Further characterization of contact sites from mitochondria of different tissues: topology of peripheral kinases

A membrane fraction of intermediate density between inner and outer membrane was isolated by density gradient centrifugation from osmotically disrupted mitochondria of rat liver, brain, and kidney. The fraction was hexokinase rich and could therefore be further purified using specific antibodies against hexokinase and immunogold labelling techniques. In agreement with recent findings the gradient fraction which cosedimented with hexokinase contained the boundary membrane contact sites because it was composed of outer and inner membrane components and beside hexokinase, was enriched also by activity of creatine kinase and nucleoside diphosphate kinase. In contrast the activity of adenylate kinase appeared to be concentrated beyond the contact sites in the outer membrane fraction. By employing surface proteolysis analysis and specific blockers of the outer membrane pore we observed that the location of the kinases relative to the membrane components in the contact fraction resembled that of intact mitochondria. This specific organization of some peripheral kinases in the contact sites suggested an important role of the voltage dependence of the outer membrane pore, in that the pore may become limiting in anion exchange because of influence of the inner membrane potential on the closely attached outer membrane. Such control of anion exchange would lead to a dynamic compartmentation at the mitochondrial surface by the formation of contact sites, which may explain the preferential utilization of cytosolic creatine by the mitochondrial creatine kinase, as postulated in the phosphocreatine shuttle.

Mitochondrial boundary membrane contact sites in brain: points of hexokinase and creatine kinase location, and control of Ca2+ transport

The location of hexokinase at the surface of brain mitochondria was investigated by electron microscopy using immuno-gold labelling techniques. The enzyme was located where the two mitochondrial limiting membranes were opposed and contact sites were possible. Disruption of the outer membrane by digitonin did not remove bound hexokinase and creatine kinase from brain mitochondria, although the activity of outer membrane markers and adenylate kinase decreased, suggesting a preferential location of both enzymes in the contact sites. In agreement with that, a membrane fraction was isolated from osmotically lysed rat brain mitochondria in which hexokinase and creatine kinase were concentrated. The density of this kinase-rich fraction was specifically increased by immuno-gold labelling of hexokinase, allowing a further purification by density gradient centrifugation. The fraction was composed of inner and outer limiting membrane components as shown by the specific marker enzymes, succinate dehydrogenase and NADH-cytochrome-c-oxidase (rotenone insensitive). As reported earlier for the enriched contact site fraction of liver mitochondria the fraction from brain mitochondria contained a high activity of glutathione transferase and a low cholesterol concentration. Moreover, the contacts showed a higher Ca2+ binding capacity in comparison to outer and inner membrane fractions. This finding may have regulatory implications because glucose phosphorylation via hexokinase activated the active Ca2+ uptake system and inhibited the passive efflux, resulting in an increase of intramitochondrial Ca2+.

Metabolic and Innate Immune Cues Merge into a Specific Inflammatory Response via the UPR

Innate immune responses are intricately linked with intracellular metabolism of myeloid cells. Toll-like receptor (TLR) stimulation shifts intracellular metabolism toward glycolysis, while anti-inflammatory signals depend on enhanced mitochondrial respiration. How exogenous metabolic signals affect the immune response is unknown. We demonstrate that TLR-dependent responses of dendritic cells (DCs) are exacerbated by a high-fatty-acid (FA) metabolic environment. FAs suppress the TLR-induced hexokinase activity and perturb tricarboxylic acid cycle metabolism. These metabolic changes enhance mitochondrial reactive oxygen species (mtROS) production and, in turn, the unfolded protein response (UPR), leading to a distinct transcriptomic signature with IL-23 as hallmark. Interestingly, chemical or genetic suppression of glycolysis was sufficient to induce this specific immune response. Conversely, reducing mtROS production or DC-specific deficiency in XBP1 attenuated IL-23 expression and skin inflammation in an IL-23-dependent model of psoriasis. Thus, fine-tuning of innate immunity depends on optimization of metabolic demands and minimization of mtROS-induced UPR.

Immunolocalization of glyceraldehyde-3-phosphate dehydrogenase, hexokinase, and carboxypeptidase Y in yeast cells at the ultrastructural level

We have developed a simple and effective method to embed whole yeast cells in Lowicryl resins with excellent ultrastructural and antigenic preservation. Using affinity-purified antibodies eluted from electrophoretically separated proteins transferred to nitrocellulose, we have shown by immunoelectron microscopy that two glycolytic enzymes, glyceraldehyde-3-phosphate dehydrogenase and hexokinase, are present in the cytoplasm and the nucleus. Carboxypeptidase Y is localized in the yeast vacuole. These results agree with earlier localization studies based on subcellular fractionation.

