Monoclonal antibodies against rat brain hexokinase. Utilization in epitope mapping studies and establishment of structure-function relationships

Identification of a mitochondrial-binding site on the N-terminal end of hexokinase II

Hexokinase II (HKII) is responsible for the first step in the glycolysis pathway by adding a phosphate on to the glucose molecule so it can proceed down the pathway to produce the energy for continuous cancer cell growth. Tumour cells overexpress the HKII enzyme. In fact, it is the overexpression of the HKII enzyme that makes the diagnosis of cancer possible when imaged by positron emission tomography (PET). HKII binds to the voltage-dependent anion channel (VDAC) located on the mitochondrial outer membrane (MOM). When bound to the MOM, HKII is blocking a major cell death pathway. Thus, HKII is responsible for two characteristics of cancer cells, rapid tumour growth and inability of cancer cells to undergo apoptosis. One method to identify novel compounds that may interfere with the HKII-VDAC-binding site is to create a molecular model using the crystal structure of HKII. However, the amino acid(s) responsible for HKII binding to VDAC are not known. Therefore, a series of truncations and point mutations were made to the N-terminal end of HKII to identify the binding site to VDAC. Deletions of the first 10 and 20 amino acids indicated that important amino acid(s) for binding were located within the first 10 amino acids. Next, a series of point mutations were made within the first 10 amino acids. It is clear from the immunofluorescence images and immunoblot results that mutating the fifth amino acid from histidine to proline completely abolished binding to the MOM.

RanBP2 modulates Cox11 and hexokinase I activities and haploinsufficiency of RanBP2 causes deficits in glucose metabolism

The Ran-binding protein 2 (RanBP2) is a large multimodular and pleiotropic protein. Several molecular partners with distinct functions interacting specifically with selective modules of RanBP2 have been identified. Yet, the significance of these interactions with RanBP2 and the genetic and physiological role(s) of RanBP2 in a whole-animal model remain elusive. Here, we report the identification of two novel partners of RanBP2 and a novel physiological role of RanBP2 in a mouse model. RanBP2 associates in vitro and in vivo and colocalizes with the mitochondrial metallochaperone, Cox11, and the pacemaker of glycolysis, hexokinase type I (HKI) via its leucine-rich domain. The leucine-rich domain of RanBP2 also exhibits strong chaperone activity toward intermediate and mature folding species of Cox11 supporting a chaperone role of RanBP2 in the cytosol during Cox11 biogenesis. Cox11 partially colocalizes with HKI, thus supporting additional and distinct roles in cell function. Cox11 is a strong inhibitor of HKI, and RanBP2 suppresses the inhibitory activity of Cox11 over HKI. To probe the physiological role of RanBP2 and its role in HKI function, a mouse model harboring a genetically disrupted RanBP2 locus was generated. RanBP2^−/− are embryonically lethal, and haploinsufficiency of RanBP2 in an inbred strain causes a pronounced decrease of HKI and ATP levels selectively in the central nervous system. Inbred RanBP2^+/− mice also exhibit deficits in growth rates and glucose catabolism without impairment of glucose uptake and gluconeogenesis. These phenotypes are accompanied by a decrease in the electrophysiological responses of photosensory and postreceptoral neurons. Hence, RanBP2 and its partners emerge as critical modulators of neuronal HKI, glucose catabolism, energy homeostasis, and targets for metabolic, aging disorders and allied neuropathies.

The Ran-binding protein 2 (RanBP2) is a large protein with several domains. Although several protein partners were found to interact with selective domains of RanBP2, none to this date were found toward its large leucine-rich domain (LD). Cell-based experiments support several roles of RanBP2 in cell function, such as the production of functional proteins, control of protein trafficking between the nuclear and cytosol compartments, and control of multiple facets underlying cell division. Still, the genetic and physiological implications of the interactions between RanBP2 and its partners and of the function of RanBP2 in a whole-animal model remain elusive. The authors report the identification of two novel mitochondrial partners of the LD of RanBP2, Cox11 and hexokinase type I (HKI); and with multidisciplinary approaches probe the role of RanBP2 and its LD on Cox11, HKI, and functions allied to these. The authors found that RanBP2 exhibits chaperone activity toward HKI and Cox11. RanBP2 and Cox11 profoundly modulate HKI activity. Moreover, partial loss-of-function of RanBP2 in a mouse model induces deficits in growth rates and breakdown of glucose, promotes the down-regulation of HKI and ATP levels selectively in the central nervous system, and impairs visual function. These findings support a critical role of RanBP2 and its partners in metabolic processes and allied disease states.

