6. METABOLISM OF PHOSPHOLIPIDS
275
is apparently not active in ether solution (38), but this enzyme has not
yet been prepared in a soluble form.
There is almost universal agreement that phospholipase A can be
stimulated by the presence of calcium ions, although there is also evidence that excess calcium can be inhibitory. The pH optima of the enzymes from all sources lie between 6.8 and 7.7, a comparatively small
variation compared with the wide divergence seen with phospholipase
B.
2. Distribution
Phospholipase A is perhaps best known as a constituent of the venom
of snakes and partly accounts for the well-known hemolytic properties
of the poisons—the lysolecithin formed by its action causing hemolysis
of erythrocytes. It is also present in the poisons and venoms of many
other animals including, e.g., wasps, bees, and scorpions. Table II shows
the species whose venoms have been characterized as containing
phospholipase A. The amount of enzyme present in the venoms varies
from species to species, but in all cases it is not the only toxin present
and often not even an important one. In some snake venoms it is
present in large amounts and can be crystallized by comparatively
simple procedures (Table II).
In mammalian tissues, phospholipase A has been most extensively
studied in the pancreas. As with snake venoms the enzyme shows considerable resistance to heat, and it has considerable solubility in
aqueous ethanol solutions (34a, 54, 55). It appears to be present in the
pancreas of most mammals, although there is little in the pancreatic
juice of ruminants, e.g., sheep or ox. It occurs in the hepatopancreatic
juice of Helix pomatia (55a). Phospholipase A has been characterized
also in pig lung (55) and in rat intestinal mucosa (56). The particulate
enzyme isolated from the latter source is activated by the addition of
free fatty acids; it is unusual in that it is inhibited by calcium ions in
low concentrations. A number of reports in the older literature (e.g.,
57) suggest that the enzyme may be present in other mammalian tissues such as liver, kidney, heart muscle, adrenals, but the enzyme was
not clearly characterized as being of the A type.
An active phospholipase A is present in the bacterium Serratia
plymuihica, especially when this is grown in a lecithin-enriched medium
(38). This bacterial enzyme is thermolabile and Ca
2+ is required for
full activity. It is possible that ferrous ions also may be needed as a
cofactor.
It is likely that a phospholipase A is present in ram spermatozoa
275
is apparently not active in ether solution (38), but this enzyme has not
yet been prepared in a soluble form.
There is almost universal agreement that phospholipase A can be
stimulated by the presence of calcium ions, although there is also evidence that excess calcium can be inhibitory. The pH optima of the enzymes from all sources lie between 6.8 and 7.7, a comparatively small
variation compared with the wide divergence seen with phospholipase
B.
2. Distribution
Phospholipase A is perhaps best known as a constituent of the venom
of snakes and partly accounts for the well-known hemolytic properties
of the poisons—the lysolecithin formed by its action causing hemolysis
of erythrocytes. It is also present in the poisons and venoms of many
other animals including, e.g., wasps, bees, and scorpions. Table II shows
the species whose venoms have been characterized as containing
phospholipase A. The amount of enzyme present in the venoms varies
from species to species, but in all cases it is not the only toxin present
and often not even an important one. In some snake venoms it is
present in large amounts and can be crystallized by comparatively
simple procedures (Table II).
In mammalian tissues, phospholipase A has been most extensively
studied in the pancreas. As with snake venoms the enzyme shows considerable resistance to heat, and it has considerable solubility in
aqueous ethanol solutions (34a, 54, 55). It appears to be present in the
pancreas of most mammals, although there is little in the pancreatic
juice of ruminants, e.g., sheep or ox. It occurs in the hepatopancreatic
juice of Helix pomatia (55a). Phospholipase A has been characterized
also in pig lung (55) and in rat intestinal mucosa (56). The particulate
enzyme isolated from the latter source is activated by the addition of
free fatty acids; it is unusual in that it is inhibited by calcium ions in
low concentrations. A number of reports in the older literature (e.g.,
57) suggest that the enzyme may be present in other mammalian tissues such as liver, kidney, heart muscle, adrenals, but the enzyme was
not clearly characterized as being of the A type.
An active phospholipase A is present in the bacterium Serratia
plymuihica, especially when this is grown in a lecithin-enriched medium
(38). This bacterial enzyme is thermolabile and Ca
2+ is required for
full activity. It is possible that ferrous ions also may be needed as a
cofactor.
It is likely that a phospholipase A is present in ram spermatozoa