Evidence for formation of a rabbit liver aldolase--rabbit liver fructose-1,6-bisphosphatase complex

Fructose-1,6-bisphosphatase: From a glucose metabolism enzyme to multifaceted regulator of a cell fate

Fructose-1,6-bisphosphatase (FBPase) is one of the ancient, evolutionarily conserved enzymes of carbohydrate metabolism. It has been described for a first time in 1943, however, for the next half a century mostly kinetic and structural parameters of animal FBPases have been studied. Discovery of ubiquitous expression of the muscle isozyme of FBPase, thus far considered to merely regulate glycogen synthesis from carbohydrate precursors, and its nuclear localisation in several cell types has risen new interest in the protein, resulting in numerous publications revealing complex functions/properties of FBPase. This review summarises the current knowledge of FBPase in animal cells providing evidence that the enzyme merits the name of moonlighting protein.

A new level of regulation in gluconeogenesis: metabolic state modulates the intracellular localization of aldolase B and its interaction with liver fructose-1,6-bisphosphatase

Understanding how glucose metabolism is finely regulated at molecular and cellular levels in the liver is critical for knowing its relationship to related pathologies, such as diabetes. In order to gain insight into the regulation of glucose metabolism, we studied the liver-expressed isoforms aldolase B and fructose-1,6-bisphosphatase-1 (FBPase-1), key enzymes in gluconeogenesis, analysing their cellular localization in hepatocytes under different metabolic conditions and their protein-protein interaction in vitro and in vivo. We observed that glucose, insulin, glucagon and adrenaline differentially modulate the intracellular distribution of aldolase B and FBPase-1. Interestingly, the in vitro protein-protein interaction analysis between aldolase B and FBPase-1 showed a specific and regulable interaction between them, whereas aldolase A (muscle isozyme) and FBPase-1 showed no interaction. The affinity of the aldolase B and FBPase-1 complex was modulated by intermediate metabolites, but only in the presence of K(+). We observed a decreased association constant in the presence of adenosine monophosphate, fructose-2,6-bisphosphate, fructose-6-phosphate and inhibitory concentrations of fructose-1,6-bisphosphate. Conversely, the association constant of the complex increased in the presence of dihydroxyacetone phosphate (DHAP) and non-inhibitory concentrations of fructose-1,6-bisphosphate. Notably, in vivo FRET studies confirmed the interaction between aldolase B and FBPase-1. Also, the co-expression of aldolase B and FBPase-1 in cultured cells suggested that FBPase-1 guides the cellular localization of aldolase B. Our results provide further evidence that metabolic conditions modulate aldolase B and FBPase-1 activity at the cellular level through the regulation of their interaction, suggesting that their association confers a catalytic advantage for both enzymes.

Interaction between calpain-1 and HSP90: new insights into the regulation of localization and activity of the protease

Here we demonstrate that heat shock protein 90 (HSP90) interacts with calpain-1, but not with calpain-2, and forms a discrete complex in which the protease maintains its catalytic activity, although with a lower affinity for Ca²⁺. Equilibrium gel distribution experiments show that this complex is composed by an equal number of molecules of each protein partner. Moreover, in resting cells, cytosolic calpain-1 is completely associated with HSP90. Since calpain-1, in association with HSP90, retains its proteolytic activity, and the chaperone is displaced by calpastatin also in the absence of Ca²⁺, the catalytic cleft of the protease is not involved in this association. Thus, calpain-1 can form two distinct complexes depending on the availability of calpastatin in the cytosol. The occurrence of a complex between HSP90 and calpain-1, in which the protease is still activable, can prevent the complete inhibition of the protease even in the presence of high calpastatin levels. We also demonstrate that in basal cell conditions HSP90 and calpain-1, but not calpain-2, are inserted in the multi-protein N-Methyl-D-Aspartate receptor (NMDAR) complex. The amount of calpain-1 at the NMDAR cluster is not modified in conditions of increased [Ca²⁺]_i, and this resident protease is involved in the processing of NMDAR components. Finally, the amount of calpain-1 associated with NMDAR cluster is independent from Ca²⁺-mediated translocation. Our findings show that HSP90 plays an important role in maintaining a given and proper amount of calpain-1 at the functional sites.

