Phase composition of lipoprotein SM/cholesterol/PtdCho affects FA specificity of sPLA2s

Abstract

We have previously reported preferential release of polyunsaturated FAs during hydrolysis of lipoprotein phosphatidylcholine (PtdCho) by group X secretory phospholipase A2 (sPLA2) and preferential release of oligounsaturated FAs during hydrolysis of lipoprotein PtdCho by group V sPLA2, but the mechanism of this selectivity has remained unknown. We now show that the rate and specificity of hydrolysis are affected by relative increases in endogenous SM and free cholesterol (FC) during the lipase digestion. The highest preference for arachidonate release from LDL and HDL by group X sPLA2 was observed for residual SM/PtdCho molar ratio of 1.2 and 0.4, compared with the respective starting ratios of 0.4 and 0.2, as measured by liquid chromatography/electrospray ionization-mass spectrometry. Group V sPLA2 showed preferential release of linoleate from LDL and HDL at SM/PtdCho ratio 1.5 and 0.6, respectively. We have attributed the change in FA specificity to segregation of molecular species of PtdCho and of sPLA2s between disordered and ordered SM/FC/PtdCho lipid phases. The increases in SM and FC during digestion with group IIA sPLA2 were more limited, and a preferential hydrolysis of any FAs was not observed. The significance of SM and FC SM and FC accumulation during sPLA2 hydrolysis of lipoprotein PtdCho has been previously overlooked.

Keywords

• secretory phospholipase A2
• phosphatidylcholine
• sphingomyelin
• phase diagrams
• liquid chromatography-mass spectrometry