6. METABOLISM OF PHOSPHOLIPIDS
277
and that this is responsible for the liberation of a free fatty from the
choline plasmalogen which they contain (57a). Evidence has also been
obtained that a similar enzyme is present in sea urchin spermatozoa
(58).
B. PHOSPHOLIPASE B
1. Properties
The preferred substrate of this enzyme is lysolecithin or lysophosphatidylethanolamine, which it attacks, liberating the esterified fatty
acid and leaving glycerylphosphorylcholine or glycerylphosphorylethanolamine. Until recently it was believed that the enzyme could not
hydrolyze diacylated phosphoglycerides, but partially purified preparations of the enzyme from Penicillium notatum can slowly hydrolyze
lecithin in the presence of certain naturally occurring activators, e.g.,
phosphatidylinositol, polyglycerol phospholipid (59), which appear to
function by introducing on the substrate surface a certain minimum
concentration of negatively charged polar groups (33, 60). No separation of the enzymatic activity toward either substrate (lysolecithin or
activated lecithin) could be obtained by electrophoresis of the enzyme
under a variety of conditions. The mold enzyme preparation can also
slowly attack phosphatidylinositol, liberating glycerylphosphorylinositol
(61).
Phospholipase B differs from other phospholipases in that its activity
toward lysolecithin is not stimulated by the addition of ether. This may
reflect the fact that lysolecithin goes into colloidal solution in water in
the form of micromicelles (62). The enzyme does not require calcium
for full activity as do the A, C, and D phospholipases.
2. Distribution and Comparison of Phospholipase B from
Various Sources
Phospholipase B is very widely distributed in nature, where it usually occurs in association with phospholipase A. Under such circumstances, lysolecithin hydrolysis always occurs at a much faster rate than
that of lecithin, a device that probably protects living cells from the
accumulation of toxic lysophosphoglycerides, which have a disrupting
effect on cell membranes.
In mammalian tissues, phospholipase B occurs in the pancreas, from
which it may be extracted and purified to a crystalline form (63). The
enzyme has been studied also in rat liver (64) and in rat intestinal
mucosa (56) and is present in rat kidney, ileum, testis, skeletal muscle,
277
and that this is responsible for the liberation of a free fatty from the
choline plasmalogen which they contain (57a). Evidence has also been
obtained that a similar enzyme is present in sea urchin spermatozoa
(58).
B. PHOSPHOLIPASE B
1. Properties
The preferred substrate of this enzyme is lysolecithin or lysophosphatidylethanolamine, which it attacks, liberating the esterified fatty
acid and leaving glycerylphosphorylcholine or glycerylphosphorylethanolamine. Until recently it was believed that the enzyme could not
hydrolyze diacylated phosphoglycerides, but partially purified preparations of the enzyme from Penicillium notatum can slowly hydrolyze
lecithin in the presence of certain naturally occurring activators, e.g.,
phosphatidylinositol, polyglycerol phospholipid (59), which appear to
function by introducing on the substrate surface a certain minimum
concentration of negatively charged polar groups (33, 60). No separation of the enzymatic activity toward either substrate (lysolecithin or
activated lecithin) could be obtained by electrophoresis of the enzyme
under a variety of conditions. The mold enzyme preparation can also
slowly attack phosphatidylinositol, liberating glycerylphosphorylinositol
(61).
Phospholipase B differs from other phospholipases in that its activity
toward lysolecithin is not stimulated by the addition of ether. This may
reflect the fact that lysolecithin goes into colloidal solution in water in
the form of micromicelles (62). The enzyme does not require calcium
for full activity as do the A, C, and D phospholipases.
2. Distribution and Comparison of Phospholipase B from
Various Sources
Phospholipase B is very widely distributed in nature, where it usually occurs in association with phospholipase A. Under such circumstances, lysolecithin hydrolysis always occurs at a much faster rate than
that of lecithin, a device that probably protects living cells from the
accumulation of toxic lysophosphoglycerides, which have a disrupting
effect on cell membranes.
In mammalian tissues, phospholipase B occurs in the pancreas, from
which it may be extracted and purified to a crystalline form (63). The
enzyme has been studied also in rat liver (64) and in rat intestinal
mucosa (56) and is present in rat kidney, ileum, testis, skeletal muscle,