Mouse spermatogenic cell-specific type 1 hexokinase (mHk1-s) transcripts are expressed by alternative splicing from the mHk1 gene and the HK1-S protein is localized mainly in the sperm tail

Unique type 1 hexokinase (HK1) mRNAs are present in mouse spermatogenic cells (mHk1-s). They encode a spermatogenic cell-specific sequence region (SSR) but not the porin-binding domain (PBD) necessary for HK1 binding to porin on the outer mitochondrial membrane. This study determined the origin of the multiple Hk1-s transcripts in mouse spermatogenic cells and verified that they are translated in mouse spermatogenic cells. It also showed that a single mHk1 gene encodes the mHk1 transcripts of somatic cells and the mHk1-sa and mHk1-sb transcripts of spermatogenic cells, that alternative exons are used during mHk1 gene expression in mouse spermatogenic cells, and that mHK1-S is translated in mouse spermatogenic cells and is localized mainly with the fibrous sheath in the tail region, not with the mitochondria in the midpiece of mouse sperm.

Changes in free and bound forms and total amount of hexokinase isozyme II of rat muscle in response to contractile activity

Increased contractile activity as induced by chronic low-frequency stimulation evoked in rat fast-twitch muscle an almost immediate increase in the ratio between structure-bound and free hexokinase. In addition, an up to 14-fold rise in total hexokinase activity occurred after two weeks of stimulation indicating that glucose phosphorylation became a limiting step of glucose utilization under these conditions. The increase in hexokinase activity was transitory as prolonged stimulation led to a leveling off and steep decline with an apparent half-life of 2.5 days after three weeks of stimulation. The transient increase in glucose phosphorylating capacity can be explained by previous observations indicating that prolonged stimulation leads to a shift from a carbohydrate-based to a fatty-acid-based energy metabolism. Using an isozyme-specific sandwich ELISA, it was shown that both increases and decreases in total hexokinase activity were matched by corresponding changes in the amount of hexokinase isozyme II protein. Increases in both total hexokinase activity (3-4-fold) and hexokinase II protein content were also observed after denervation in rat fast-twitch muscle. In view of reports in the literature, it is suggested that the elevations in hexokinase II observed with increased contractile activity and denervation relate to enhanced glucose uptake and utilization.

Molecular analysis of Plasmodium falciparum hexokinase

Hexokinase, a key glycolytic enzyme, is involved in the initial phosphorylation reaction of imported glucose and specific blocking of this activity may therefore arrest the development of malaria parasites. We describe here the cloning of a single copy hexokinase gene of Plasmodium falciparum (PfHK) from cDNA or genomic DNA libraries. The deduced amino acid sequence of PfHK has 26% identity with human hexokinase I and its predicted molecular mass assigns it as an invertebrate type isoenzyme of hexokinase. A single 1.5-kb exon is translated from a 3-kb mRNA in asexual stages of the parasite. In contrast to aldolase and GPI, the gene for this glycolytic enzyme is located on chromosome 8. Poly- and monoclonal antibodies against recombinant PfHK support our cloning results at the protein level as they detect a protein of the predicted size and isoelectric point by Western blotting in parasite protein samples. Moreover, polyclonal rabbit IgG against recombinant PfHK partially inhibits the hexokinase activity of a P. falciparum lysate which provides direct proof that the gene cloned encodes hexokinase of the parasite.

Membrane potential-dependent conformational changes in mitochondrially bound hexokinase of brain

Previously characterized monoclonal antibodies (Mabs) were used in a study of Type I hexokinase from rat brain. Based on the relative reactivity of these Mabs with soluble and mitochondrially bound forms, binding to mitochondria was shown to affect specific epitopic regions in both N- and C-terminal halves of the enzyme and to modulate conformational changes induced by binding of the ligands, Glc or ATP. Reactivities with Mabs recognizing epitopes in two defined regions of the N-terminal half and one defined region of the C-terminal half of the mitochondrially bound enzyme were selectively affected by mitochondrial membrane potential, or by addition of oligomycin, carboxyatractyloside, or bongkrekic acid. The Glc-6-P analog, 1 ,5-anhydroglucitol-6-P, was much more effective as a competitive inhibitor against extramitochondrial ATP than against intramitochondrial ATP generated by oxidative phosphorylation. These results provide further insight into the role of hexokinase-mitochondrial interactions in regulation of cerebral glucose metabolism.

