Phospholipase A2 activity in the fat body of the tobacco hornworm Manduca sexta

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Abstract
We report on phospholipase A2 (PLA2) activity in homogenates prepared from fat bodies of the tobacco hornworm Manduca sexta. PLA2 activity is responsible for hydrolyzing fatty acids from the sn-2 position of phospholipids. The rate of hydrolysis increased with increasing homogenate protein concentration up to ∼︁ 320 μg protein/ml reaction volume. Higher protein concentrations did not appreciably increase the rate of PLA2 activity. As seen in some, but not all PLA2s from mammalian sources, hydrolyzing activity increased linearly with time. The fat body activity was sensitive to pH (optimal activity at pH 8–9) and temperature (optimal activity at ∼︁40°C). The activity was associated with fat body rather than hemolymph, because no activity was detected in cell-free serum. The fat body PLA2 activity differs from the majority of PLA2s with respect to calcium requirements. Whereas most PLA2s are calcium-independent. A few others are known to require submicromolar calcium concentrations. The fat body activity appears to be calcium independent. These data show that a PLA2 activity that can hydrolyze arachidonic acid from the sn-2 position of phospholipids is associated with the tobacco hornworm fat body. The biological significance of this activity relates to biosynthesis of eicosanoids. Pharmacological inhibition of PLA2 impairs the ability of this insect to respond to bacterial infections. Since the impairment can be reversed by treatment with exogenous arachidonic acid, the PLA2 activity may be an important step in eicosanoid biosynthesis.

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