Metabolic control at the cytosol-mitochondria interface in different growth phases of CHO cells

Metabolism at the cytosol-mitochondria interface and its regulation is of major importance particularly for efficient production of biopharmaceuticals in Chinese hamster ovary (CHO) cells but also in many diseases. We used a novel systems-oriented approach combining dynamic metabolic flux analysis and determination of compartmental enzyme activities to obtain systems level information with functional, spatial and temporal resolution. Integrating these multiple levels of information, we were able to investigate the interaction of glycolysis and TCA cycle and its metabolic control. We characterized metabolic phases in CHO batch cultivation and assessed metabolic efficiency extending the concept of metabolic ratios. Comparing in situ enzyme activities including their compartmental localization with in vivo metabolic fluxes, we were able to identify limiting steps in glycolysis and TCA cycle. Our data point to a significant contribution of substrate channeling to glycolytic regulation. We show how glycolytic channeling heavily affects the availability of pyruvate for the mitochondria. Finally, we show that the activities of transaminases and anaplerotic enzymes are tailored to permit a balanced supply of pyruvate and oxaloacetate to the TCA cycle in the respective metabolic states. We demonstrate that knowledge about metabolic control can be gained by correlating in vivo metabolic flux dynamics with time and space resolved in situ enzyme activities.

Expression, purification, crystallization and preliminary X-ray diffraction studies of phosphoglycerate mutase from Staphylococcus aureus NCTC8325

Phosphoglycerate mutase (PGM) is a key enzyme in carbohydrate metabolism. It takes part in both glycolysis and gluconeogenesis. PGM from pathogenic Staphylococcus aureus (NCTC8325) was cloned in pQE30 expression vector overexpressed in Escherichia coli M15 (pREP4) cells and purified to homogeneity. The protein was crystallized from two different conditions, (i) 0.1 M HEPES pH 7.5, 20%(w/v) polyethylene glycol 10,000 and (ii) 0.2 M NaCl, 0.1 M bis-tris pH 6.5, 25%(w/v) polyethylene glycol 3350, at 25°C by the sitting-drop vapour-diffusion method. Crystals grown at pH 7.5 diffracted to 2.5 Å resolution and belonged to the orthorhombic space group P2(1)2(1)2, with unit-cell parameters a = 77.0, b = 86.11, c = 94.07 Å. Crystals from the second condition at pH 6.5 diffracted to 2.00 Å resolution. These crystals belonged to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 73.21, b = 81.75, c = 89.18 Å. X-ray diffraction data have been collected and processed to the maximum resolution to determine the structure of PGM. The structure has been solved by molecular replacement and structure refinement is now in progress.

Expression, purification, crystallization and preliminary X-ray diffraction studies of phosphoglycerate kinase from methicillin-resistant Staphylococcus aureus MRSA252

Phosphoglycerate kinase (PGK) from methicillin-resistant Staphylococcus aureus MRSA252 has been cloned in pQE30 expression vector, overexpressed in Escherichia coli SG13009 (pREP4) cells and purified to homogeneity. The protein was crystallized from 0.15 M CaCl(2), 0.1 M HEPES-NaOH pH 6.8, 20%(w/v) polyethylene glycol 2000 at 298 K by the hanging-drop vapour-diffusion method. The crystals belonged to space group P2(1), with unit-cell parameters a = 45.14, b = 74.75, c = 58.67 Å, β = 95.72°. X-ray diffraction data have been collected and processed to a maximum resolution of 2.3 Å. The presence of one molecule in the asymmetric unit gives a Matthews coefficient (V(M)) of 2.26 Å(3) Da(-1) with a solvent content of 46%. The structure has been solved by molecular replacement and structure refinement is now in progress.

Association of Calpastatin with Inactive Calpain

It is generally accepted that the Ca(2+)-dependent interaction of calpain with calpastatin is the most relevant mechanism involved in the regulation of Ca(2+)-induced proteolysis. We now report that a calpain-calpastatin association can occur also in the absence of Ca(2+) or at very low Ca(2+) concentrations, reflecting the physiological conditions under which calpain retains its inactive conformational state. The calpastatin binding region is localized in the non-inhibitory L-domain containing the amino acid sequences encoded by exons 4-7. This calpastatin region recognizes a calpain sequence located near the end of the DII-domain. Interaction of calpain with calpastatins lacking these sequences becomes strictly Ca(2+)-dependent because, under these conditions, the transition to an active state of the protease is an obligatory requirement. The occurrence of the molecular association between Ca(2+)-free calpain and various recombinant calpastatin forms has been demonstrated by the following experimental results. Addition of calpastatin protected calpain from trypsin digestion. Calpain was coprecipitated when calpastatin was immunoprecipitated. The calpastatin molecular size increased following exposure to calpain. The two proteins comigrated in zymogram analysis. Furthermore, calpain-calpastatin interaction was perturbed by protein kinase C phosphorylation occurring at sites located at the exons involved in the association. At a functional level, calpain-calpastatin interaction at a physiological concentration of Ca(2+) represents a novel mechanism for the control of the amount of the active form of the protease potentially generated in response to an intracellular Ca(2+) influx.