Properties of human erythrocyte hexokinase related to cell age

The kinetic, electrophoretic and immunological properties of hexokinase from human erythrocytes have been studied in relation to cell age. No differences in kinetic behaviour between hexokinase partly purified from reticulocytes, 10% youngest cells, normal red cell population or from 10% oldest cells were observed. The stability of the enzyme preparations showed little differences; hexokinase from the 10% youngest cells was the most labile enzyme, followed respectively by the enzyme from reticulocytes, normal red cell population and the 10% oldest cells. The electrophoretic pattern of erythrocyte hexokinase changed during senescence. The hexokinase activity located in the second band from the anode is shifted to the third with increasing cell age. The molecular specific acitivity of the enzyme from the 10% youngest cells, the normal red cell population and the 10% oldest cells remains the same, while the molecular specific activity of hexokinase from reticulocytes was much lower.

Disposition of mitochondrially bound hexokinase at the membrane surface, deduced from reactivity with monoclonal antibodies recognizing epitopes of defined location

A panel of monoclonal antibodies against rat brain hexokinase (ATP: D-hexose 6-phosphotransferase, EC 2.7.1.1) has been employed to investigate the orientation of the mitochondrially bound enzyme on the mitochondrial surface. Based on their ability to immunoprecipitate truncated forms of the protein, obtained by in vitro translation of truncated versions of the mRNA, the epitopes for seven monoclonal antibodies were mapped to regions consisting of 20-50 amino acid residues within the sequence of the N-terminal half of the enzyme. There is extensive sequence similarity between the N- and C-terminal halves of this enzyme, which is thought to have evolved by a process of gene duplication and fusion. However, these antibodies react selectively with epitopes in the N-terminal half, and thus epitopic regions for several of these antibodies could be further defined by eliminating from consideration regions showing substantial sequence similarity with the C-terminal half. The epitope for one of the monoclonal antibodies, designated 4D4, was shown to involve the extreme N-terminus of the enzyme; selective proteolytic modification of this region resulted in loss of immunoreactivity. Relative location of epitopes for three other antibodies, designated 2B, 1C5, and 4C5, within a 20-residue segment was deduced from effects of modifying sulfhydryl residues within this segment on immunoreactivity. Thus, by a combination of sequence analysis and experimental methods, the epitopes for these seven antibodies could be localized to defined regions within the overall sequence. The ability of these antibodies to prevent binding of hexokinase to mitochondria, and their ability to recognize the mitochondrially bound enzyme, provided a basis for assessing the relative proximity of the corresponding epitopes to the mitochondrial surface when the enzyme was bound. The disposition of the bound enzyme on the mitochondrial surface was deduced by relating these results to the proposed structure for brain hexokinase.

Functional consequences of mutation of highly conserved serine residues, found at equivalent positions in the N- and C-terminal domains of mammalian hexokinases

Despite the extensive sequence similarity between the N- and C-terminal halves of the 100-kDa molecular weight mammalian hexokinases (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1), reflecting their evolutionary origin by duplication and fusion of a gene coding for a smaller ancestral hexokinase, there is evidence for a functional division, with the C-terminal domain retaining a catalytic role while the N-terminal domain serves a regulatory function [binding of the product inhibitor, glucose 6-phosphate) (Glc-G-P)]. Conversion of Ser603 to Ala in the C-terminal domain of rat Type I hexokinase, expressed in COS-1 cells, resulted in drastic reduction of catalytic activity; Ser603 is analogous to Ser158, a residue of critical catalytic importance in the homologous yeast hexokinase. In contrast, conversion of Ser155 to Ala in the N-terminal domain (analogous to Ser603 in the C-terminal domain) of rat Type I hexokinase had no effect on catalytic activity or on inhibition of the enzyme by the Glc-6-P analog, 1,5-anhydroglucitol-6-P. Immunoreactivity with monoclonal antibodies recognizing conformationally sensitive epitopes was not affected, indicating that neither mutation resulted in gross structural perturbation. These results are consistent with the assignment of catalytic function, involving Ser603, to the C-terminal domain, and demonstrate that the analogous Ser155 is not critical for either catalytic or regulatory function. The Type I isozyme, expressed in COS-1 cells, retained the ability to bind to mitochondria in a Glc-6-P-sensitive manner, as previously found with the enzyme isolated from mammalian tissues.

