding among other products phosphorylinositol and free inositol (103, 104).
In mammalian tissues, a number of enzymes have been described
which hydrolyze sphingomyelin (cf., e.g., 105), but none of these have
been completely characterized. There is some evidence, however, that
spleen extracts can hydrolyze sphingomyelin with the formation of a
ceramide and phosphorylcholine (106).
A number of workers have advanced claims that tissues such as liver
and spleen possess phospholipid dehydrogenases (cf., e.g., 107, 108), as
it was observed that the reduction of indophenol dyes was increased by
the addition of lecithin or sphingomyelin. Such an oxidation, however,
cannot be properly understood until the enzymatic substrate and reaction
products have been fully characterized.
III. The Pathways of Phospholipid Metabolism in Vivo
The characterization of enzymes in vitro gives an invaluable guide
as to the mechanism by which metabolism occurs in the living cell, but
it by no means proves that the same pathway is operative in vivo. In
mammalian tissues, however, there is certain additional evidence which
281
enzyme and its substrate may be controlled by electrostatic forces analogous to those of the Penicillium notatum phospholipase B-lecithin interaction (33, 60).
The pH optimum of the enzyme lies between 4.8 and 6 and seems
to depend less on the source than on the mode of determination of
activity. In general its resistance to heat appears to be less than that of
phospholipases A and C (94-96).
Two of the richest sources of phospholipase D are the Savoy cabbage
and brussels sprouts, and in these it appears to be selectively located
in the inner leaves (93). Smaller activities have also been described in
the turnip, potato, beet root, and pea plants (93) as well as in rutabago,
spinach, green stringbean, sweet corn, and onion (96). In cereals it is
found in wheat, barley, and oats (100), and it has also been described
in the latex of Hevea brasiliensis (101).
F. OTHER ENZYME SYSTEMS THAT DEGRADE PHOSPHOLIPIDS
An enzyme has been detected in ox pancreas (61) and rat liver (102)
which catalyzes the hydrolysis of monophosphoinositide to phosphorylinositol and a diglyceride (monophosphoinositidase C). Impure preparations of this enzyme require Ca
2+ ions for full activity, and they are
inactive toward lecithin or phosphatidylserine, although lysolecithin is
attacked. Diphosphoinositide, the complex inositol-containing lipid of
the nervous system, can be broken down rapidly by homogenates from
guinea pig brain, liver, kidney, and spleen, yielding among other products phosphorylinositol and free inositol (103, 104).
In mammalian tissues, a number of enzymes have been described
which hydrolyze sphingomyelin (cf., e.g., 105), but none of these have
been completely characterized. There is some evidence, however, that
spleen extracts can hydrolyze sphingomyelin with the formation of a
ceramide and phosphorylcholine (106).
A number of workers have advanced claims that tissues such as liver
and spleen possess phospholipid dehydrogenases (cf., e.g., 107, 108), as
it was observed that the reduction of indophenol dyes was increased by
the addition of lecithin or sphingomyelin. Such an oxidation, however,
cannot be properly understood until the enzymatic substrate and reaction
products have been fully characterized.
III. The Pathways of Phospholipid Metabolism in Vivo
The characterization of enzymes in vitro gives an invaluable guide
as to the mechanism by which metabolism occurs in the living cell, but
it by no means proves that the same pathway is operative in vivo. In
mammalian tissues, however, there is certain additional evidence which