280
R. M. C. DAWSON
properties [e.g., the pH optimum of the C. perfringens enzyme, 8.3, is
higher than that of the C. bifermentans, 6.2 (83)] and differences in
their toxicity (86). This indicates therefore that the enzymatic active
centers are not identical even though the antigenic groups on their
molecules may be similar.
The phospholipase in Bacillus cereus and B. cereus var. mycoides
has been characterized as of the G type. In both these organisms the
phospholipases are immunologically related, but they are not so related
to C. perfringens phospholipase C (87). The phospholipase C of B.
cereus is probably the "thromboplastinase" isolated from strains of
this organism, which greatly reduces the thromboplastic activity of a
tissue suspension (75b).
Recent evidence suggests that phospholipase C can occur in higher
plants. Barley mitochondria have been shown to attack lecithin with the
liberation of phosphorylcholine (88); this may explain the high concentration of phosphorylcholine found in barley seedlings (89). Kates
(90) has found that spinach chloroplasts can hydrolyze lecithin, liberating phosphorylcholine.
E. PHOSPHOLIPASE D: PROPERTIES AND DISTRIBUTION
Phospholipase D, which catalyzes the hydrolysis of choline from
lecithin, leaving a phosphatidic acid, appears to be confined to the
higher plants. The enzyme was first demonstrated in the carrot and
cabbage (91), and it is probably responsible for the phosphatidic acid
which older work had suggested was a constituent of cabbage leaves
(92). The enzyme from cabbage acts readily upon phosphatidylethanolamine and more slowly upon lysolecithin (93, 94). It appears to exist
in the plant cells as an insoluble enzyme associated with the plastids
(95), but a soluble form can be readily demonstrated in the cabbage
(93) as well as in the carrot (96) and in cottonseed (94).
The soluble enzyme requires the presence of calcium for maximal
activity (93, 96). Both the insoluble plastid enzyme and the soluble
enzyme of the carrot and cabbage are greatly stimulated by the addition
of ether to the reaction mixture (93, 95). On the other hand, the phospholipase D of cottonseed does not appear to be activated by this
solvent (94). The plastid enzyme is activated also by other solvents,
e.g., linear aliphatic ethers, ketones, and esters as well as anionic detergents (97). Solvents that stimulate the reaction have been shown to
produce coalescence between lecithin micelles and chloroplasts (98).
The partially purified soluble enzyme from cabbage is stimulated by
water-soluble phosphodiesters (93) as well as by phosphatidylinositol
(99). These observations suggest that the reaction between the soluble
R. M. C. DAWSON
properties [e.g., the pH optimum of the C. perfringens enzyme, 8.3, is
higher than that of the C. bifermentans, 6.2 (83)] and differences in
their toxicity (86). This indicates therefore that the enzymatic active
centers are not identical even though the antigenic groups on their
molecules may be similar.
The phospholipase in Bacillus cereus and B. cereus var. mycoides
has been characterized as of the G type. In both these organisms the
phospholipases are immunologically related, but they are not so related
to C. perfringens phospholipase C (87). The phospholipase C of B.
cereus is probably the "thromboplastinase" isolated from strains of
this organism, which greatly reduces the thromboplastic activity of a
tissue suspension (75b).
Recent evidence suggests that phospholipase C can occur in higher
plants. Barley mitochondria have been shown to attack lecithin with the
liberation of phosphorylcholine (88); this may explain the high concentration of phosphorylcholine found in barley seedlings (89). Kates
(90) has found that spinach chloroplasts can hydrolyze lecithin, liberating phosphorylcholine.
E. PHOSPHOLIPASE D: PROPERTIES AND DISTRIBUTION
Phospholipase D, which catalyzes the hydrolysis of choline from
lecithin, leaving a phosphatidic acid, appears to be confined to the
higher plants. The enzyme was first demonstrated in the carrot and
cabbage (91), and it is probably responsible for the phosphatidic acid
which older work had suggested was a constituent of cabbage leaves
(92). The enzyme from cabbage acts readily upon phosphatidylethanolamine and more slowly upon lysolecithin (93, 94). It appears to exist
in the plant cells as an insoluble enzyme associated with the plastids
(95), but a soluble form can be readily demonstrated in the cabbage
(93) as well as in the carrot (96) and in cottonseed (94).
The soluble enzyme requires the presence of calcium for maximal
activity (93, 96). Both the insoluble plastid enzyme and the soluble
enzyme of the carrot and cabbage are greatly stimulated by the addition
of ether to the reaction mixture (93, 95). On the other hand, the phospholipase D of cottonseed does not appear to be activated by this
solvent (94). The plastid enzyme is activated also by other solvents,
e.g., linear aliphatic ethers, ketones, and esters as well as anionic detergents (97). Solvents that stimulate the reaction have been shown to
produce coalescence between lecithin micelles and chloroplasts (98).
The partially purified soluble enzyme from cabbage is stimulated by
water-soluble phosphodiesters (93) as well as by phosphatidylinositol
(99). These observations suggest that the reaction between the soluble