Monoclonal Antibodies Inducing Conformational Changes on the Antigen Molecule

Monoclonal antibodies (MoAbs) are extensively used as biological tools because of their invariable specificity. However, the interpretation of results can be misled by the behaviour of MoAb displaying allosteric effects, i.e. long-range conformational changes on the antigen (Ag). It has been shown that some MoAbs are able to modify the spatial structure of the corresponding protein Ag, affecting in this way its biological activity as well as its binding to a second MoAb. Thus, a researcher using a MoAb as a tool to investigate some features of an antigenic molecule must be aware of the possible positive or negative allosteric properties of the antibody.

Functional interactions between the noncovalently associated N- and C-terminal halves of mammalian Type I hexokinase

The 100 kDa Type I isozyme of mammalian hexokinase has evolved by duplication and fusion of a gene encoding an ancestral 50 kDa hexokinase. Although the N- and C-terminal halves are similar in sequence, they differ in function, catalytic activity being associated only with the C-terminal half while the N-terminal half serves a regulatory role. The N- and C-terminal halves of rat Type I hexokinase have been coexpressed in M + R 42 cells. The halves associate noncovalently to produce a 100 kDa form that exhibits characteristics seen with the intact Type I isozyme but not with the isolated catalytic C-terminal half, i.e., characteristics that are influenced by interactions between the halves. These include a decreased K(m) for the substrate ATP and the ability of P(i) to antagonize inhibition by Glc-6-P or its analog, 1-5-anhydroglucitol-6-P. Thus, functional interactions between the N- and C-terminal halves do not require their covalent linkage.

Kinetic and regulatory properties of HK I(+), a modified form of the type I isozyme of mammalian hexokinase in which interactions between the N- and C-terminal halves have been disrupted

A modified form (HK I(+)) of rat Type I hexokinase (HK I) has been expressed. HK I(+) contains a centrally located polyalanine insert which, along with the known helical propensity of adjacent sequence, was expected to lead to alpha-helix formation, with resulting distension of the molecule and disruption of interactions between the N- and C-terminal halves. The properties of HK I(+) are consistent with this expectation and with previous proposals that (1) inhibition of HK I by Glc-6-P or its analogs and antagonism of this inhibition by P(i) result from competition of these ligands for a binding site in the N-terminal half of HK I, with resulting conformational changes propagated through interactions with the catalytic C-terminal half, and (2) binding of Glc-6-P to a site in the C-terminal half of HK I is obstructed by interactions between the halves, present in HK I but not HK I(+).

Developmental regulation of genes mediating murine brain glucose uptake

We examined the molecular mechanisms that mediate the developmental increase in murine whole brain 2-deoxyglucose uptake. Northern and Western blot analyses revealed an age-dependent increase in brain GLUT-1 (endothelial cell and glial) and GLUT-3 (neuronal) membrane-spanning facilitative glucose transporter mRNA and protein concentrations. Nuclear run-on experiments revealed that these developmental changes in GLUT-1 and -3 were regulated posttranscriptionally. In contrast, the mRNA and protein levels of the mitochondrially bound glucose phosphorylating hexokinase I enzyme were unaltered. However, hexokinase I enzyme activity increased in an age-dependent manner suggestive of a posttranslational modification that is necessary for enzymatic activation. Together, the postnatal increase in GLUT-1 and -3 concentrations and hexokinase I enzymatic activity led to a parallel increase in murine brain 2-deoxyglucose uptake. Whereas the molecular mechanisms regulating the increase in the three different gene products may vary, the age-dependent increase of all three constituents appears essential for meeting the increasing demand of the maturing brain to fuel the processes of cellular growth, differentiation, and neurotransmission.