Different involvement for aldolase isoenzymes in kidney glucose metabolism: aldolase B but not aldolase A colocalizes and forms a complex with FBPase

The expression of aldolase A and B isoenzyme transcripts was confirmed by RT-PCR in rat kidney and their cell distribution was compared with characteristic enzymes of the gluconeogenic and glycolytic metabolic pathway: fructose-1,6-bisphosphatase (FBPase), phosphoenol pyruvate carboxykinase (PEPCK), and pyruvate kinase (PK). We detected aldolase A isoenzyme in the thin limb and collecting ducts of the medulla and in the distal tubules and glomerula of the cortex. The same pattern of distribution was found for PK, but not for aldolase B, PEPCK, and FBPase. In addition, co-localization studies confirmed that aldolase B, FBPase, and PEPCK are expressed in the same proximal cells. This segregated cell distribution of aldolase A and B with key glycolytic and gluconeogenic enzymes, respectively, suggests that these aldolase isoenzymes participate in different metabolic pathways. In order to test if FBPase interacts with aldolase B, FBPase was immobilized on agarose and subjected to binding experiments. The results show that only aldolase B is specifically bound to FBPase and that this interaction was specifically disrupted by 60 microM Fru-1,6-P2. These data indicate the presence of a modulated enzyme-enzyme interaction between FBPase and isoenzyme B. They affirm that in kidney, aldolase B specifically participates, along the gluconeogenic pathway and aldolase A in glycolysis.

Enzyme co-localization in pea leaf chloroplasts: glyceraldehyde-3-P dehydrogenase, triose-P isomerase, aldolase and sedoheptulose bisphosphatase

Nearest neighbor analysis of immunocytolocalization experiments indicates that the enzymes glyceraldehyde-3-P dehydrogenase, triose-P isomerase and aldolase are located close to one another in the pea leaf chloroplast stroma, and that aldolase is located close to sedoheptulose bisphosphatase. Direct transfer of the triose phosphates between glyceraldehyde-3-P dehydrogenase and triose-P isomerase, and from glyceraldehyde-3-P dehydrogenase and triose-P isomerase to aldolase, is then a possibility, as is direct transfer of sedoheptulose bisphosphate from aldolase to sedoheptulose bisphosphatase. Spatial organization of these enzymes may be important for efficient CO(2) fixation in photosynthetic organisms. In contrast, there is no indication that fructose bisphosphatase is co-localized with aldolase, and direct transfer of fructose bisphosphate from aldolase to fructose bisphosphatase seems unlikely.

Inhibition of fructose-1,6-bisphosphatase by a new class of allosteric effectors

A highly constrained pseudo-tetrapeptide (OC252-324) further defines a new allosteric binding site located near the center of fructose-1,6-bisphosphatase. In a crystal structure, pairs of inhibitory molecules bind to opposite faces of the enzyme tetramer. Each ligand molecule is in contact with three of four subunits of the tetramer, hydrogen bonding with the side chain of Asp187 and the backbone carbonyl of residue 71, and electrostatically interacting with the backbone carbonyl of residue 51. The ligated complex adopts a quaternary structure between the canonical R- and T-states of fructose-1,6-bisphosphatase, and yet a dynamic loop essential for catalysis (residues 52-72) is in a conformation identical to that of the T-state enzyme. Inhibition by the pseudo-tetrapeptide is cooperative (Hill coefficient of 2), synergistic with both AMP and fructose 2,6-bisphosphate, noncompetitive with respect to Mg2+, and uncompetitive with respect to fructose 1,6-bisphosphate. The ligand dramatically lowers the concentration at which substrate inhibition dominates the kinetics of fructose-1,6-bisphosphatase. Elevated substrate concentrations employed in kinetic screens may have facilitated the discovery of this uncompetitive inhibitor. Moreover, the inhibitor could mimic an unknown natural effector of fructose-1,6-bisphosphatase, as it interacts strongly with a conserved residue of undetermined functional significance.

