Metabolism of phosphatidylglycerol and bis(monoacylglycero)-phosphate in macrophage subcellular fractions

Bis(monoacylglycero)phosphate (BMP) is synthesized from exogenous phosphatidylglycerol (PG) by macrophages (Cochran, F. R., Roddick, V. L., Connor, J. R., Thornburg, J. T., and Waite, M. (1987) J. Immunol. 138, 1877-1883). Previous work from our laboratory showed that arachidonic acid in BMP was released by the macrophages upon challenge of the cells with PMA (Cochran, F. R., Connor, J. R., Roddick, V. L., and Waite, M. (1985) Biochem. Biophys. Res. Commun. 130, 800-806). Here we extend those studies using a model cultured cell line of macrophages, RAW 264.7. When PG labeled with 32P- and 3H]glycerol in both moieties was added to the culture medium, 32P/3H]BMP was synthesized in a time-dependent manner. Fractionation of cell homogenates on a discontinuous sucrose gradient in which the light membranes were floated from dense sucrose showed an enrichment of 3H]BMP in light membrane fractions. The precursor 3H]PG was also found in the light fractions but, relative to the 3H]BMP, was more abundant in the denser membrane fractions. The appearance of 3H]PG and 3H]BMP in the light membrane fraction was time-dependent which suggested that the initial uptake and metabolism of 3H]PG was into the denser membranes. Incubation of the light membranes under conditions that are optimal for the lysosomal phospholipase A1 led to significant metabolism of 3H]PG. Both degradation of 3H]PG to water-soluble compounds and its conversion to acylphosphatidylglycerol occurred while no lyso-PG was detected. On the other hand, little BMP was found to be degraded. From these studies we postulate that in lysosomes acylphosphatidylglycerol is a precursor of BMP and that the previously reported turnover of arachidonic acid by BMP may occur via transacylation rather than hydrolysis.