Chiral synthesis of a dithiolester analog of phosphatidylcholine as a substrate for the assay of phospholipase A2.

The synthesis of a dithiolester analog of phosphatidylcholine, 1,2-bis(heptanoylthio)-1,2-dideoxy-sn-glycerol-3-phosphocholine (thio PC), is described. Starting with 1-trityl-sn-glycerol (prepared from D-mannitol), tosylation followed by displacement with potassium methyl xanthate gave a trithiocarbonate. Reductive cleavage of the latter gave a 1,2-dithiol which was then acylated, detritylated, and esterified with choline phosphate. Hydrolysis of thio PC by phospholipase A2 (Naja naja) indicated 95% chiral purity. The rate of hydrolysis as a function of substrate concentration showed a sharp increase at about 0.17 mM, the critical micellar concentration of the lipid. A spectrophotometric assay of phospholipase A2 (by measurement of released thiol groups in the presence of dithionitrobenzoic acid) was quite sensitive. As little as 1 ng of enzyme was detected, representing a rate of about 0.2 nmol of substrate hydrolyzed per min.