Isolation and properties of mitochondrial aspartate aminotransferase from red muscle of grey mullet, Mugil auratus Risso

Aglycemic HepG2 Cells Switch From Aminotransferase Glutaminolytic Pathway of Pyruvate Utilization to Complete Krebs Cycle at Hypoxia

Human hepatocellular carcinoma HepG2 cells are forced to oxidative phosphorylation (OXPHOS), when cultured in aglycemic conditions at galactose and glutamine. These Oxphos cells represent a prototype of cancer cell bioenergetics with mixed aerobic glycolysis and OXPHOS. We aimed to determine fractions of (i) glutaminolytic pathway involving aminotransferase reaction supplying 2-oxoglutarate (2OG) to the Krebs cycle vs. (ii) active segment of the Krebs cycle with aconitase and isocitrate dehydrogenase-3 (ACO-IDH3), which is typically inactive in cancer cells due to the citrate export from mitochondria. At normoxia, Oxphos cell respiration was decreased down to ~15 and ~10% by the aminotransferase inhibitor aminooxyacetate (AOA) or with AOA plus the glutamate-dehydrogenase inhibitor bithionol, respectively. Phosphorylating to non-phosphorylating respiration ratios dropped from >6.5 to 1.9 with AOA and to zero with AOA plus bithionol. Thus, normoxic Oxphos HepG2 cells rely predominantly on glutaminolysis. Addition of membrane-permeant dimethyl-2-oxoglutarate (dm2OG) to inhibited cells instantly partially restored respiration, evidencing the lack of 2OG-dehydrogenase substrate upon aminotransferase inhibition. Surprisingly, after 72 hr of 5% O₂ hypoxia, the AOA (bithionol) inhibition ceased and respiration was completely restored. Thus in aglycemic HepG2 cells, the hypoxia-induced factor (HIF) upregulation of glycolytic enzymes enabled acceleration of glycolysis pathway, preceded by galactolysis (Leloir pathway), redirecting pyruvate via still incompletely blocked pyruvate dehydrogenase toward the ACO-IDH3. Glycolytic flux upregulation at hypoxia was evidently matched by a higher activity of the Leloir pathway in Oxphos cells. Hypoxic Oxphos cells increased 2-fold the NADPH oxidase activity, whereas hypoxic glycolytic cells decreased it. Oxphos cells and glycolytic cells at 5 mM glucose decreased their reduced glutathione fraction. In contrast to aglycemic cells, glycolytic HepG2 cells decreased their respiration at hypoxia despite the dm2OG presence, i.e., even at unlimited respiratory substrate availability for 72 hr at 5% O₂, exhibiting the canonical HIF-mediated adaptation. Nevertheless, their ATP content was much higher with dm2OG as compared to its absence during hypoxic adaptation. Thus, the metabolic plasticity of cancer cells is illustrated under conditions frequently established for solid tumors in vivo, such as aglycemia plus hypoxia. Consequently, a wide acceptance of the irreversible and exclusive Warburg phenotype in cancer cells is incorrect.

Interaction between dendrimers and regulatory proteins. Comparison of effects of carbosilane and carbosilane–viologen–phosphorus dendrimers

For nanoparticles to be used successfully in biomedical application, their interactions with biological fluids need to be investigated, in which they will react with proteins and other macromolecules.

Substrate inhibition mode of omega-transaminase from Vibrio fluvialis JS17 is dependent on the chirality of substrate

Substrate inhibition is a common phenomenon in enzyme chemistry, which is observed only with a fast-reacting substrate enantiomer. We report here for the first time substrate inhibition of an enantioselective enzyme by both substrate enantiomers. The enantioselective substrate inhibition, i.e., different mode of inhibition by each substrate enantiomer, of (S)-specific omega-transaminase was found with various chiral amines. A kinetic model based on ping-pong bi-bi mechanism has been developed and kinetic parameters were measured. The kinetic model reveals that the inhibition by (R)-amine results from formation of Michaelis complex with enzyme-pyridoxal 5'-phosphate, whereas the inhibition by (S)-amine results from the formation of the complex with enzyme-pyridoxamine 5'-phosphate. Substrate inhibition constants (K(SI)) of each (S)-enantiomer of four chiral amines showed a linear correlation with those of cognate (R)-amines. Such a correlation was also found between the K(SI) values and Michaelis constants of (S)-amines. These correlations indicate that recognition mechanisms and active site structures of both enzyme-pyridoxal 5'-phosphate, enzyme-pyridoxamine 5'-phosphate are similar. Taken together with the results, high propensity for non-productive substrate binding strongly suggests that binding pockets of the omega-transaminase is loosely defined, which accounts for the enantioselective substrate inhibition.

Selective determination of fish aspartate aminotransferase isoenzymes by their differential sensitivity to proteases

Various proteases (proteinase K, subtilisin, trypsin and chymotrypsin) were used to study the selective inactivation of the aspartate aminotransferase (EC 2.6.1.1) isoenzymes of grey mullet (Mugil auratus Risso; Osteichthyes). The cytosolic isoenzyme was significantly inactivated by proteinase K, subtilisin and chymotrypsin, while the mitochondrial isoenzyme was sensitive only to proteinase K and to high doses of trypsin. Further identification of the aspartate aminotransferase isoenzymes was based on their discrete sensitivity toward chymotrypsin. Chymotrypsin (1 mg/ml) successfully inhibited purified cytosolic aspartate aminotransferase as well as cytosolic isoenzyme from plasma, whereas the mitochondrial form persisted unaffected. Similar results were obtained when examining liver and red muscle homogenates. This method revealed that the increased total activity of aspartate aminotransferase in fish plasma with induced acute liver injury, was partially a result of the mitochondrial isoenzyme leakage from damaged tissue.