Role of the phospholipid environment in modulating the activity of the rat brain synaptic plasma membrane calcium ATPase

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Abstract
The role of the phospholipid environment in modulating the activity of the rat brain synaptic plasma membrane (SPM) Ca2+-ATPase was investigated by its reconstitution into different phospholipids. Retention of activity of the solubilized Ca2+-ATPase depended on addition of exogenous phospholipids. As the cholate concentration used for solubilization of native SPM increased, a larger excess of exogeneous phospholipids, relative to membrane protein, had to be added to maintain optimal activity. Highest ATP-dependent Ca2+ transport activity was obtained when reconstitution was carried out in calf brain phospholipids (BPLs) followed by soybean phospholipids (SPLs) and the lowest in egg PC; reconstitution at a 40:1 weight ratio of exogenous phospholipids to native SPM protein resulted in ATP-dependent Ca2+ transport of 40.0 .+-. 4.16, 23.4 .+-. 8.48, and 11.54 .+-. 2.31 nmol of Ca2+ (mg of protein)-1 (5 min)-1, respectively. Partial substitution of egg PC with BPLs led to an increase in the activity of the reconstituted Ca2+ pump. The highest ATP-dependent Ca2+ uptake was obtained when ratios of 15:25 or 10:30 egg PC to BPLs were used. Testing the individual phospholipids participating in the BPL mixture showed that addition of PS to egg PC led to a consistent increase in Ca2+ pump activity. Substitution of 50% of the PC with PS resulted in a 3.8-fold higher ATP-dependent Ca2+ uptake than that obtained in egg PC alone. No other phospholipid tested-PE, SM, or PI-had a similar effect. Increasing the proportion of PS within the BPL mixture above its original content led to a gradual decrease in the reconstituted SPM Ca2+ pump activity. Enrichment of asolectin with PS led first to increased Ca2+ pump activity; then, as the proportion of PS increased, Ca2+ transport of the reconstituted pump decreased. An increased proportion of PE, SM, or PI within the BPLs or asolectin, above their original contents, resulted in decreased Ca2+ transport. These results indicate that optimal SPM Ca2+ pump activity requires the combined presence of a critical amount of PC and PS within the reconstituted membrane.

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