Hexokinase isoenzymes from the Novikoff hepatoma. Purification, kinetic and structural characterization, with emphasis on hexokinase C

The purification to homogeneity of hexokinases B and C from the cytosol of rat Novikoff hepatoma was achieved by a protocol using an initial chromatography on Blue 2-agarose to separate the isoenzymes from each other. After that step each hexokinase was subjected to chromatography on DEAE-cellulose, hydroxyapatite and Sephacryl S-300, followed by re-chromatography on hydroxyapatite. The final preparations of hexokinases B and C had specific activities of 86 and 23.5 units/mg of protein respectively, and gave single bands on electrophoresis under non-denaturing conditions or in SDS/polyacrylamide gels. Mr values of about 100,000 were found for both isoenzymes either by Sephacryl S-300 chromatography or by SDS/polyacrylamide-gel electrophoresis. Values of apparent Km for glucose and ATP of pure hexokinase B were similar to those reported for the enzyme from other sources. The apparent Km value for glucose of hexokinase C was 0.025 mM. Marked inhibition of hexokinase C by glucose concentrations above 0.2 mM was found. The effect was partially relieved by ATP concentrations above 1 mM and was independent of pH. Glucose 6-phosphate was inhibitory, but the Ki value (0.18 mM) is higher than those reported for other animal hexokinases. The amino acid composition of hexokinase C was found to be similar to those reported for hexokinases B and D. Also, an immune serum directed against hexokinase A was able, at low dilutions, to bind hexokinases B and C. An immune serum directed against hexokinase C was able, at low dilutions, to bind hexokinase B and also, but weakly, hexokinase A.

Antigenic cross-reactivities between mammalian hexokinases

Immunoelectrophoresis and gel diffusion techniques have demonstrated that, contrary to previous reports, antigenic cross-reactivity does exist between the three major rat hexokinase isoenzymes. Immune inhibition and immune precipitation assays performed in parallel generally failed to detect this cross-reactivity and possible reasons for this failure are discussed. The inability of the latter assay systems to detect cross-reactivity would seem to account for the discrepancies between the present results and those obtained in earlier studies.

A gel electrophoresis method for epitope mapping studies with monoclonal antibodies

The linear polyacrylamide gradient gel electrophoresis method of Lambin and Fine (1979, Anal. Biochem. 98, 160-168) has been adapted for estimation of the molecular weights of antigen-antibody complexes, thereby providing information useful in epitope mapping studies with monoclonal antibodies. The method has been applied to mapping of the epitopes recognized by four different monoclonal antibodies raised against rat brain hexokinase (1984, Finney et al., J. Biol. Chem., 259, 8232-8237). The results obtained with the gel electrophoresis method were in agreement with epitope mapping studies conducted by the molecular sieve high-performance liquid chromatographic method of Crawford et al., (1982, Proc. Natl. Acad. Sci. USA 79, 7031-7035). Epitope mapping by the gel electrophoretic method offers several advantages in terms of speed, convenience, and economy when compared with alternative procedures that have been described.

LL95 monoclonal antibody mimics functional effects of ZP3 on mouse sperm: evidence that the antigen recognized is not hexokinase

