Purification and physical properties of hexokinase from human erythrocytes

Insights into the ameliorative effect of oleic acid in rejuvenating phenylhydrazine induced oxidative stress mediated morpho-functionally dismantled erythrocytes

Phenylhydrazine (PHZ), an intermediate in the synthesis of fine chemicals is toxic for human health and environment. Despite of having severe detrimental effects on different physiological systems, exposure of erythrocytes to PHZ cause destruction of haemoglobin and membrane proteins leading to iron release and complete haemolysis of red blood cells (RBC). Involvement of oxidative stress behind such action triggers the urge for searching a potent antioxidant. The benefits of consuming olive oil is attributed to its 75% oleic acid (OA) content in average. Olive oil is the basic component of Mediterranean diet. Hence, OA has been chosen in our present in vitro study to explore its efficacy against PHZ (1 mM) induced alterations in erythrocytes. Four different concentrations of OA (0.01 nM, 0.02 nM, 0.04 nM and 0.06 nM) were primarily experimented with, among which 0.06 nM OA has shown to give maximal protection. This study demonstrates the capability of OA in preserving the morphology, intracellular antioxidant status and the activities of metabolic enzymes of RBCs that have been diminished by PHZ, through its antioxidant mechanisms. The results of the present study firmly establish OA as a promising antioxidant for conserving the health of erythrocyte from PHZ toxicity which indicate toward future possible use of OA either singly or in combination with other dietary components for protection of erythrocytes against PHZ induced toxic cellular changes.

Insight into the mechanism of galactokinase: Role of a critical glutamate residue and helix/coil transitions

Galactokinase, the enzyme which catalyses the first committed step in the Leloir pathway, has attracted interest due to its potential as a biocatalyst and as a possible drug target in the treatment of type I galactosemia. The mechanism of the enzyme is not fully elucidated. Molecular dynamics (MD) simulations of galactokinase with the active site residues Arg-37 and Asp-186 altered predicted that two regions (residues 174-179 and 231-240) had different dynamics as a consequence. Interestingly, the same two regions were also affected by alterations in Arg-105, Glu-174 and Arg-228. These three residues were identified as important in catalysis in previous computational studies on human galactokinase. Alteration of Arg-105 to methionine resulted in a modest reduction in activity with little change in stability. When Arg-228 was changed to methionine, the enzyme's interaction with both ATP and galactose was affected. This variant was significantly less stable than the wild-type protein. Changing Glu-174 to glutamine (but not to aspartate) resulted in no detectable activity and a less stable enzyme. Overall, these combined in silico and in vitro studies demonstrate the importance of a negative charge at position 174 and highlight the critical role of the dynamics in to key regions of the protein. We postulate that these regions may be critical for mediating the enzyme's structure and function.

Tetrameric structure of mitochondrially bound rat brain hexokinase: a crosslinking study

Rat brain hexokinase (ATP:D-hexose-6-phosphotransferase; EC 2.7.1.1) was derivatized with sulfosuccinimidyl-2-(m-azido-o-nitrobenzamido)ethyl-1,3'-dithiopro pionate (SAND), a photosensitive and cleavable crosslinking agent. The catalytic activity and mitochondrial binding properties of the enzyme were only marginally affected by reaction with SAND. When the derivatized enzyme was bound to liver mitochondria, photolysis resulted in extensive formation of a single crosslinked species with estimated molecular mass 460 kDa. This was determined to contain only hexokinase and thus represents a tetramer of the 116 kDa (apparent molecular mass in gel system used) monomeric enzyme. Although small amounts of tetramer were detected after photolysis of relatively high concentrations of derivatized enzyme in free solution, tetramer formation was greatly enhanced when the enzyme was bound to mitochondria. No evidence of dimeric or trimeric structures was seen even when only a small fraction of the available binding sites on the mitochondrial membrane were occupied. It is thus concluded that tetramer formation is closely linked with binding of the enzyme to the outer mitochondrial membrane and, more specifically, to the pore structure through which metabolites traverse this membrane. It is speculated that a tetrameric structure surrounding the mitochondrial pores may facilitate interactions between the hexokinase reaction and oxidative phosphorylation, mediated by the adenine nucleotides which are common intermediates in these reactions.

Hexokinase type I multiplicity in human erythrocytes

Hexokinase I in human erythrocytes exists in multiple molecular forms that differ in isoelectric points. By means of Western blotting and immunodetection of total glucose-phosphorylating activity by using an antibody raised in rabbit against homogeneous human placenta hexokinase I, a single protein band was detected. Identical results were also obtained by immunoaffinity chromatography of the partially purified enzyme. Separation of the three major hexokinase I subtypes (Ia, Ib and Ic) by h.p.l.c. ion-exchange chromatography and immunodetection following electrophoretic blotting confirmed that each hexokinase subtype showed the same apparent Mr of 112,000, which is the value obtained for the high-Mr hexokinase I from human placenta. Purification of erythrocyte hexokinase by a combination of several procedures including dye-ligand and affinity chromatography that were previously successfully applied to the purification of other mammalian hexokinases type I produced a 35,000-fold-purified enzyme that showed several contaminants after SDS/polyacrylamide-gel electrophoresis. Only one of these peptides was found to be recognized by anti-(hexokinase I) IgG, suggesting that proteolytic degradation does not occur and that hexokinases Ia, Ib and Ic have the same apparent Mr.

Purification, properties, and evidence for two subtypes of human placenta hexokinase type I

In human placenta 85% of total hexokinase activity (EC 2.7.1.1) was found in a soluble form. Of this, 70% is hexokinase type I while the remaining 30% is hexokinase type II. All the bound hexokinase is type I. Soluble hexokinase I was purified 11,000-fold by a combination of ion-exchange chromatography, affinity chromatography, and dye-ligand chromatography. The specific activity was 190 units/mg protein with a 75% yield. The enzyme shows only one band in nondenaturing polyacrylamide gel electrophoresis that stains for protein and enzymatic activity; however, two components (with Mr 112,000 and 103,000) were constantly seen in sodium dodecyl sulfate-gel electrophoresis. Many attempts were made to separate these two proteins under native conditions; however, only one peak of activity was obtained when the enzyme was submitted to gel filtration (Mr 118,000), preparative isoelectric focusing (pI 5.9), anion-exchange chromatography, hydroxylapatite chromatography, and affinity chromatography on immobilized dyes and immobilized glucosamine. The high and low molecular weight hexokinases show the same isoelectric point under denaturing conditions as determined by two-dimensional gel electrophoresis. Each hexokinase subtype was obtained by preparative sodium dodecyl sulfate electrophoresis followed by electroelution. Monospecific antibodies raised in rabbits against electroeluted high and low molecular weight hexokinases were not able to recognize the native enzymes but each of them detected both hexokinases on immunoblots. Amino acid compositions and peptide mapping by limited proteolysis of the high and low molecular weight hexokinases were also performed and suggested a strong homology between these two subtypes of human hexokinase I.