In vivo incorporation of cytidine-monophosphate-ciliatine into rat liver lipids

Further observations on the uptake and effects of phosphonates in perfused rat liver studied by (31)P-NMR

We examined the route of uptake of 2-aminoethylphosphonate (NEthPo) and of phenylphosphonate (PhePo; 10 mM each) in perfused liver by (31)P-NMR. Uptake of NEthPo was concentrative. The rate of uptake was reduced to 21 +/- 2% (n = 3; all percentages refer to control rates) by substituting choline for Na(+), and to 21 +/- 4% (n = 3), 32 +/- 6% (n = 5) and 70 +/- 5% (n = 3) by replacing Cl(-) by gluconate, SO(4)(2-) or NO(3)(-), respectively. Taurine (20 mM) reduced NEthPo uptake to 38 +/- 6% (n = 3). The data are consistent with uptake of NEthPo by the Na(+)-coupled Cl(-)-dependent beta-amino acid transporter. A small fraction of NEthPo was incorporated into phospholipid. PhePo uptake evolved over 1 h towards levels of the membrane-permeant volume marker dimethyl methylphosphonate. Uptake depended on H(+), and was inhibited by 4, 4'-diisothiocyanato-stilbene-2,2'-disulphonic acid (100 microM), bumetanide and furosemide (1 mM each) and alpha-cyano-4-OH-cinnamic acid (5 mM) to 31 +/- 4% (n = 4), 28 +/- 4% (n = 4), 27 +/- 5% (n = 6) and 40 +/- 7% (n = 4), respectively. These characteristics of PhePo uptake are reminiscent of H(+)-coupled monocarboxylate transport. The monocarboxylates, lactate and acetate (20 mM), and the substrate analogue, phenylalanine (20 mM), were not inhibitory, while benzoic acid (20 mM) slightly inhibited (to 82 +/- 5%; n = 4) PhePo uptake. The tested phosphonates (10 mM) did not significantly affect hepatic extraction of [(3)H]-cholate or [(3)H]-taurocholate (25 microM each; 1:3 bile salt:albumin). The monocarboxylate analogue, PhePo (10 mM), did not significantly interfere with disposal of lactate (0.3-5 mM).

[Biosynthesis and metabolism of phosphonolipids and phospholipids in rat hepatocytes and Saccharomyces cerevisiae]

In rat hepatocytes, ciliatine (2 aminoéthylphosphonic acid) is incorporated into phosphonolipid (PnE) by the same pathway leading from phosphorylethanolamine to phospholipid (PE). The two resulting lipids are isolated from mitochondria and microsomes. The rates of biosynthesis are quite comparable; the processes of trimethylation and of in vitro transfer in the presence of a specific exchange protein are very similar. In yeast, on the other hand, the uptake of the two precursors is very slight, suggesting that the direct cytidylic pathway of phospholipid biosynthesis is strongly repressed. Despite small amounts of PE and PnE produced, methylation occurs with a good yield. The good incorporation of ethanolamine may be understood by a base-exchange mechanism. The natural PE biosynthesis is achieved through the decarboxylation of phosphatidylserine, and followed by methylation leading to phosphatidylcholine. The use of very small amounts of precursors does not modify the natural course of phospholipid biosynthesis.