Transphosphatidylation by immobilized phospholipase D in aqueous media.

Phospholipase D (PLD) from Streptomyces sp. was immobilized by covalent binding to aminopropyl-glass activated by glutardialdehyde and to the macroporous synthetic polymer VA-Epoxy Biosynth (from Riedel-de Häen, Seelze, Germany) pre-activated by epoxy groups. The immobilized PLDs were examined for the synthesis of phosphatidylglycerol (PG) from soybean (Glycine max) phosphatidylcholine (PC) in purely aqueous solutions in comparison with commonly used diethyl ether/buffer systems. In contrast with general assumptions, the transphosphatidylation was shown to yield a high percentage of PG, even in pure buffer. With PLD immobilized to VA-Epoxy Biosynth, the formation of phosphatidic acid (PA) is insignificant, while the yield of PG amounts to 60%. With PLD immobilized to porous glass (average pore diameter 17 nm), higher yields of PG (72%) are reached, but the formation of PA also increases (up to 10%). In comparison with the reaction in the diethyl ether/buffer system, however, the conversion of PC into PG proceeds much more slowly. Detergents such as Triton X-100 accelerate the reaction.

Hexadecylphosphocholine and 2-modified 1,3-diacylglycerols as effectors of phospholipase D.

The kinetic behaviour of phospholipase D (PLD) from cabbage has been studied in the presence of several substrate-like compounds such as hexadecylphosphocholine (HPC) and 1,3-didodecanoylglycero-2-phosphatides. 1,3-Didodecanoyl- glycero-2-phosphocholine (1,3-DiC12PC) was found being not cleft by PLD, whereas HPC is hydrolyzed by PLD with small rate. The plot of initial velocity vs. substrate concentration for HPC is more sigmoidal than those for the common substrate phosphatidylcholine (PC)/sodium dodecylsulfate (SDS) (1:0.5) or the short-chain 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DiC6PC). The anionic amphiphiles 1,3-didodecanoylglycero-2-sulfate and 1,3-didodecanoylglycero-2-phosphate act as activators of PLD towards PC similar to SDS. In contrast, 1,3-DiC12PC shows inhibitory properties with an increase in the sigmoidicity of the initial velocity as a function of substrate concentration in the PC/SDS assay. Also HPC inhibits the hydrolysis of PC/SDS, whereas it acts as activator or inhibitor in the hydrolysis of DiC6PC. The results suggest that PLD possesses two substrate-binding sites, where one binds substrate in function of an effector without catalytic activity while the other is the catalytic site. HPC and 1,3-DiC12PC are assumed to compete with the substrate for both binding sites with effects depending on the ratio of concentrations and affinities of substrates and effectors.

Alkylphosphate esters as inhibitors of phospholipase D.

Alkylphosphate esters were shown to be potent inhibitors of phospholipase D. Using phosphatidyl choline/sodium dodecylsulfate (2:1) as substrate, IC50 values were determined for alkylphosphocholines of different chain length (C10-C18) and for various octadecylphosphate esters with different polar head groups. The inhibitory potency strongly increased with increasing chain length of the alkyl chain. The substitution of choline for heterocyclic nitrogen compounds or for 2-trimethylarsonio-ethanol also affected the inhibition of phospholipase D. Octadecylphosphocholine proved to be the most efficient inhibitor (IC50 = 6.4 microM).