In the mouse, a 95 kD sperm protein has been identified as a putative receptor for the zona pellucida glycoprotein ZP3. The 95 kD sperm protein is a tyrosine kinase substrate, with phosphorylation on tyrosine stimulated upon zona pellucida binding. The latter finding is observed not only in live cells but also in isolated sperm membranes and in an electroeluted 95 kD protein. Stimulation of 95 kD protein tyrosine phosphorylation by zona pellucida is completely abolished by tyrosine kinase inhibitors, which effectively inhibit the sperm acrosome reaction. Since receptor oligomerization by ZP3 is essential for acrosome reaction triggering, we hypothesized that application of an external crosslinking agent will lead to the acrosome reaction, even in the absence of natural ligand ZP3. Here, we report the generation of a mouse monoclonal antibody (mAb) raised against the 95 kD protein. This antibody, termed LL95, mimics the bioactivities of ZP3 in inhibiting sperm-zona binding and inducing the acrosome reaction. The latter depends on receptor oligomerization. Immunolocalization revealed that the LL95 antigen is restricted to the head surface in the acrosomal region of live sperm. Thus, LL95 fulfills several criteria predicted for an antibody that recognizes a sperm receptor for the zona pellucida. Recently, it was reported that the amino acid sequence of the 95 kD protein we described corresponds to a mouse hepatoma hexokinase (Kalab et al., 1994: J Biol Chem 269:3810-3817). Although both hexokinase and LL95 antigen migrate at 95 kD in nonreducing gels, we show here that LL95 does not recognize hexokinase. Identification of different proteins is clear where hexokinase is a 116 kD protein and LL95 recognizes sperm proteins of 110 and 130 kD. Moreover, mAb anti-phosphotyrosine immunoprecipitates LL95 antigen under conditions where hexokinase is absent. Use of anti-hexokinase antibodies in gamete interaction assays failed to demonstrate any effect on either sperm-zona binding or acrosome reaction triggering. Finally, antihexokinase antibodies bind to a sperm tail antigen, thus direct involvement of hexokinase in gamete interaction seems improbable.

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.

The hexokinase isoenzyme PII of Saccharomyces cerevisiae ia a protein kinase

The HXK2 gene product has an important role in controlling carbon catabolite repression in Saccharomyces cerevisiae. We have raised specific antibodies against the hexokinase PII protein and have demonstrated that it is a 58 kDa phosphoprotein with protein kinase activity. The predicted amino acid sequence of the HXK2 gene product has significant homology to the conserved catalytic domain of mammalian and yeast protein kinases. Protein kinase activity was located in a different domain of the protein from the hexose-phosphorylating activity. The hexokinase PII protein level remained unchanged in P2T22D mutant cells (hxk1 HXK2 glk1) growing in a complex medium with glucose. The protein kinase activity of hexokinase PII is regulated by the glucose concentration of the culture medium. Exit from the carbon catabolite repression phase and entry into derepression phase may be controlled, in part, by modulation of the 58 kDa protein kinase activity by changes in cyclic AMP concentration.

Hexokinase shift to mitochondria is associated with an increased sensitivity to glucose in rat pancreatic islets

When rat pancreatic islets are incubated in 5.5 or 16.7 mmol/l glucose for 3 h, an increased sensitivity is observed in islets pre-exposed to high glucose, as indicated by a shift to the left of the glucose dose-response curve (EC50 7.1 +/- 0.9 and 11.5 +/- 1.2 in high- and low-glucose-exposed islets, respectively; n = 5, P < 0.05). To investigate the mechanism(s) responsible for this effect, we measured hexokinase and glucokinase activity both in the cytosolic fraction and in a mitochondrion-enriched fraction, since binding to the outer mitochondrial membrane has been reported to result in an increased enzyme activity. In islets cultured at 16.7 mmol/l glucose, the cytosolic hexokinase activity was similar to control islets, but mitochondrial enzyme activity was significantly increased (124 +/- 7 vs. 51 +/- 9 nmol x microg(-1) x 90 min(-1), P < 0.01). As a consequence, the cytosolic:mitochondrial fraction ratio was altered in comparison with control islets. In contrast, glucokinase activity in the two groups of islets was similar in the cytosolic fraction and undetectable in the mitochondrial fraction. Hexokinase I quantitation by Western blot confirmed the enzyme translocation from the free cytosolic to the mitochondria-bound form in islets cultured at 16.7 mmol/l glucose. Glucose-induced alterations were reversible after 1 h exposure to 5.5 mmol/l glucose. Moreover, in islets exposed to 16.7 mmol/l glucose, inhibition of hexokinase binding to mitochondria by the addition of 20 nmol/l dicyclohexylcarbodiimide resulted in no increase of glucose sensitivity (EC50 10.9 +/- 0.4, n = 3, similar to that of control islets). These data indicate that after chronic exposure to high glucose, the beta-cell becomes more sensitive to glucose before eventually getting desensitized. This increased sensitivity is associated with (and may be due to) an increased hexokinase activity secondary to a subcellular shift of the enzyme from the free cytosolic to the mitochondria-bound, more active form.

