Investigation of the interaction of pig muscle lactate dehydrogenase with acidic phospholipids at low pH

New Era of Diacylglycerol Kinase, Phosphatidic Acid and Phosphatidic Acid-Binding Protein

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol (DG) to generate phosphatidic acid (PA). Mammalian DGK consists of ten isozymes (α–κ) and governs a wide range of physiological and pathological events, including immune responses, neuronal networking, bipolar disorder, obsessive-compulsive disorder, fragile X syndrome, cancer, and type 2 diabetes. DG and PA comprise diverse molecular species that have different acyl chains at the sn-1 and sn-2 positions. Because the DGK activity is essential for phosphatidylinositol turnover, which exclusively produces 1-stearoyl-2-arachidonoyl-DG, it has been generally thought that all DGK isozymes utilize the DG species derived from the turnover. However, it was recently revealed that DGK isozymes, except for DGKε, phosphorylate diverse DG species, which are not derived from phosphatidylinositol turnover. In addition, various PA-binding proteins (PABPs), which have different selectivities for PA species, were recently found. These results suggest that DGK–PA–PABP axes can potentially construct a large and complex signaling network and play physiologically and pathologically important roles in addition to DGK-dependent attenuation of DG–DG-binding protein axes. For example, 1-stearoyl-2-docosahexaenoyl-PA produced by DGKδ interacts with and activates Praja-1, the E3 ubiquitin ligase acting on the serotonin transporter, which is a target of drugs for obsessive-compulsive and major depressive disorders, in the brain. This article reviews recent research progress on PA species produced by DGK isozymes, the selective binding of PABPs to PA species and a phosphatidylinositol turnover-independent DG supply pathway.

Polyunsaturated fatty acid-containing phosphatidic acids selectively interact with L-lactate dehydrogenase A and induce its secondary structural change and inactivation

Phosphatidic acid (PA) consists of various molecular species that have different fatty acyl chains at the sn-1 and sn-2 positions; and consequently, mammalian cells contain at least 50 structurally distinct PA molecular species. However, the different roles of each PA species are poorly understood. In the present study, we attempted to identify dipalmitoyl (16:0/16:0)-PA-binding proteins from mouse skeletal muscle using liposome precipitation and tandem mass spectrometry analysis. We identified L-lactate dehydrogenase (LDH) A, which catalyzes conversion of pyruvate to lactate and is a key checkpoint of anaerobic glycolysis critical for tumor growth, as a 16:0/16:0-PA-binding protein. LDHA did not substantially associate with other phospholipids, such as phosphatidylcholine, phosphatidylserine, phosphatidylglycerol, phosphatidylinositol, phosphoinositides and cardiolipin at physiological pH (7.4), indicating that LDHA specifically bound to PA. Interestingly, 18:0/18:0-, 18:0/20:4- and 18:0/22:6-PA also interacted with LDHA, and their binding activities were stronger than 16:0/16:0-PA at pH 7.4. Moreover, circular dichroism spectrometry showed that 18:0/20:4- and 18:0/22:6-PA, but not 16:0/16:0- or 18:0/18:0-PA, significantly reduced the α-helical structure of LDHA. Furthermore, 18:0/20:4- and 18:0/22:6-PA attenuated LDH activity. Taken together, we demonstrated for the first time that LDHA is a PA-binding protein and is a unique PA-binding protein that is structurally and functionally controlled by associating with 18:0/20:4- and 18:0/22:6-PA.

Lactate Dehydrogenase C Produces S-2-Hydroxyglutarate in Mouse Testis

Metabolomics is a valuable tool for studying tissue- and organism-specific metabolism. In normal mouse testis, we found 70 μM S-2-hydroxyglutarate (2HG), more than 10-fold greater than in other tissues. S-2HG is a competitive inhibitor of α-ketoglutarate-dependent demethylation enzymes and can alter histone or DNA methylation. To identify the source of testis S-2HG, we fractionated testis extracts and identified the fractions that actively produced S-2HG. Through a combination of ion exchange and size exclusion chromatography, we enriched a single active protein, the lactate dehydrogenase isozyme LDHC, which is primarily expressed in testis. At neutral pH, recombinant mouse LDHC rapidly converted both pyruvate into lactate and α-ketoglutarate into S-2HG, whereas recombinant human LDHC only produced lactate. Rapid S-2HG production by LDHC depends on amino acids 100-102 being Met-Val-Ser, a sequence that occurs only in the rodent protein. Other mammalian LDH can also produce some S-2HG, but at acidic pH. Thus, polymorphisms in the Ldhc gene control testis levels of S-2HG, and thereby epigenetics, across mammals.

Phenolic lipids affect the activity and conformation of acetylcholinesterase from Electrophorus electricus (Electric eel)

Phenolic lipids were isolated from rye grains, cashew nutshell liquid (CNSL) from Anacardium occidentale, and fruit bodies of Merrulius tremellosus, and their effects on the electric eel acetylcholinesterase activity and conformation were studied. The observed effect distinctly depended on the chemical structure of the phenolic lipids that were available for interaction with the enzyme. All of the tested compounds reduced the activity of acetylcholinesterase. The degree of inhibition varied, showing a correlation with changes in the conformation of the enzyme tested by the intrinsic fluorescence of the Trp residues of the protein.

Ultracentrifugation studies of the location of the site involved in the interaction of pig heart lactate dehydrogenase with acidic phospholipids at low pH. A comparison with the muscle form of the enzyme

Lactate dehydrogenase (LDH) from the pig heart interacts with liposomes made of acidic phospholipids most effectively at low pH, close to the isoelectric point of the protein (pH = 5.5). This binding is not observed at neutral pH or high ionic strength. LDH-liposome complex formation requires an absence of nicotinamide adenine dinucleotides and adenine nucleotides in the interaction environment. Their presence limits the interaction of LDH with liposomes in a concentration-dependent manner. This phenomenon is not observed for pig skeletal muscle LDH. The heart LDH-liposome complexes formed in the absence of nicotinamide adenine dinucleotides and adenine nucleotides are stable after the addition of these substances even in millimolar concentrations. The LDH substrates and studied nucleotides that inhibit the interaction of pig heart LDH with acidic liposomes can be ordered according to their effectiveness as follows: NADH > NAD > ATP = ADP > AMP > pyruvate. The phosphorylated form of NAD (NADP), nonadenine nucleotides (GTP, CTP, UTP) and lactate are ineffective. Chemically cross-linked pig heart LDH, with a tetrameric structure stable at low pH, behaves analogously to the unmodified enzyme, which excludes the participation of the interfacing parts of subunits in the interaction with acidic phospholipids. The presented results indicate that in lowered pH conditions, the NADH-cofactor binding site of pig heart LDH is strongly involved in the interaction of the enzyme with acidic phospholipids. The contribution of the ATP/ADP binding site to this process can also be considered. In the case of pig skeletal muscle LDH, neither the cofactor binding site nor the subunit interfacing areas seem to be involved in the interaction.