Limited proteolysis of bovine muscle and heart lactate dehydrogenase is inhibited by phospholipid liposome interaction

Bacterial Interference With Lactate Dehydrogenase Assay Leads to an Underestimation of Cytotoxicity

Models to study host-pathogen interactions in vitro are an important tool for investigating the infectious disease process and evaluating the efficacy of antimicrobial compounds. In these models, the viability of mammalian cells is often determined using the lactate dehydrogenase (LDH) cytotoxicity assay. In the present study we evaluated whether bacteria could interfere with the LDH assay. As a model for host-pathogen interactions, we co-cultured lung epithelial cells with eight bacteria encountered in the lower respiratory tract. We show that LDH activity is affected by Pseudomonas aeruginosa, Klebsiella pneumoniae, Stenotrophomonas maltophilia, and Streptococcus pneumoniae, and that this depends on the density of the start inoculum and the duration of infection. Two different mechanisms were discovered through which bacteria interfered with LDH activity, i.e., acidification of the cell culture medium (by K. pneumoniae and S. pneumoniae) and protease production (by P. aeruginosa and S. maltophilia). In addition, we developed and validated a modified protocol to evaluate cytotoxicity using the LDH assay, where bacterial interference with LDH quantification is avoided.

Lysophosphatidic acid affinity chromatography reveals pyruvate kinase as a specific LPA-binding protein

Lysophosphatidic acid is a pleiotropic lipid signaling molecule that evokes a broad array of cellular responses including proliferation, tumor cell invasion, neurite retraction, cytoskeletal rearrangements and smooth muscle contraction. Generally, lysophosphatidic acid triggers physiological responses through interaction with specific plasma membrane receptors called LPA 1-4. There is, however, increasing evidence in support of intracellular proteins that interact with LPA. We employed Affigel-immobilized LPA to isolate cytoplasmic proteins that interact with this lysophospholipid. Among the proteins retained by this affinity matrix, pyruvate kinase, clathrin heavy chain and heat shock protein 70 (Hsp70) were identified by mass spectrometry. Isothermal titration calorimetry showed that pyruvate kinase contains one binding site for LPA (Ka approx. 10(6) M(-1)). Furthermore, LPA dissociates enzymatically active pyruvate-kinase tetramers into less active dimers, and is maximally active at concentrations close to its critical micelle concentration. These effects were not mimicked by other lysophospholipids. Co-immunoprecipitation experiments showed that pyruvate kinase interacts with clathrin, and confocal imaging revealed co-localization between clathrin and pyruvate kinase in the perinuclear region of cells. Our data suggest that pyruvate kinase partly exists in complex with clathrin in subcellular membranous areas, and that locally increased LPA levels can trigger inactivation of the metabolic enzyme.

Interaction of bovine heart pyruvate kinase with phospholipids

The interaction between bovine heart pyruvate kinase and liposomes was investigated for various phospholipids as function of pH, and salt concentration using steady-state kinetics and ultracentrifugation. Liposomes made from erythrocyte total lipid fraction and individual phospholipids were used. Pyruvate kinase specific activity increases upon the interaction with the phospholipids. The activation is specifically sensitive to presence of phosphatidylserine in liposomes. L-serine, and phospho-L-serine which are main components of phosphatidylserine head group show also some activation effect. Efficient adsorption of pyruvate kinase to phosphatidylserine liposomes occurs in the pH range 6.0-8.0 and at low ionic strength. Interaction with phosphatidylserine liposomes results in the change of Vmax and Km values for phospho enol pyruvate without marked effect on Km value for ADP, and Hill coefficients for both substrates. The interaction does not seem to influence the cooperativity between binding sites.

Interaction of phospholipids with proteins and peptides. New advances 1990

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.