Novel Anti-Hexokinase 1 Antibodies Are Associated With Poor Prognosis in Patients With Primary Biliary Cholangitis

INTRODUCTION

Antibodies to hexokinase 1 (HK1) and kelch-like 12 (KLHL12) have been identified as potential biomarkers in primary biliary cholangitis (PBC), and this study assesses changes of these antibodies over time and if they are associated with clinical outcomes.

METHODS

Two hundred fifty-four PBC patients (93.3% female, 51 ± 12.3 years old) were tested for anti-HK1 and anti-KLHL12, antimitochondrial (AMA), anti-gp210, and anti-sp100 antibodies. One hundred sixty-nine patients were tested twice and 49 three times within 4.2 (0.8-10.0) years. Biochemistry and clinical features at diagnosis, response to therapy, events of decompensation, and liver-related death or transplantation were evaluated.

RESULTS

Anti-HK1 and anti-KLHL2 were detected in 46.1% and 22.8% patients, respectively. AMA were positive in 93.7%, anti-sp100 in 26.4%, and anti-gp210 in 21.3% of patients. Anti-HK1 and anti-KLHL12 positivity changed over time in 13.3% and 5.5% of patients, respectively. Anti-HK1 or anti-KLHL12 were present in 37.5% of AMA-negative patients, and in 40% of AMA, anti-gp210, and anti-sp100 negative. No significant differences were observed between those with or without HK1 and KLHL12 antibodies, but transplant-free survival and time to liver decompensation were significantly lower in patients anti-HK1 positive (P = 0.039; P = 0.04) and in those anti-sp100 positive (P = 0.01; P = 0.007). No changes in survival and events of liver decompensation were observed according to the positivity of AMA, anti-KLHL12, or anti-gp210 antibodies.

DISCUSSION

HK1 and KLHL12 antibodies are present in 40% of PBC patients who are seronegative by the conventional PBC-specific antibodies. The novel antibodies remain rather steady during the course of the disease, and HK1 antibodies are associated with unfavourable outcomes.

Discrimination of antigenic site and thiol-inhibitor-sensitive site of hexokinase isoenzymes

Rabbit antiserum was prepared against hexokinase isoenzyme type I which was purified from rat brain mitochondria. The antiserum inhibited the activity of the mitochondrial hexokinase type I as well as that of the cytosolic type I enzyme prepared from rat brain, kidney and spleen. It did not, however, inhibit the activity of type II hexokinase from muscle and spleen or that of the type III enzyme from spleen. The results suggest that all hexokinase type I isoenzymes may have a common antigenic site irrespective of their sources, though their responses to a thiol inhibitor are different.

Role of hexokinase in the regulation of erythrocyte hexose monophosphate pathway under oxidative stress

Human erythrocytes overloaded with homogeneous human hexokinase (up to 15-times the activity of normal RBC) show almost unmodified rates of glucose metabolized in the HMP, however hexokinase-loaded RBC are able to metabolize 1.5 fold more glucose than controls through the HMP when an oxidizing agent like methylene blue (5 to 100 microM) is present. Similarly, RBC loaded with inactivating anti-hexokinase IgG (12 +/- 3% residual hexokinase activity) show HMP rates unchanged under resting conditions, but only 12% of the HMP rate found in normal controls under oxidative stress. These data provide clear evidence that the HMP rate under conditions of oxidative stress is controlled by hexokinase activity and suggest that RBC from patients with hexokinase deficiency are not able to increase the HMP rate under oxidative stress like erythrocytes from individuals with G6PD deficiency.

Schistosoma mansoni hexokinase: cDNA cloning and immunogenicity studies

DNA encoding a Schistosoma mansoni hexokinase (SHEX) was amplified from cDNA by the polymerase chain reaction using opposing oligonucleotide primers designed to hybridize with two short segments of hexokinase coding sequences that are well-conserved through evolution. The resulting DNA fragment was then used as a probe to identify a full-length hexokinase cDNA clone. SHEX cDNA encodes a 50-kDa protein that is approximately 46% homologous to rat hexokinase, 40% to rat glucokinase, and 34% to yeast hexokinase A. SHEX coding DNA was expressed within Escherichia coli cells and the 50-kDa recombinant product (rSHEX) was partially purified. Mice repeatedly immunized with rSHEX produced antibodies which recognize rSHEX but this offered no significant protection against subsequent cercarial challenge. On Western blots, rSHEX is weakly recognized by antisera against rat brain hexokinase but not by sera from three strains of mice experimentally infected with S. mansoni parasites or from numerous human schistosomiasis patients. Thus, unlike other reported S. mansoni glycolytic enzymes, hexokinase appears to be poorly immunogenic during schistosome infection and of limited potential as a vaccine candidate.

