Elabbadi N, Ancelin M L, Vial H J
UMR CNRS 5539, Département Biologie Santé, Université Montpellier II case 107, Place E. Bataillon, 34095 Montpellier Cedex 5, France.
Biochem J. 1997 Jun 1;324 ( Pt 2)(Pt 2):435-45. doi: 10.1042/bj3240435.
Erythrocytes infected with Plasmodium falciparum or Plasmodium knowlesi efficiently incorporated radioactive serine into phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn) and phosphatidylcholine (PtdCho). Serine was also metabolized into ethanolamine (Etn) and phosphorylethanolamine (P-Etn) via direct serine decarboxylation; this is a major phenomenon since together these metabolites represent 60% of total radioactive water-soluble metabolites. They were identified by reverse-phase HPLC and two TLC-type analyses and confirmed by alkaline phosphatase treatment, which depleted the radioactive P-Etn peak completely with a concomitant increase in that of Etn. In the presence of 5 microM labelled serine, radioactivity appeared in Etn and P-Etn after a 25 min lag period, and isotopic equilibrium was reached at 40 and 95 min respectively. There was a similar lag period for PtdEtn formation, which accumulated steadily for at least 180 min. Incorporation of serine into phospholipids and water-soluble metabolites increased in the presence of up to 500 microM external serine. An apparent plateau was then reached for all metabolites except intracellular serine and Etn. Exogenous Etn (at 20 microM) induced a concomitant dramatic decrease in serine incorporation into P-Etn and all phospholipids, but not into Etn. Increasing exogenous serine to 100 microM decreased the incorporation of radioactive Etn into PtdEtn by only 30%, and the PtdCho level was not affected. 2-Hydroxyethylhydrazine significantly decreased serine incorporation into P-Etn and PtdEtn, whereas Etn was accumulated. No concomitant inhibition of PtdSer or PtdCho labelling from serine occurred, even when PtdEtn formation was decreased by 95%. This indicates that the PtdEtn pool derived from direct serine decarboxylation differed from that derived from PtdSer decarboxylation, and the latter appeared to be preferentially used for PtdCho biosynthesis. Hydroxylamine also inhibited phosphorylation of serine-derived Etn but not that of exogenous Etn. The rate of PtdSer synthesis from 10 microM L-serine was 3.1+/-0.5 and 2.95+/-1.3 nmol/5 h per 10(10) infected cells, whereas L-serine decarboxylation accounted for 7.1+/-1.5 and 9.9+/-3 nmol/5 h per 10(10) infected cells for P. falciparum and P. knowlesi respectively (means+/-S.E.M.). The serine decarboxylating reaction was not detected in other higher eukaryotic cells such as mouse fibroblasts and human lymphocytes. Finally, these results also indicate compartmentalization of phospholipid metabolism in Plasmodium-infected erythrocytes.
感染恶性疟原虫或诺氏疟原虫的红细胞能有效地将放射性丝氨酸掺入磷脂酰丝氨酸（PtdSer）、磷脂酰乙醇胺（PtdEtn）和磷脂酰胆碱（PtdCho）中。丝氨酸还可通过直接丝氨酸脱羧作用代谢为乙醇胺（Etn）和磷酸乙醇胺（P-Etn）；这是一个主要现象，因为这些代谢产物加起来占总放射性水溶性代谢产物的60%。它们通过反相高效液相色谱法和两种薄层层析类型分析进行鉴定，并通过碱性磷酸酶处理得到证实，碱性磷酸酶处理使放射性P-Etn峰完全消失，同时Etn峰增加。在存在5微摩尔标记丝氨酸的情况下，经过25分钟的延迟期后，Etn和P-Etn中出现放射性，分别在40分钟和95分钟达到同位素平衡。PtdEtn形成也有类似的延迟期，其至少稳定积累180分钟。在存在高达500微摩尔外部丝氨酸的情况下，丝氨酸掺入磷脂和水溶性代谢产物的量增加。除细胞内丝氨酸和Etn外，所有代谢产物随后都达到了明显的平稳期。外源性Etn（20微摩尔）导致丝氨酸掺入P-Etn和所有磷脂中的量同时显著减少，但不影响Etn。将外源性丝氨酸增加到100微摩尔时，放射性Etn掺入PtdEtn中的量仅减少30%，且PtdCho水平不受影响。2-羟乙基肼显著降低丝氨酸掺入P-Etn和PtdEtn中的量，而Etn则积累。即使PtdEtn形成减少95%，也未同时抑制丝氨酸对PtdSer或PtdCho的标记。这表明直接丝氨酸脱羧产生的PtdEtn池与PtdSer脱羧产生的不同，后者似乎优先用于PtdCho的生物合成。羟胺也抑制丝氨酸衍生的Etn的磷酸化，但不抑制外源性Etn的磷酸化。10微摩尔L-丝氨酸合成PtdSer的速率为每10^10个感染细胞3.1±0.5和2.95±1.3纳摩尔/5小时，而对于恶性疟原虫和诺氏疟原虫，L-丝氨酸脱羧作用分别为每10^10个感染细胞7.1±1.5和9.9±3纳摩尔/5小时（平均值±标准误）。在其他高等真核细胞如小鼠成纤维细胞和人类淋巴细胞中未检测到丝氨酸脱羧反应。最后，这些结果还表明疟原虫感染的红细胞中磷脂代谢存在区室化。
