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R. M. C. DAWSON
acyl ester bond of lysolecithin as its preferred substrate; phosphoHpase
C, as the enzyme that catalyzes the hydrolysis of lecithin at the ester
bond between the diglyceride and phosphorylchoHne moieties; and
phosphoHpase D, between the phosphatidic acid and choline moieties.
Some authorities classify the latter two enzymes in the reverse order,
i.e., phosphoHpase C as the enzyme that hydrolyzes choline from
lecithin, but the present system is preferred as it is more systematic
and is in accord with the order in which the enzymes were discovered.
The metabolic steps catalyzed by these phospholipases and the associated enzymes responsible for the further degradation of the hydrolysis
products are shown diagrammatically in Fig. 2.
A. PHOSPHOLIPASE A
1. Properties
Phospholipase A catalyzes the hydrolysis of lecithin with the formation of lysolecithin and a single fatty acid. Until recently it was thought
that the α-fatty acid was the one removed by snake venom phospholipase A, but recent evidence suggests that it may be the ß fatty acid
(32). The enzyme is quite specific for lecithin with the L-configuration
and «-structure (34). With lecithin isolated from natural sources, e.g.,
ovolecithin, the enzyme preferentially removes unsaturated fatty acids,
which could merely reflect a positional localization of these on the
lecithin molecule. However, lecithins containing fully saturated fatty
acids are readily attacked (35).
As well as lecithin, phospholipase A will also attack phosphatidylethanolamine (34, 34a). There is also some evidence that phosphatidylserine, phosphatidic acid, ethanolamine plasmalogen, and choline plasmalogen are also attacked, although phosphatidylinositol is not hydrolyzed (34, 34a, 35).
The phospholipase A enzymes of venoms and the pancreas are unusual in two respects. First, they have a remarkable resistance toward
heat, especially below pH 7, and the venom enzyme will even stand
boiling at pH 5.9 for some minutes (36). Secondly, their catalytic activity is greatly stimulated in the presence of ether. The enzyme appears
in fact to form an ether-soluble enzyme substrate complex so that it
readily works in wet ethyl ether containing dissolved lecithin substrate
(35, 37). Under these conditions the lysolecithin formed is precipitated
from the ether, carrying down the enzyme with it, but the enzyme continues to catalyze the hydrolysis of the lecithin substrate until breakdown is complete. Bacterial phospholipase A from Serratia plymuthica
R. M. C. DAWSON
acyl ester bond of lysolecithin as its preferred substrate; phosphoHpase
C, as the enzyme that catalyzes the hydrolysis of lecithin at the ester
bond between the diglyceride and phosphorylchoHne moieties; and
phosphoHpase D, between the phosphatidic acid and choline moieties.
Some authorities classify the latter two enzymes in the reverse order,
i.e., phosphoHpase C as the enzyme that hydrolyzes choline from
lecithin, but the present system is preferred as it is more systematic
and is in accord with the order in which the enzymes were discovered.
The metabolic steps catalyzed by these phospholipases and the associated enzymes responsible for the further degradation of the hydrolysis
products are shown diagrammatically in Fig. 2.
A. PHOSPHOLIPASE A
1. Properties
Phospholipase A catalyzes the hydrolysis of lecithin with the formation of lysolecithin and a single fatty acid. Until recently it was thought
that the α-fatty acid was the one removed by snake venom phospholipase A, but recent evidence suggests that it may be the ß fatty acid
(32). The enzyme is quite specific for lecithin with the L-configuration
and «-structure (34). With lecithin isolated from natural sources, e.g.,
ovolecithin, the enzyme preferentially removes unsaturated fatty acids,
which could merely reflect a positional localization of these on the
lecithin molecule. However, lecithins containing fully saturated fatty
acids are readily attacked (35).
As well as lecithin, phospholipase A will also attack phosphatidylethanolamine (34, 34a). There is also some evidence that phosphatidylserine, phosphatidic acid, ethanolamine plasmalogen, and choline plasmalogen are also attacked, although phosphatidylinositol is not hydrolyzed (34, 34a, 35).
The phospholipase A enzymes of venoms and the pancreas are unusual in two respects. First, they have a remarkable resistance toward
heat, especially below pH 7, and the venom enzyme will even stand
boiling at pH 5.9 for some minutes (36). Secondly, their catalytic activity is greatly stimulated in the presence of ether. The enzyme appears
in fact to form an ether-soluble enzyme substrate complex so that it
readily works in wet ethyl ether containing dissolved lecithin substrate
(35, 37). Under these conditions the lysolecithin formed is precipitated
from the ether, carrying down the enzyme with it, but the enzyme continues to catalyze the hydrolysis of the lecithin substrate until breakdown is complete. Bacterial phospholipase A from Serratia plymuthica