Functional organization of mammalian hexokinases: characterization of the rat type III isozyme and its chimeric forms, constructed with the N- and C-terminal halves of the type I and type II isozymes

Previous studies have shown that catalytic function is associated with both halves of the Type II isozyme of mammalian hexokinase, while the Type I isozyme is functionally differentiated into a catalytic C-terminal half and regulatory N-terminal half. The Type III isozyme has now been shown to be similar to the Type I isozyme in its functional organization. Chimeras composed of the N-terminal half of Type III hexokinase and the C-terminal half of either Type I or Type II hexokinase have activities that can be attributed to the C-terminal half and are similar in activity to chimeras composed of the C-terminal half of Type III and the intrinsically inactive N-terminal domain of Type I or the inactivated (by site-directed mutation) N-terminal half of Type II hexokinase. Virtually no activity was seen with chimeras constructed with the N-terminal half of the Type III isozyme and catalytically inactive (by site-directed mutation) C-terminal halves of Type I or Type II hexokinase. Substrate inhibition by Glc is seen only with the Type III isozyme and with chimeric forms containing the C-terminal half of Type III hexokinase and the N-terminal half of Type I or Type II isozyme, the latter inactivated by site-directed mutation; this is attributed to conformational changes induced by binding of Glc to a low affinity site in the N-terminal half, with subsequent effect on catalytic activity of the C-terminal half. These results also provide further insight into the role of interactions (or lack of interactions) between the N- and C-terminal halves in the inhibition of the Type I-III isozymes by Glc-6-P, its antagonism by low concentrations of Pi, and the inhibition seen at higher concentrations of Pi.

Metabolic modulation of hexokinase association with mitochondria in living smooth muscle cells

Hexokinase isoform I binds to mitochondria of many cell types. It has been hypothesized that this association is regulated by changes in the concentrations of specific cellular metabolites. To study the distribution of hexokinase in living cells, fluorophore-labeled functional hexokinase I was prepared. After microinjection into A7r5 smooth muscle cells, hexokinase localized to distinct structures identified as mitochondria. The endogenous hexokinase demonstrated a similar distribution with the use of immunocytochemistry. 2-Deoxyglucose elicited an increase in glucose 6-phosphate (G-6-P) and a decrease in ATP levels and diminished hexokinase binding to mitochondria in single cells. 3-O-methylglucose elicited slowly developing decreases in all three parameters. In contrast, cyanide elicited a rapid decrease in both ATP and hexokinase binding. Analyses of changes in metabolite levels and hexokinase binding indicate a positive correlation between binding and cell energy state as monitored by ATP. On the other hand, only in the presence of 2-deoxyglucose was the predicted inverse correlation between binding and G-6-P observed. Unlike the relatively large changes in distribution observed with the fluorescent-injected hexokinase, cyanide caused only a small decrease in the localization of endogenous hexokinase with mitochondria. These findings suggest that changes in the concentrations of specific metabolites can alter the binding of hexokinase I to specific sites on mitochondria. Moreover, the apparent difference in sensitivity of injected and endogenous hexokinase to changes in metabolites may reflect the presence of at least two classes of binding mechanisms for hexokinase, with differential sensitivity to metabolites.

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.

Human pancreatic lipase. Importance of the hinge region between the two domains, as revealed by monoclonal antibodies

Several monoclonal antibodies (mAbs) were prepared against human pancreatic lipase (HPL). Two enzyme-linked immunosorbent assay (ELISA) procedures were set up for screening hybridomas producing specific antibodies. Four mAbs (81-23, 146-40, 315-25, and 320-24) of the IgG1 isotype were found to react with HPL in both simple sandwich and double sandwich ELISAs, while mAb 248-31, of the IgG2b isotype, reacted only with HPL in a double sandwich ELISA. The results of Western blot analysis carried out with native and SDS-denatured HPLs indicated that mAb 248-31 recognized only native HPL, while all the other mAbs recognized both forms of HPL. Since mAb 248-31 did not recognize SDS-denatured HPL, it was not possible to localize its epitope. To carry out epitope mapping along the primary sequence of HPL, four fragments (14, 26, 30, and 36 kDa) resulting from a limited chymotryptic cleavage of HPL were characterized by Western blotting as well as N-terminal amino acid sequence analysis. Of the above five anti-HPL mAbs, four (81-23, 248-31, 315-25, and 320-24) were found to inhibit the lipolytic activity of HPL (in both the presence and absence of bile salts and colipase), while mAb 146-40 had no inhibitory effects. The epitope recognized by mAb 146-40 was found to be located in the N-terminal domain (Lys1-Phe335). Combined immunoinactivation and epitope mapping studies showed that three inhibitory mAbs (81-23, 315-25, and 320-24) recognize overlapping epitopes from the hinge region between the N- and C-terminal domains of HPL, belonging to the 26-kDa fragment. In the presence of lipids, a significant decrease has been observed in the bending angle between the N- and C-terminal domains of the HPL tertiary structure (van Tilbeurgh, H., Egloff, M. P., Martinez, C., Rugani, N., Verger, R. and Cambillau, C. (1993) Nature 362, 814-820). From the present immunochemical data, we further propose that locking the hinge movement with mAbs may induce lipase immunoinactivation.

