Abstract
N-Acylphosphatidylethanolamine (NAPE)-hydrolyzing phospholipase D (NAPE-PLD) is a membrane-bound enzyme which releases the endocannabinoid anandamide and other bioactive N-acylethanolamines from their corresponding NAPEs in animal tissues. Our previous studies showed that NAPE-PLD solubilized from the membrane is remarkably stimulated by millimolar concentrations of Ca2+ while the membrane-bound form is much less sensitive to Ca2+. This finding suggested that certain membrane constituents diminished the stimulatory effect of Ca2+. In the present studies, we examined the effects of membrane fractions from COS-7 cells and brain tissue on the purified recombinant rat NAPE-PLD, and found that heat-stable membrane component(s) dose-dependently activated NAPE-PLD up to 4.8–5.0 fold. In the presence of the membrane fractions, however, the stimulatory effect of Ca2+ on the purified NAPE-PLD was considerably reduced. When it was examined if the membrane fractions can be replaced with various pure phospholipids, phosphatidylethanolamine activated NAPE-PLD up to 3.3 fold, which was followed by decrease in the stimulatory effects of Ca2+ and several other divalent cations. These results suggest that membrane components including phosphatidylethanolamine keep the membrane-associated form of NAPE-PLD constitutively active.
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