Characterization of Phospholipase A2 Activity Enriched in the Nerve Growth Cone

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Abstract
Nerve growth cones isolated from fetal rat brain exhibit in their cytosol a robust level of phospholipase A2 activity hydrolyzing phosphatidylinositol (PI) and phosphatidylethanolamine (PE) but not phosphatidylcholine (PC). Western blot analysis with an antibody to the well‐characterized cytosolic phospholipase A2 (mol wt 85,000) reveals only trace amounts of this PC‐ and PE‐selective enzyme in growth cones. By gel filtration on Superose 12, growth cone phospholipase A2 activity elutes essentially as two peaks of high molecular mass, at ∼65 kDa and at well over 100 kDa. Anion exchange chromatography completely separates a PI‐selective from a PE‐selective activity, indicating the presence of two different, apparently monoselective phospholipase A2 species. The PI‐selective enzyme, the predominant phospholipase A2 activity in whole growth cones, is enriched greatly in these structures relative to their parent fractions from fetal brain. This phospholipase A2 is resistant to reducing agents and is found in the cytosol as well as membrane‐associated in the presence of Ca2+. However, its catalytic activity is Ca2+‐independent regardless of whether the enzyme is associated with pure substrate or mixed‐lipid growth cone vesicles. The PE‐selective phospholipase A2 in growth cones was studied in less detail but shares with the PI‐selective enzyme several properties, including intracellular localization, the existence of cytosolic and membrane‐associated forms, and Ca2+ independence. Our data indicate growth cones contain two high‐molecular‐weight forms of phospholipase A2 that share many properties with known, Ca2+‐independent cytosolic phospholipase A2 species but that appear to be monoselective for PI and PE, respectively. In particular, the PI‐selective enzyme may represent a new member of the growing family of cytoplasmic phospholipase A2. The enrichment of the PI‐selective phospholipase A2 in growth cones suggests it plays a major role in the regulation of growth cone function.

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