Epitope mapping and immunoinactivation of human gastric lipase using five monoclonal antibodies

Five monoclonal antibodies (mAb) directed against human gastric lipase (HGL) have been produced by hybridization of myeloma cells with spleen cells of BALB/c immunized mice. All these mAb belong to the IgG1 class with a kappa light chain. The effects of these mAb on the enzymic activity of HGL were studied and used to define three classes of antibodies, depending upon their immunoinactivation properties. As determined by ELISA and immunoinactivation studies, four overlapping epitopes were found to be part of the functional sites of the enzyme. The mAb appear to be suitable probes for studying the lipid binding and catalytic domains of HGL. The results of the ELISA additivity test were used to describe tentatively the epitopes of HGL in terms of a schematic spatial map.

Localization of the type III isozyme of hexokinase at the nuclear periphery

The distribution of the type III isozyme of hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) in rat kidney, liver, spleen, lung, and brain was determined immunohistochemically, using a monoclonal antibody generated against the enzyme purified from rat Novikoff hepatoma. In all tissues, specific cell types exhibited intense staining at the nuclear periphery, as confirmed by analysis using confocal microscopy. Isolated nuclei from kidney or liver were devoid of detectable type III hexokinase, although the enzyme was found in the "soluble" fraction from kidney or liver homogenates; these results suggest that the type III isozyme is associated in a labile manner with the external surface of the nucleus, with this association being disrupted by conventional homogenization and nuclear isolation procedures. The nuclear localization of the type III isozyme contrasts with previously demonstrated association of the type I and II isozymes with mitochondria. The physiological significance of a nuclear localization for the type III isozyme remains unclear. However, it was noted that many of the cells exhibiting prominent nuclear staining for type III hexokinase are endothelial or epithelial cells, suggesting a possible relationship between nuclear type III hexokinase and transport functions which are prominent in such cells.

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.

Remarkably enhanced expression of the type II hexokinase in rat hepatoma cell line AH130

Expression of mRNAs encoding hexokinase isozymes was studied in various cells such as rat brain, liver, skeletal muscle, kidney and heart, and the rat hepatoma cell line AH130 by Northern blotting. High specific expression of type II hexokinase was observed only with AH130 cells. In contrast, specific expression of type I hexokinase was detected in energy-requiring normal tissue cells such as brain and heart. These results suggest that the expression of hexokinase isozyme in the tumor cells is different from that in normal cells.

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.

Differential scanning calorimetric study of rat brain hexokinase: domain structure and stability

Rat brain hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) has been studied by differential scanning calorimetry. In "high-ionic-strength" buffer (50 mM Tris-Cl, 0.5 mM EDTA, 10 mM monothioglycerol, pH 8.5), and assuming two-state behavior with calorimetric enthalpy equal to van't Hoff enthalpy, the endotherm could be deconvoluted into two transitions with Tm values of about 48 and 51 degrees C and enthalpies of about 109 and 112 kcal/mol, respectively. A similar endotherm was seen when glucose or glucose 6-phosphate was present, except that Tm values for both transitions were increased. The glucose analog, N-acetylglucosamine, had no observable effect on the endotherm, which is in agreement with previous studies indicating that this ligand, unlike glucose and glucose 6-phosphate, does not induce conformational changes that lead to increased stability of the enzyme. In "low-ionic-strength" buffer (5 mM Tris-Cl, 0.5 mM EDTA, 10 mM monothioglycerol, pH 8.5), the transitions were partially resolved even in the absence of ligands, with Tm values of about 49 and 55 degrees C. Due to difficulties with erratic baseline behavior under the low-ionic-strength conditions, enthalpies were not routinely determined, but these appeared to be similar to those seen in high-ionic-strength buffer. Also similar was the increase in stability, as reflected by the increase in Tm for both transitions, when glucose or glucose 6-phosphate was present. Correlation of these transitions with specific regions of the molecule was established by analysis of enzyme in which the domain corresponding to the first transition was selectively denatured by a partial scan in the calorimeter. Subsequent rescanning of these samples showed only the second transition, confirming the selective denaturation of the domain corresponding to the first transition and retention of the folded structure of the second domain. Discrimination between denatured (first transition) and undenatured (second transition) domains was based on the markedly increased susceptibility of the denatured region to tryptic digestion; regions of the molecule that retained their folded structure and resistance to proteolysis were identified by immunoblotting techniques using monoclonal antibodies recognizing epitopes having defined locations within the overall sequence. Based on this analysis, the first transition corresponds to unfolding of the C-terminal half of the molecule, with the second transition resulting from unfolding of the more stable N-terminal half. The order of unfolding could be reversed in the presence of ATP-Mg2+ and N-acetylglucosamine, conditions which have been shown to result in selective stabilization of the C-terminal domain.