IRF5 regulates airway macrophage metabolic responses

Interferon regulatory factor 5 (IRF5) is a master regulator of macrophage phenotype and a key transcription factor involved in expression of proinflammatory cytokine responses to microbial and viral infection. Here, we show that IRF5 controls cellular and metabolic responses. By integrating ChIP sequencing (ChIP-Seq) and assay for transposase-accessible chromatin using sequencing (ATAC)-seq data sets, we found that IRF5 directly regulates metabolic genes such as hexokinase-2 (Hk2). The interaction of IRF5 and metabolic genes had a functional consequence, as Irf5-/- airway macrophages but not bone marrow-derived macrophages (BMDMs) were characterized by a quiescent metabolic phenotype at baseline and had reduced ability to utilize oxidative phosphorylation after Toll-like receptor (TLR)-3 activation, in comparison to controls, ex vivo. In a murine model of influenza infection, IRF5 deficiency had no effect on viral load in comparison to wild-type controls but controlled metabolic responses to viral infection, as IRF5 deficiency led to reduced expression of Sirt6 and Hk2. Together, our data indicate that IRF5 is a key component of AM metabolic responses following influenza infection and TLR-3 activation.

Hexokinase II may be dispensable for CD4 T cell responses against a virus infection

Activation of CD4 T cells leads to their metabolic reprogramming which includes enhanced glycolysis, catalyzed through hexokinase enzymes. Studies in some systems indicate that the HK2 isoform is the most up regulated isoform in activated T cells and in this report the relevance of this finding is evaluated in an infectious disease model. Genetic ablation of HK2 was achieved in only T cells and the outcome was evaluated by measures of T cell function. Our results show that CD4 T cells from both HK2 depleted and WT animals displayed similar responses to in vitro stimulation and yielded similar levels of Th1, Treg or Th17 subsets when differentiated in vitro. A modest increase in the levels of proliferation was observed in CD4 T cells lacking HK2. Deletion of HK2 led to enhanced levels of HK1 indicative of a compensatory mechanism. Finally, CD4 T cell mediated immuno-inflammatory responses to a virus infection were similar between WT and HK2 KO animals. The observations that the expression of HK2 appears non-essential for CD4 T cell responses against virus infections is of interest since it suggests that targeting HK2 for cancer therapy may not have untoward effects on CD4 T cell mediated immune response against virus infections.

Epitopic regions recognized by monoclonal antibodies against rat brain hexokinase: Association with catalytic and regulatory function

Using direct and competitive epitope mapping methods, 23 monoclonal antibodies (Mabs) against rat brain hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) were divided into nine groups, each recognizing epitopes within defined surface regions of the N- or C-terminal domains; the latter have been associated with regulatory or catalytic functions, respectively. Reactivity of Mabs with the isolated domains was also studied. Based on the effect of various ligands on immunoreactivity, specific regions involved in ligand-induced conformational changes were identified. Adjacent epitopic regions, designated Regions F and G and located in the N- and C-terminal domains, respectively, were selectively affected by inhibitory hexose 6-phosphates (or analogs), marking these regions as being involved in transmission of the conformational signal from the regulatory N-terminal domain to the catalytic C-terminal domain. Consistent with this, the Ki for inhibition of the enzyme by the glucose 6-phosphate analog, 1,5-anhydroglucitol-6-phosphate, was markedly increased by Mabs binding in these regions, but unaffected by Mabs binding elsewhere in the molecule. Reactivity with Mabs recognizing conformationally sensitive epitopes in Region H of the C-terminal domain was greatly decreased by binding of substrate hexoses that induce closure of a cleft in the catalytic domain; selective recognition of the "open cleft" conformation, thereby preventing closure of the cleft required for progression of the catalytic cycle, can account for the marked decrease in Vmax that results from binding of these Mabs. Reactivity with Mabs binding to Region H was also decreased in the presence of inhibitory hexose 6-phosphates, implying that cleft closure was also induced by the latter; this is consistent with the suggestion that limitation of access to the C-terminal ATP binding site, resulting from cleft closure, is a factor in inhibition of the enzyme.