Biosynthetic preparation of radioactively labelled ethanolamine plasmalogen (1-O-[1'-14C]octadec-1'-enyl-2-acyl-sn-glycero-3-phosphoethanola mine) using a protozoan cell culture

Metabolism of ether lysophospholipids in Leishmania donovani promastigotes

Ether lysophospholipids, 1-O-[1'-14C]octadec-1'-enyl-sn-glycero-3-phosphoethanolamine (A) and -phosphocholine (B) as well as 1-O-[1'-14C]octadecyl-sn-glycero-3-phosphoethanolamine (C) and -phosphocholine (D) were taken up rapidly and metabolized extensively in Leishmania donovani promastigotes. Degradation to neutral lipids occurred first, followed by incorporation into phospholipids. Incubation of the cells with (A) or (B) revealed the stability of the O-[1-14C]octadec-1-enyl group up to 15 h, indicating the absence of any O-alk-1-enyl cleavage enzymes. Most of the radioactivity was found in 1-O-alkenyl-2-acyl-glycerophosphoethanolamine and 1-O-alkenyl-2,3-diacylglycerol. 1-O-Alkenyl-2-acyl-glycerophosphocholine was detected only after incubation with substrate (B). In contrast to the O-alk-1-enyl residue, the O-[1-14C]octadecyl moiety in substrate (C) and (D) could be converted into the O-[1-14C]octadec-1-enyl moiety or cleaved, yielding labelled acyl groups. Following 5 h incubation with substrate (C), most of the incorporated radioactivity was associated with 1-O-[1'-14C]octadec-1'-enyl-2-acyl-glycerophosphoethanolamine, 1-O-[1'-14C]octadecyl-2,3-diacylglycerol and 1-O-[1'-14C]octadecyl-2-acyl-glycerophosphoinositol. After 15 h minor amounts of label appeared in diacyl glycerophosphocholine. Similar labelling patterns were obtained with the choline analogue (D), except that 1-O-[1'-14C]octadecyl-2-acyl-glycerophosphocholine was found additionally. Incubations of the four labelled ether lysophospholipids with cell homogenates showed the presence of a lysophospholipase D and a phosphohydrolase. There was no specificity towards different ether residues or phosphobase moieties. Formation of alkyl- and alkenylglycerol, respectively, was stimulated by Mg2+ ions and the phosphohydrolase was inhibited by NaF. The results support the conclusion that the specific pattern of ether phospholipids in L. donovani cells is due to a pronounced specificity of the biosynthetic enzymes. Enzymes of the catabolic reactions are of low specificity or absent, such as plasmalogenases.

Cytotoxicity of ester and ether lysophospholipids on Leishmania donovani promastigotes

The cytotoxic activity of four ester lysophospholipids, three ether lysophospholipids, and two radylglycerols on Leishmania donovani promastigotes was determined by measuring the inhibition of cell growth. The 1-acyl lysophospholipids reduced cell growth to 50% of controls at concentrations of 6.4-10.9 microM. In contrast, 1-O-alkenyl-sn-glycero-3-phosphoethanolamine, 1-O-hexadecyl-sn-glycero-3-phosphocholine, and 1-O-hexadecyl-sn-glycerol already showed a 50% inhibition of growth at concentrations between 2.1 and 2.8 microM. Moreover, the unnatural alkyl lysophospholipid analogue 1-O-octadecyl-2-methoxy-sn-glycero-3-phosphocholine was even 10-fold more toxic. Incubations of L. donovani promastigotes with radioactively labelled ether lysophospholipids revealed a rapid uptake of these compounds and their incorporation into cellular lipids at a non-toxic concentration of 1.0 microM. An accumulation of the lysophospholipids in the cell due to insufficient metabolism may be the cause of its cytotoxic effect. The sensitivity of L. donovani cells towards ether lysophospholipids was found to be similar to that reported for tumor cells.

Conversion of radiolabelled ethanolamine plasmalogen into the dimethylethanolamine and choline analogue via transphosphatidylation by phospholipase D from cabbage

Starting from biosynthetically prepared ethanolamine plasmalogen 14C-labelled in the O-alkenyl moiety, choline and dimethylethanolamine plasmalogen were prepared by transphosphatidylation utilizing phospholipase D from cabbage. Investigation of the time course of the reaction showed that transphosphatidylation was simultaneously accompanied by hydrolysis of both the substrate and the desired product, resulting in a maximum of product yield after 1-3 h under the reaction conditions investigated. Optimal reaction conditions gave yields of 40% and 62% (of total radioactivity) respectively for the purified choline and dimethylethanolamine derivatives.