STEROL AND STERYL ESTER REGULATION OF PHOSPHOLIPASE A(2) FROM THE MOSQUITO PARASITE LAGENIDIUM-GIGANTEUM

Lagenidium giganteum, a facultative parasite of mosquito larvae, canno t synthesize sterols, and requires an exogenous source of these lipids in order to enter its reproductive cycle. This parasite grows vegetat ively in the absence of sterols, but requires cholesterol or structura lly related compounds to produce motile zoospores, which are the only stage capable of infecting mosquitoes. Sterols structurally related to chotesterol and some steryl esters inhibited the activity of L. gigan teum phospholipase A(2) (PLA(2)), an enzyme that hydrolyzes fatty acid s from the sn-2 position of glycerophospholipids. Sterols that induce reproduction inhibited L. giganteum PLA(2) activity, while sterols and steroids that do not support sporulation had minimal effect. Most ste ryl esters had no effect on enzyme activity, but cholesteryl arachidon ate (CA) was a potent inhibitor of parasite PLA(2). Not all enzymes pa rtly purified using a DEAE-Sephacel column were affected by these lipi ds, demonstrating selective inhibition of specific enzymes. Potency wa s enhanced by up to several orders of magnitude if epoxy fatty acids w ere esterified to the cholesterol nucleus. The steryl ester pool was d ynamic during morphogenesis, and the fatty acid composition of the ste ryl esters did not mimic total cell or membrane (glycerophospholipid) fatty acid composition as L. giganteum proceeded through its growth cy cle. Synthesis of CA and monoepoxy CA by the parasite was confirmed us ing electrospray mass spectrometry and collision-induced dissociation. Steryl derivatives selectively inhibited PLA(2) enzymes from bovine p ancreas, snake venom, and human cytoplasmic 85-kDa PLA(2).