Use of protein-protein interactions in affinity chromatography

Biospecific recognition between proteins is a phenomenon that can be exploited for designing affinity-chromatographic purification systems for proteins. In principle, the approach is straightforward, and there are usually many alternative ways, since a protein can be always found which binds specifically enough to the desired protein. Routine immunoaffinity chromatography utilizes the recognition of antigenic epitopes by antibodies. However, forces involved in protein-protein interactions as well the forces keeping the three-dimensional structures of proteins intact are complicated, and proteins are easily unfolded by various factors with unpredictable results. Because of this and because of the generally high association strength between proteins, the correct adjustment of binding forces between an immobilized protein and the protein to be purified as well as the release of bound proteins in biologically active form from affinity complexes are the main problem. Affinity systems involving interactions like enzyme-enzyme, subunit-oligomer, protein-antibody, protein-chaperone and the specific features involved in each case are presented as examples. This article also aims to sketch prospects for further development of the use of protein-protein interactions for the purification of proteins.

Muscle aldolase decreases muscle FBPase sensitivity toward AMP inhibition

Muscle aldolase bound to muscle FBPase (K(d) = 8.7 microM) decreases the latter's sensitivity towards AMP inhibition. I(0.5) of muscle FBPase was increased from 0.06 microM to 0.65 microM when determined in the presence of 10 microM of muscle aldolase. In the presence of 10 microM of liver aldolase I(0.5) of liver FBPase was increased only twofold, from 11.0 microM to 21.7 microM. The effect of muscle aldolase on liver FBPase and liver aldolase on muscle FBPase is rather negligible. Aldolase slightly affected interaction of FBPase with magnesium ions decreasing K(a) and Hill constant (n). No effect of aldolase on FBPase pH optimum was observed.

Chromatographic methods to study protein-protein interactions

Proteins and enzymes are now generally thought to be organized within the cell to form clusters in a dynamic and versatile way, and heterologous protein-protein interactions are believed to be involved in virtually all cellular events. Therefore we need appropriate tools to detect and study such interactions. Chromatographic techniques prove to be well suited for this kind of investigation. Real complexes formed between proteins can be studied by classic gel filtration. When enzymes are studied, active enzyme gel chromatography is a useful alternative. A variant of classic gel filtration is gel filtration equilibrium analysis, which is similar to equilibrium dialysis. When the association formed is only dynamic and equilibrates very rapidly, either the Hummel-Dryer method of equilibrium gel filtration or large-zone equilibrium filtration sometimes allows the interactions to be analyzed, both qualitatively and quantitatively. Very often, however, interactions between enzymes and proteins can only be evidenced in vitro in media that mimic the intracellular situation. Immobilized proteins are excellent tools for this type of research. Several examples are indeed known where the immobilization of an enzyme on a solid support does not affect its real properties, but rather changes its environment in such a way that the diffusion becomes limiting. Affinity chromatography using immobilized proteins allows the analysis of heterologous protein-protein interactions, both qualitatively and quantitatively. A useful alternative appears to be affinity electrophoresis. The latter technique, however, is exclusively qualitative. All these techniques are described and illustrated with examples taken from the literature.

Foundations of metabolic organization: coherence as a basis of computational properties in metabolic networks

Biological organization is based on the coherent energy transfer allowing for macromolecules to operate with high efficiency and realize computation. Computation is executed with virtually 100% efficiency via the coherent operation of molecular machines in which low-energy recognitions trigger energy-driven non-equilibrium dynamic processes. The recognition process is of quantum mechanical nature being a non-demolition measurement. It underlies the enzymatic conversion of a substrate into the product (an elementary metabolic phenomenon); the switching via separation of the direct and reverse routes in futile cycles provides the generation and complication of metabolic networks (coherence within cycles is maintained by the supramolecular organization of enzymes); the genetic level corresponding to the appearance of digital information is based on reflective arrows (catalysts realize their own self-reproduction) and operation of hypercycles. Every metabolic cycle via reciprocal regulation of both its halves can generate rhythms and spatial structures (resulting from the temporally organized depositions from the cycles). Via coherent events which percolate from the elementary submolecular level to organismic entities, self-assembly based on the molecular complementarity is realized and the dynamic informational field operating within the metabolic network is generated.

