Assays of mitochondrial phospholipase A activity and mitochondrial swelling demonstrated that the phospholipase A activity is related to the swelling under the experimental conditions used. Both were stimulated by added free fatty acid and CaCl(2), not affected greatly by the addition of monoacyl phosphoglycerides, and inhibited by EDTA. The amount of fatty acid hydrolyzed from endogenous phosphatidyl ethanolamine and phosphatidyl choline during swelling was calculated to be 20-30 times less than the amount of added free fatty acid that gave comparable swelling. Under the experimental conditions about 4% of the phospholipid was hydrolyzed. Mitochondrial swelling was studied by electron microscopy and turbidity measurements. The results found were in agreement, whether oleic acid was present or not, except for those values obtained after very brief incubation (1 min) and after incubation for longer than 35 min. The lack of direct proportion between swelling and the concentration of lysosomes present indicated that the swelling is related mainly to mitochondrial phospholipase A, although swelling due to contaminating lysosomes cannot be excluded entirely. The temperature dependence of spontaneous, fatty acid-induced, or CaCl(2)-induced swelling suggested that enzymatic activities are responsible for swelling.

Animals↗

PubMed1969

Differentiation of phospholipases A in mitochondria and lysosomes of rat liver.

Highly purified mitochondria from rat liver contain a phospholipase A that catalyzes removal of 2-fatty acids, with a pH optimum above pH 8.0. Lysosomal preparations appeared to have two phospholipases A associated with them, one with a pH optimum at about pH 4.0, the second between pH 6.0 and 7.0. Mitochondrial phospholipase A hydrolyzed exogenous phospholipid as fast as or faster than endogenous phospholipid. The difference in specific radioactivity of (14)C-ethanolamine-labeled endogenous mitochondrial phospholipid before and after incubation indicates that a fraction of mitochondrial phosphatidyl ethanolamine is hydrolyzed more rapidly than the mitochondrial phospholipids as a whole. Acyl bond hydrolysis of exogenous and endogenous phospholipid by mitochondria was stimulated by free fatty acid, Ca(++), or in certain cases, monoacyl phospholipids or by treatments that disrupt the mitochondrial membrane. Of various fatty acids tested, lauric, myristic, oleic, and linoleic were most effective. ADP and ATP inhibited mitochondrial phospholipase, probably because they compete for Ca(++). Mg(++) also behaved as a competitive inhibitor; the effect was overcome by relatively little Ca(++).

Adenine Nucleotides↗

PubMed1967