Hexokinase-binding properties of the mitochondrial VDAC protein: inhibition by DCCD and location of putative DCCD-binding sites

The outer mitochondrial membrane receptor for hexokinase binding has been identified as the VDAC protein, also known as mitochondrial porin. The ability of the receptor to bind hexokinase is inhibited by pretreatment with dicyclohexylcarbodiimide (DCCD). At low concentrations, DCCD inhibits hexokinase binding by covalently labeling the VDAC protein, with no apparent effect on VDAC channel-forming activity. The stoichiometry of [14C]-DCCD labeling is consistent with one to two high-affinity DCCD-binding sites per VDAC monomer. A comparison between the sequence of yeast VDAC and a conserved sequence found at DCCD-binding sites of several membrane proteins showed two sites where the yeast VDAC amino acid sequence appears to be very similar to the conserved DCCD-binding sequence. Both of these sites are located near the C-terminal end of yeast VDAC (residues 257-265 and 275-283). These results are consistent with a model in which the C-terminal end of VDAC is involved in binding to the N-terminal end of hexokinase.

Rat brain hexokinase: the hydrophobic N-terminus of the mitochondrially bound enzyme is inserted in the lipid bilayer

Mitochondrially bound rat brain hexokinase was labeled with the photoactivatable reagent, 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine. This highly hydrophobic reagent is strongly partitioned into the hydrophobic environment of the membrane core, and thus selectively labels segments of a protein that penetrate this region of the membrane. Labeling of hexokinase was shown to be restricted to the N-terminal region of the molecule. Approximately 80% of the radiolabel was removed by treatment of the enzyme with chymotrypsin, which preferentially cleaves a hydrophobic 9-residue sequence at the extreme N-terminus of the enzyme, and it is considered likely that the remaining 20% was associated with two additional hydrophobic residues, immediately adjacent to this segment but not susceptible to cleavage by chymotrypsin. Labeling of the enzyme was shown to be dependent on maintenance of the association with the membrane. These results are consistent with a model in which binding of hexokinase involves insertion of an 11-residue hydrophobic N-terminal "tail," possibly existing in alpha-helical secondary structure, into the hydrophobic core of the membrane.

The interaction of phosphorylated sugars with human hexokinase I

Glucose 6-phosphate as well as several other hexose mono- and diphosphates were found by kinetic studies to be competitive inhibitors of human hexokinase I (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) versus MgATP. Limited proteolysis by trypsin does not destroy the hexokinase activity but produces as well-defined peptide map when the digested enzyme is electrophoresed in the presence of sodium dodecyl sulfate. MgATP at subsaturating concentration protects hexokinase from trypsin digestion, while phosphorylated sugars, Mg2+, glucose and inorganic phosphate have no effect. Addition of glucose 6-phosphate to the MgATP-hexokinase complex at a concentration 100-times higher than its Ki was not able to reverse the MgATP-induced conformation of hexokinase, suggesting that the binding of glucose 6-phosphate and MgATP are not mutually exclusive. Similar evidence was also obtained by studies of the induced modifications of ultraviolet spectra of hexokinase by the binding of MgATP, glucose 6-phosphate and both compounds. Among a library of monoclonal antibodies produced against rat brain hexokinase I and that recognize human placenta hexokinase I, one (4A6) was found to be able to modify the Ki of glucose 6-phosphate (from 25 to 140 microM) for human hexokinase I. The same antibody also weakens the inhibition by all the other hexoses phosphate studied without affecting the apparent Km for MgATP (from 0.6 to 0.75 mM) or for glucose. These data support the view for the binding of glucose 6-phosphate at a regulatory site on the enzyme.