Localization of the fructose 1,6-bisphosphatase at the nuclear periphery

The localization of fructose 1,6-bisphosphatase (D-Fru-1,6-)2-1-phosphohydrolase, EC 3.1.3.11) in rat kidney and liver was determined immunohistochemically using a polyclonal antibody raised against the enzyme purified from pig kidney. The immunohistochemical analysis revealed that the bisphosphatase was preferentially localized in hepatocytes of the periportal region of the liver and was absent from the perivenous region. Fructose-1,6-bisphosphatase was also preferentially localized in the cortex of the kidney proximal tubules and was absent in the glomeruli, loops of Henle, collecting and distal tubules, and in the renal medulla. As indicated by immunocytochemistry using light microscopy and confirmed with the use of reflection confocal microscopy, the enzyme was preferentially localized in a perinuclear position in the liver and the renal cells. Subcellular fractionation studies followed by enzyme activity assays revealed that a majority of the cellular fructose-1,6-bisphosphatase activity was associated to subcellular particulate structures. Overall, the data support the concept of metabolic zonation in liver as well as in kidney, and establish the concept that the Fructose-1,6-bisphosphatase is a particulate enzyme that can not be considered a soluble enzyme in the classical sense.

Phosphoglycerate-kinase-glyceraldehyde-3-phosphate-dehydrogenase interaction. Molecular mass studies

When rabbit muscle phosphoglycerate kinase (PGK; a 48-kDa monomeric protein) and glyceraldehyde-3-phosphate dehydrogenase (GraPDH; a 145-kDa homotetrameric protein) are present together in solution in the proportion of 1 mol PGK/1 mol GraPDH monomer (total protein 0.2-1.0 mg/ml), an 80--82-kDa protein species is observed by gel-penetration (dilution factor) method and by the conventional procedure of elution from a gel column. Individually, PGK and GraPDH do not exhibit any self association or dissociation in the concentration range employed. Electrophoresis of the 80-82-kDa peak eluted from the gel column shows a single protein band with mobility intermediate between those of GraPDH and PGK. In titration experiments by the gel-penetration method, plots of dilution factor of PGK (or GraPDH) activity versus GraPDH (or PGK) concentration shows two linear portions intersecting at approximately 1 mol GraPDH monomer/1 mol PGK. From the molecular-mass values and the titration experiments, it has been suggested that, in solution, these enzymes form a complex consisting of 1 molecule of PGK and one monomeric subunit of GraPDH (expected molecular mass 84 kDa). Its dissociation constant has been estimated to be equal to or less than 13 nM. The complex is dissociated in the presence of KCl or NADH, with approximately half dissociation at 0.1 M salt or 0.25 mM NADH. At 0.1 M KCl, the complex is completely dissociated by adding ATP, NADH or 3-phosphoglycerate. AMP, ADP, NAD+, glyceraldehyde-3-phosphate, phosphate ions and fructose-1,6-bisphosphate reverse the effect of KCl.

Severe acidosis in a neonate with pulmonary valve stenosis: a possible stress inducer of a fatal syndrome of fructose-1, 6-biphosphatase and aldolase deficiency

A neonate is described whose clinical condition rapidly and irreversibly deteriorated on day two. He developed a profound acidosis, hypoglycaemia and a shock-like syndrome. The infant was centrally cyanosed and had a systolic murmur from a moderately severe pulmonary valve stenosis and a small atrial septal defect. The overwhelming acidosis was inconsistent with the severity of the congenital heart defects and as no infection was found a metabolic cause was sought. Liver tissue obtained at autopsy shortly after death on day four, showed deficiencies of fructose-1, 6-biphosphatase and aldolase.

A simple approach to detect active-site-directed enzyme-enzyme interactions. The aldolase/glycerol-phosphate-dehydrogenase enzyme system

A novel approach has been elaborated to identify the mechanism of intermediate transfer in interacting enzyme systems. The aldolase/glycerol-3-phosphate-dehydrogenase enzyme system was investigated since complex formation between these two enzymes had been demonstrated. The kinetics of dihydroxyacetone phosphate conversion catalyzed by the dehydrogenase in the absence and presence of aldolase was analyzed. It was found that the second-order rate constant (kcat/Km) of the enzymatic reaction decreases due to the formation of a heterologous complex. The decrease could be attributed to an increase of the Km value since kcat did not change in the presence of aldolase. In contrast, an apparent increase in the second-order rate constant of dihydroxyacetone phosphate conversion by the dehydrogenase was observed if the triose phosphate was produced by aldolase from fructose 1,6-bisphosphate (consecutive reaction). Moreover, no effect of dihydroxyacetone phosphate on the dissociation constant of the heterologous enzyme complex could be detected by physico-chemical methods. The results suggest that the endogenous dihydroxyacetone phosphate produced by aldolase complexed with dehydrogenase is more accessible for the dehydrogenase than the exogenous one, the binding of which is impeded due to steric hindrance by bound aldolase.

