1,3-Diacylglycero-2-phosphocholines--synthesis, aggregation behaviour and properties as inhibitors of phospholipase D

Purification, sequencing and characterization of phospholipase D from Indian mustard seeds

Phospholipase D (PLD; E.C. 3.1.4.4) is widespread in plants where it fulfills diverse functions in growth and in the response to stresses. The enzyme occurs in multiple forms that differ in their biochemical properties. In the present paper PLD from medicinally relevant Indian mustard seeds was purified by Ca(2+)-mediated hydrophobic interaction and anion exchange chromatography to electrophoretic homogeneity. Based on mass-spectrometric sequence analysis of tryptic protein fragments, oligonucleotide primers for cloning genomic DNA fragments that encoded the enzyme were designed and used to derive the complete amino acid sequence of this PLD. The sequence data, as well as the molecular properties (molecular mass of 92.0 kDa, pI 5.39, maximum activity at pH 5.5-6.0 and Ca(2+) ion concentrations ⩾60 mM), allowed the assignment of this enzyme to the class of α-type PLDs. The apparent kinetic parameters Vmax and Km, determined for the hydrolysis of phosphatidylcholine (PC) in an aqueous mixed-micellar system were 356±15 μmol min(-1) mg(-1) and 1.84±0.17 mM, respectively. Phosphate analogs such as NaAlF4 and Na3VO4 displayed strong inhibition of the enzyme. Phosphatidylinositol 4,5-bisphosphate had a strong activating effect at 2-10 mM CaCl2. PLD was inactivated at temperatures >45 °C. The enzyme exhibited the highest activity toward PC followed by phosphatidylethanolamine and phosphatidylglycerol. PCs with short-chain fatty acids were better substrates than PCs with long fatty acid chains. Lyso-PC was not accepted as substrate.

Calcium-induced membrane microdomains trigger plant phospholipase D activity

Plant alpha-type phospholipase D proteins are calcium-dependent, lipolytic enzymes. The morphology of the aggregates of their phospholipid substrate fundamentally defines the interaction between the enzyme and the surface. Here we demonstrate that the Ca(2+)-induced generation of membrane microdomains dramatically activates alpha-type phospholipase D from white cabbage. 500-fold stimulation was observed upon incorporation of 10 mol % 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphate (POPA) into 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) vesicles in the presence of Ca(2+) ions. Enhanced association of PLDalpha2 with phospholipid surfaces containing anionic components was indicated by lag phase analysis and film balance measurements. Differential scanning calorimetry showed that the POPA-specific activation correlates with the phase behavior of the POPC/POPA vesicles in the presence of Ca(2+) ions. We conclude from the results that the Ca(2+)-induced formation of POPA microdomains is the crucial parameter that facilitates the binding of PLD to the phospholipid surface and suggest that this effect serves as a cellular switch for controlling PLD activity.

Hydrophilic carotenoids: surface properties and aggregation behavior of a highly unsaturated carotenoid lysophospholipid

The water dispersibility of a hydrophobic carotenoid has been greatly enhanced by using it as the acyl part in the synthesis of a highly unsaturated lysophospholipid. Dynamic light scattering has revealed the formation of stable aggregates with an average hydrodynamic radius of a few nanometers, and absorption spectra show that the aggregates can withstand the addition of ethanol or acetonitrile until the volume fraction of water falls below 70 and 62%, respectively. The properties of the carotenoid phospholipids have been characterized by determining surface tension, critical micelle concentration, surface concentration, molecular area, free energy of adsorption and micellation, adsorption-micellar energy relationship, and equilibrium constants.

Proteolytic sensitivity of a recombinant phospholipase D from cabbage: identification of loop regions and conformational changes

A recombinant phospholipase D from white cabbage (PLD2) composed of 812 amino acid residues was studied by site-directed mutagenesis and limited proteolysis to obtain first information on its tertiary structure. Limited proteolysis by thermolysin resulted in the formation of some large fragments of PLD2. From mass spectrometry and N-terminal sequencing of the peptides, the cleavage sites could be identified (1. Thr41-Ile42, 2. Asn323-Leu324 or Gly287-Leu288 and Ser319-Ile320 in case of the mutant L324S-PLD2). This suggested an exposed loop in the C2 domain of PLD2 and a large flexible region close to the N-terminal side of the first catalytic (HKD) motif. Calcium ions, the substrate 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and the competitive inhibitor 1,3-dipalmitoylglycero-2-phosphocholine influenced the proteolytic cleavage. Calcium ions exerted a destabilizing effect on the conformation of PLD2.