31P nuclear magnetic resonance spectroscopy (31P NMR) was used to monitor the hydrolysis of phospholipids in human serum high-density lipoprotein-3 (HDL3) by «-phospholipase A2 purified from Crotalus adamanteus venom. The 31P NMR spectra obtained at regular intervals during incubation of HDL3 with the enzyme indicated that phosphatidylcholine was completely converted into lysophosphatidylcholine. The proton-decoupled line widths of the phosphatidylcholine in untreated HDL3, and of the lysophosphatidylcholine produced by the enzymatic digestion of the lipoprotein particle, were ca. 7.5 Hz over the entire course of the reaction. Moreover, the chemical shifts of phosphatidylcholine and lysophosphatidylcholine (+1.11 and 0.64 ppm, respectively, with 0.0 ppm assigned to 85% H3PO4) were invariant under these conditions. From these studies, it was determined that the «-phospholipase A2 catalyzed hydrolysis of phosphatidylcholine in HDF3 follows first-order kinetics, in confirmation 1 he lipid composition of the various classes of human serum lipoproteins has been determined. In all classes, phospholipids, specifically phosphatidylcholine (PC),1 are the predominant polar lipid components (for recent reviews, cf. Morrisett et al., 1975). Despite this abundance of compositional information, neither the function nor structural arrangement of the phospholipids in these complexes is well understood.Several laboratories have been investigating the structural arrangement of the components of HDL3 and of other serum lipoproteins by means of a wide array of chemical and physical techniques (cf. Morrisett et al., 1975). Several lines of evidence indicate that the phospholipids are oriented on the surface of f From the