Rat brain hexokinase: location of the allosteric regulatory site in a structural domain at the N-terminus of the enzyme

After denaturation in 0.6 M guanidine hydrochloride, rat brain hexokinase becomes highly susceptible to proteolysis by trypsin. Glucose 6-phosphate (Glc-6-P) and its analog, 1,5-anhydroglucitol 6-phosphate, selectively protect the N-terminal half of the molecule from proteolysis. These compounds do not protect the C-terminal half of the molecule, nor do they protect enzyme activity; the Glc analog, N-acetylglucosamine, does protect the C-terminal domain and catalytic activity, but does not prevent proteolysis of the N-terminal half of the molecule. These results are consistent with previous work [M. Nemat-Gorgani and J. E. Wilson (1986) Arch. Biochem. Biophys. 251, 97-103; D. M. Schirch and J. E. Wilson (1987) Arch. Biochem. Biophys. 254, 385-396] demonstrating that binding sites for both hexose and nucleotide substrates, and thus catalytic function, are associated with a 40-kDa domain located at the C-terminus of the enzyme. They further demonstrate that the binding site for the allosteric effector, Glc-6-P, lies in the N-terminal half of the molecule and is distinct from the catalytic site. Using protection against proteolysis as a reflection of binding, it is shown that the Glc-6-P binding site in the N-terminal region has all the characteristics described for the allosteric effector site on this enzyme in terms of affinity for Glc-6-P, specificity, and synergistic interactions with the hexose binding site in the C-terminal region of the molecule. This disposition of catalytic and regulatory functions in discrete halves of the molecule is consistent with suggestions by several investigators that mammalian hexokinases evolved by a process of duplication and fusion of an ancestral gene coding for a hexokinase similar to the present-day yeast enzyme, with the regulatory site of mammalian hexokinases having evolved from what was originally a catalytic site.

Rat brain hexokinase: location of the substrate hexose binding site in a structural domain at the C-terminus of the enzyme

A glucose analog, N-(bromoacetyl)-D-glucosamine (GlcNBrAc), previously used to label the glucose binding sites of rat muscle Type II and bovine brain Type I hexokinases, also inactivates rat brain hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) with pseudo-first-order kinetics. Inactivation occurs predominantly via a "specific" pathway involving formation of a complex between hexokinase and GlcNBrAc, but significant nonspecific (i.e., without prior complex formation) inactivation also occurs, and equations to describe this behavior are derived. Inactivation is dependent on deprotonation of a residue with an alkaline pKa, consistent with the modified residue being a sulfhydryl group as reported to be the case with the hexokinase of bovine brain. The affinity label modifies three residues (per molecule of enzyme) at indistinguishable rates, but only one of these residues appears to be critical for activity. Amino acid analysis of the modified enzyme indicates derivatization of three cysteine residues; there was no indication of modification of other residues potentially reactive with haloacetyl derivatives. Kinetic analysis and effects of protective ligands were consistent with location of the critical sulfhydryl at the glucose binding site. Peptide mapping techniques permitted localization of the critical residue, and thus the glucose binding site, in a 40-kDa domain at the C-terminus of the enzyme. This is the same domain recently shown to include the ATP binding site. Thus, catalytic function is assigned to the C-terminal domain of rat brain hexokinase.

Monoclonal antibodies against the voltage-sensitive sodium channel from rat skeletal muscle: mapping antibody binding sites

Twenty-one monoclonal antibodies specific for the rat skeletal muscle voltage-sensitive sodium channel have been characterized according to subunit reactivity, recognition of carbohydrates, and mutual binding interactions. All antibodies recognize the 260-kDa alpha-subunit of the sodium channel on immunoblots. N-Acetylneuraminic acid inhibited the binding of five antibodies in a concentration-dependent manner, but five other monosaccharides known to be components of the channel had no effect on antibody binding. Competition studies using biosynthetically labeled antibodies separated these 21 antibodies into groups recognizing at least nine distinct domains. Through common interactions between domains, these could in turn be associated into two larger topologically related regions. One region encompasses seven interacting domains and 16 antibodies. This region is probably extracellular by virtue of the interaction of one subgroup with N-acetylneuraminic acid, and may represent a particularly immunogenic region on this channel protein.

