Transacylase and phospholipases in the synthesis of bis(monoacylglycero)phosphate

Bis(monoacylglycero)phosphate (monoacyl-sn-glycero-1-phospho-1'-monoacyl-sn-glycerol) is a unique lipid that represents greater than 15% of the total phospholipid of the resident alveolar macrophage, Bis(monoacylglycero)phosphate is not synthesized de novo but rather is derived from phosphatidylglycerol of the lung surfactant. There are two enantiomers of bis(monoacylglycero)phosphate synthesized, but only the sn-1, and not the sn-3, enantiomer accumulates in vivo. We recently published a scheme in which the sn-3 enantiomer was an intermediate in the synthesis of the final sn-1 bis(monoacylglycero)phosphate. Here we further expand the understanding of the biosynthesis of bis(monoacylglycero)phosphate by examining the proposed first two steps of the pathway. A phospholipase A and a transacylase activity are partially separated by gel permeation chromatography. Both are optimally active in the acid pH range that supports the concept that they function in the lysosome-endosome compartment of the cell. Independently, these two enzyme systems are incapable of converting phosphatidylglycerol into sn-3 bis(monoacylglycero)phosphate. However, combination of the two partially purified enzymes reestablishes the synthesis of sn-3 bis(monoacylglycero)phosphate from phosphatidylglycerol. The results presented here support our hypothesis that the phospholipase and transacylase are separate enzymes essential to the synthesis of bis(monoacylglycero)phosphate.