Short-chain phosphatidylethanolamines: physical properties and susceptibility of the monomers to phospholipase A2 action

The homologous series of optically active short-chain phosphatidylethanolamines (PE) from dibutyryl-PE to dioctanoyl-PE was synthesized. In addition, two monomeric short-chain phospholipid analogues that are not degraded by phospholipase A2 (1,2-bis[(butylcarbamyl)oxy]-sn-glycero-3-phosphocholine and the corresponding ethanolamine derivative) were synthesized. In contrast to the short-chain phosphatidylcholines (PC), short-chain PE's have defined solubilities in water. No break below the solubility limit was found in surface tension plots, suggesting that these compounds exist as monomers in aqueous solution. Only when a significant fraction of the molecules is negatively charged can they form micelles by themselves. Cobra venom phospholipase A2 hydrolyzes monomeric short-chain PE's at about the same rate as short-chain PC's but hydrolyzes long-chain PC's much more rapidly than long-chain PE's. The hydrolysis of short-chain PE's is found to be activated by phosphocholine-containing compounds only in the presence of an interface; in its absence phosphocholine-containing compounds can act as competitive inhibitors. Possible explanations for this phenomenon are considered.