Degradation artefacts during sample preparation for sodium dodecyl sulphate polyacrylamide gel electrophoresis

Preparation of samples for sodium dodecyl sulphate polyacrylamide gel electrophoresis routinely involves heating the protein in solution containing detergent and reducing agent for at least two minutes. Here we show that this treatment causes fragmentation of the protein glycogen phosphorylase, whether purified or as a component of a skeletal muscle preparation. The fragments are detected as minor bands on western blots and represent the products of discrete breakage point in the peptide sequence. Protease inhibitors cannot suppress the fragmentation. Such small amounts of immunoreactive fragments may be incorrectly identified on western blots as contaminants that were originally present in the antigen preparation. They may also be a source of ambiguity in studies that search for degradation intermediates during proteolysis.

A modified ELISA that selectively detects monoclonal antibodies recognizing native antigen

A modified ELISA procedure is described which permits selective detection of monoclonal antibodies reactive with the native antigen. The essence of the method is the use of immobilization methods that do not result in denaturation of the antigen during adsorption to the microtiter plates. This was accomplished by using polyclonal antibodies, adsorbed to the wells of the microtiter plate, as the immobilizing agent.

Rat brain hexokinase: location of the substrate nucleotide binding site in a structural domain at the C-terminus of the enzyme

8-Azido-ATP serves as a substrate for rat brain hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1). Irradiation of hexokinase in the presence of this photoactivatable ATP analog results in inactivation of the enzyme. ATP and hexose 6-phosphates (Glc-6-P, 1,5-anhydroglucitol-6-P) previously shown to competitively inhibit nucleotide binding protect the enzyme from photoinactivation; other hexose 6-phosphates do not. Hexoses (Glc, Man) previously shown to enhance nucleotide binding also protect against photoinactivation; other hexoses do not. These effects of hexoses and hexose 6-phosphates can be interpreted in terms of the conformational changes previously shown to result from the binding of these ligands and to influence the characteristics of the nucleotide binding site (M. Baijal and J. E. Wilson (1982) Arch. Biochem. Biophys. 218, 513-524). Limited tryptic cleavage of the enzyme produces three major fragments having molecular weights of about 10K, 40K, and 50K, and thought to represent major structural domains within the enzyme (P. G. Polakis and J. E. Wilson (1984) Arch. Biochem. Biophys. 234, 341-352). Tryptic cleavage of the enzyme, photoinactivated in the presence of 14C-labeled azido-ATP, discloses prominent labeling of the 10K and 40K domains, which are known to originate from the N- and C-terminal regions, respectively. Labeling of the 40K domain is influenced by ligands in a manner that corresponds to the effectiveness of these ligands in protecting against photoinactivation whereas labeling of the 10K domain is not affected by these same ligands. It is concluded that the 40K domain includes the binding site for nucleotide substrates. More refined two-dimensional peptide mapping techniques demonstrate that the predominant site of labeling is a peptide segment, molecular weight approximately 20K, that is located in the central and/or C-terminal region of the 40K domain. Labeling of the 10K domain is attributed to nonspecific interaction of azido-ATP with the hydrophobic sequence shown to be located at the N-terminus of brain hexokinase (P. G. Polakis and J. E. Wilson (1985) Arch. Biochem. Biophys. 236, 328-337).

Hexokinase A from mammalian brain: comparative peptide mapping and immunological studies with monoclonal antibodies

Immunological reactivity of partially purified hexokinase A (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) from brain of several vertebrate species has been compared by using enzyme-linked immunosorbent assay and seven monoclonal antibodies raised against the rat brain enzyme. The epitopes recognized by three of these antibodies have been rather widely conserved among the species examined (rat, mouse, guinea pig, rabbit, cat, dog, sheep, cow, pig, chicken), while this was not the case for the epitopes recognized by the other antibodies, which differed markedly in their distribution among these species. The domain structure of these enzymes has been examined by peptide mapping (after limited tryptic digestion) in conjunction with immunoblotting techniques employing monoclonal antibodies. The results indicate that the overall domain structure of these enzymes is similar to that previously described for rat brain hexokinase A, but that there are significant differences in the size of these domains in enzymes from different species.