Cytosols (105000 X g supernatant) from seven rat tissues were assayed for Ca²⁺-independent phospholipase\ud A₂ activity with either 1-acyl-2-[1-¹⁴C]linoleoyl-sn-glycero-3-phosphocholine, 1-acyl-2-[l-¹⁴C]linoleoyl-snglycero-\ud 3-phosphoethanohunine or 1-0-hexadecyl-2-[9,10-³H₂]oleoyl-sn-glycero-3-phosphocholine as substrate.\ud Low but consistent activities ranging from l0-120 pmol /min per mg protein were found in all\ud tissues. The highest activities were present in liver, lung and brain. Total activities in mU/g wet weight were\ud rather constant, ranging from 0.43 (heart) to 1.36 (liver). The soluble enzyme from rat lung cytosol was\ud further investigated and was found to be capable of hydrolyzing microsomal membrane-associated substrates\ud without exhibiting much selectivity for phosphatidylcholine species. Comparative gel filtration experiments\ud of cytosol prepared from non-perfused and perfused lungs indicated that part of the Ca²⁺-independent\ud phospholipase A₂ originated from blood cells, but most of it was derived from lung cells. Lung cytosol also\ud contained Ca²⁺-dependent phospholipase A₂ activity, a small part of which originated from blood cells,\ud presumably platelets. The major amount of Ca²⁺ -dependent phospholipase A₂ activity, however, came from\ud lung cells. Neither this enzyme nor the Ca²⁺-independent phospholipase A₂ from lung tissue showed\ud immunological cross-reactivity with monoclonal antibodies against Ca²⁺-dependent phospholipase A ₂ isolated\ud from rat liver mitochondria
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