Further characterization of the diacylglycerol-phosphatidylethanolamine exchange reaction catalyzed by cell-free extracts of Escherichia coli

The conditions for phosphatidylethanolamine (PE)-diacylglycerol (DAG) exchange catalysed by cell-free extracts of Escherichia coli were studied using 14C- or 3H-analogues of both these lipids. The reaction, examined with either labelled PE or labelled DAG, occurred without co-factor addition and was inhibited by Ca2+ and Mg2+. Detergents such as Triton X-100 greatly enhanced the activity; however, the optimal concentration of this agent depended on the lipid substrate concentration. The exchange-catalysing enzyme involved in these extracts appeared to be very specific for DAG and PE, since no other labelled phospholipid or acylglycerol derivative formed radioactive product under the assay conditions tested. Again, endogenous 3H]PE present in the enzyme source, but no other endogenous lipid, was converted to labelled DAG in the presence of added 1,2-dioleoyl-sn-glycerol. The Vmax value for the conversion of labelled PE to DAG was very similar to the Vmax value found for the conversion of labelled DAG to PE as would be expected in the case of an exchange reaction being responsible for both conversions. However, the Km value for PE was appreciably larger than that for DAG. The enzyme involved, displayed a broad acyl chain specificity as could be judged from: (1) the ability of various species of DAG and PE to stimulate the exchange; (2) the suitability of lipid substrates prepared from widely different biological sources; and (3) the interchange of acyl groups that occurred between dimyristoyl PE and dilauroylglycerol. As would be expected for an exchange reaction, the incorporation of lauroyl groups into PE occurred without an increase in the total fatty acid content of this phospholipid. The results of the present study confirm and further characterize the PE-DAG exchange reaction of E. coli.