Phosphatidylcholine formation is the predominant lipid biosynthetic event in the hemoparasite Babesia bovis

This work examines the lipid composition and metabolism of bovine red blood cells infected by apicomplexan Babesia parasites, organisms closely relate d to Plasmodium sp. We found that erythrocytes infected with Babesia bovis (i-RBC) accumulate lipids and show striking increases in phosphatidylcholin e. phosphatidic acid, diacylglycerol and cholesteryl esters as compared to uninfected erythrocytes cultured under the same conditions (n-RBC). A simil ar pattern was observed in cultures of erythrocytes infected with Babesia b igemina. The lipid profile of purified B. bovis merozoites showed that phos phatidylcholine is the most abundant phospholipid in this parasite (31.8% /- 2.8 of total phospholipid), markedly differing from bovine n-RBC, in whi ch it is only a minor component (4.8% +/- 0.6). B. bovis cultures incorpora te radiolabeled choline into complex lipids, especially phosphatidylcholine . with minor amounts recovered in sphingomyelin and lysophosphatidylcholine . When [C-14] stearate was used as precursor, the labeling pattern again ga ve the highest incorporation into phosphatidylcholine, with lesser incorpor ation in sphingomyelin, phosphatidylinositol, phosphatidylethanolamine and phosphatidic acid. Diacylglycerol and small amounts of cholesteryl esters w ere the only labeled neutral lipids found. B. bovis also incorporates [H-3] myo-inositol into phosphatidylinositol. Parallel incubations with n-RBC as a control yielded no incorporation into either polar or neutral lipids wit h ally precursor. These results indicate that the lipid changes observed in i-RBC can be explained on the basis of the lipid biosynthetic activities o f the babesial parasite. Gas chromatography-mass spectrometry (GC-MS) analy sis of fatty acid methyl esters from phospholipids of i-RBC and n-RBC showe d the same qualitative composition in both. However. i-RBC had higher ratio s of saturated to unsaturated fatty acids and B. bovis cultures did not des aturate [C-14] stearate. Cholesterol was the only sterol detected by GC-MS. Phospholipase A(2) treatment of i-RBC and n-RBC revealed no enhanced hemol ytic effects in i-RBC, suggesting that the erythrocyte membrane phospholipi d composition is essentially unaltered by the parasite. Labeling of i-RBC o r n-RBC with [I-125] Bolton-Hunter resulted in an enhanced phosphatidylseri ne labeling in i-RBC. This study provides the first data on B, bovis lipid constitution and biosynthesis. They show that phosphatidylcholine formation is the main biosynthetic process in these cells. The striking differences in the contents of phosphatidylcholine between host erythrocytes and the pa rasite suggests that it may be a useful target for both chemotherapy and im munoprophylaxis against bovine babesiosis. (C) 2000 Elsevier Science B.V. A ll rights reserved.