Isomerization of bis(monoacylglycero)phosphate by acyl migration

Bis(monoacylglycero)phosphates (BMPs) are biologically functional acidic lipids present in late endosomes and lysosomes. We recently reported that lysosomal phospholipase A2 (LPLA2, PLA2G15), the lysosomal enzyme mediating BMP catabolism, degrades BMP isomers with distinct substrate specificity. Specifically, sn-(3-oleoyl-2-hydroxy)-glycerol-1-phospho-sn-1'-(3'-oleoyl-2'-hydroxy)-glycerol (S,S-(3,3'-diC_18:1)-BMP) is a significantly better substrate for LPLA2 than S,S-(2,2'-diC_18:1)-BMP. S,S-(2,2'-diC_18:1)-BMP is generally considered the only biologically relevant BMP isomer. We investigated the isomerization of S,S-(2,2'-diC_18:1)-BMP to (S,S-(3,3'-diC_18:1)-BMP) in vitro and in cells. Thin-layer chromatography was used to distinguish S,S-(3,3'-diC_18:1)-BMP from S,S-(2,2'-diC_18:1)-BMP. S,S-(2,2'-diC_18:1)-BMP/1,2-di-O-(9Z-octadecenyl)-sn-glycero-3-phosphocholine liposomes were incubated at varying pH in the presence or absence of test substances. First, we studied bovine serum albumin, which is known to promote isomerization of 1-acyl-2-lysophosphatidylcholine. The formation of S,S-(3,3'-diC_18:1)-BMP in the presence of albumin increased in a time-dependent and albumin concentration-dependent manner under neutral conditions and was dependent on pH and the molar ratio of S,S-(2,2'-diC_18:1)-BMP in liposomes. Treatment of isomeric products generated during isomerization reaction with sn-1,3-specific lipase produced both oleic acid but also lyso-phosphatidylglycerol, indicating that the conversion of S,S-(2,2'-diC_18:1)-BMP to S,S-(3,3'-diC_18:1)-BMP is preceded via S,S-(2,3'-diC_18:1)-BMP. S,S-(3,3'-diC_18:1)-BMP formed was preferentially degraded by LPLA2 over the S,S-(2,2'-diC_18:1)-BMP. Proteins such as HSP70 and human serum albumin and metal ions such as Fe³⁺ and Zn²⁺ acted as cofactors promoting the isomerization of S,S-(2,2'-diC_18:1)-BMP under neutral conditions. At baseline, RAW 264.7 cells showed nonnegligible amounts of sn-1,3-specific lipase-sensitive BMPs. However, lipase-sensitive BMPs were increased by exposure to chloroquine or NH₄Cl, suggesting that cells undergo S,S-(2,2'-diacyl)-BMP isomerization upon alkalinization of intracellular acidic compartments.