Isolation and characterization of a mutant liver aldolase in adult hereditary fructose intolerance. Identification of the enzyme variant by radioassay in tissue biopsy specimens

Hereditary fructose intolerance (HFI) is a metabolic disorder caused by enzymic deficiency of aldolase B, a genetically distinct cytosolic isoenzyme expressed exclusively in liver, kidney, and intestine. The molecular basis of this enzyme defect has been investigated in three affected individuals from a nonconsanguineous kindred, in whom fructose-l-phosphate aldolase activities in liver or intestinal biopsy samples were reduced to 2-6% of mean control values. To identify a putative enzyme mutant in tissue extracts, aldolase B was purified from human liver by affinity chromatography and monospecific antibodies were prepared from antiserum raised in sheep. Immunodiffusion gels showed a single precipitin line common to pure enzyme and extracts of normal liver and intestine, but no reaction with extracts of brain, muscle, or HFI liver. However, weak positive staining for aldolase in hepatocyte and enterocyte cytosol was demonstrated by indirect immunofluorescence of HFI tissues. This was abolished by pretreatment with pure enzyme protein. Accordingly, a specific radioimmunoassay (detection limit 7.5 ng) was established to quantify immunoreactive aldolase B in human biopsy specimens. Extracts of tissue from affected patients gave 10-25% immunoreactive enzyme in control samples; immunoreactive aldolase in intestinal extracts from four heterozygotes was reduced (to 55%) when compared with seven samples from normal control subjects (P < 0.05). In extracts of HFI tissues, there was a sevenfold reduction in apparent absolute specific activity (1.02 vs. 8.82 U/mg) of immunoreactive fructose-l-phosphate aldolase B, but the apparent specific activity in heterozygotes (7.71 U/mg) was only slightly impaired. Displacement radioimmunotitration of aldolase B in liver supernatants showed a significant (P < 0.005) decrease in antibody avidity for immunoreactive protein in HFI tissue when compared with the pure enzyme or extract of normal control liver. Immunoaffinity chromatography on antialdolase B-Sepharose facilitated isolation and purification of enzyme from liver biopsy specimens. Active aldolase in normal liver, with substrate activity ratios and Michaelis constants identical to biochemically purified human enzyme, could be recovered from antibody columns. Chromatography on monospecific Fab' antialdolase B enabled pure enzyme protein to be retrieved quantitatively from normal control and HFI liver: direct chemical assay showed 1.88 and 1.15 mg aldolase protein/g of tissue, respectively. This confirmed that the catalytic properties of the HFI aldolase were profoundly impaired with specific activities of fructose-l-phosphate cleavage of 7.21 and 0.07 U/mg, respectively. Radioimmunoassay gave estimates of 7.66 and 1.18 U/mg, respectively. Sodium dodecyl sulfate-polyacrylamide electrophoresis indicated that immunopurified aldolase from HFI liver possessed a single subunit size similar to material from control liver extracts: M(r) 39,100 vs. 37,900+/-700 (SD) D, respectively. Electrofocusing under denaturing conditions of aldolase isolated in parallel from control and HFI liver revealed the same complement of subunits and, despite qualitative differences in distribution of bands during degradation, no additional charged species. Fructose phosphate aldolase deficiency in hereditary fructose intolerance is attended by the synthesis of an immunoreactive, but functionally and structurally modified enzyme variant that results from a restricted genetic mutation.

Limited proteolysis of liver aldolase and fructose 1,6-bisphosphatase by lysosomal proteinases: effect on complex formation

Cathepsin M, which catalyzes inactivation of both rabbit liver fructose-1,6-bisphosphate aldolase (EC 4.1.2.13) and rabbit liver fructose 1,6-bisphosphatase (Fru-P2ase; EC 3.1.3.11), has been characterized as a peptidyl peptidase. Modification of the COOH terminus of aldolase by cathepsin M or by Fru-P2ase converting enzyme 2 abolishes its ability to bind to phosphocellulose P11 and to form the complex with Fru-P2ase. On the other hand, modification of the COOH terminus of Fru-P2ase does not affect its interaction with aldolase. This property is lost, however, when Fru-P2ase is modified in the NH2-terminal region by the converting enzyme or by subtilisin. The results suggest that interaction of aldolase and Fru-P2ase may involve the exposed COOH-terminal region of the former and an exposed proteinase-sensitive region located between residues 57 and 67 of the latter.