Immunochemical evidence for the presence of advanced glycation end products in human lens proteins and its positive correlation with aging

Prolonged incubation of protein with reducing sugar proceeds through a series of reactions involving early stage products to the advanced glycation end products with fluorescence, brown color, and cross-linking. Known collectively as the Maillard reaction, these changes have been suggested as factors in diabetic complications and the aging process. The early stage products have been demonstrated in vivo, but evidence for the presence in vivo of the advanced glycation end products has been limited. We sought to provide immunochemical evidence by the preparation and use of polyclonal and monoclonal antibodies to these end products (Horiuchi, S., Araki, N., and Morino, Y. (1991) J. Biol. Chem. 266, 7329-7332) as probes to identify and quantitate such compounds in human lens crystallins. Neither of the antibodies reacted with extracts from infant lenses, but fractions from adult lenses showed a significant reactivity, correlating with lens age. Our findings provide the first immunochemical evidence that human lens crystallins contain advanced glycation end products and that these products increase with tissue age.

Exposure of rat peritoneal macrophages to acetylated low density lipoprotein results in release of plasma membrane cholesterol. An efficient substrate for esterification by acyl-CoA:cholesterol acyltransferase

In J774 macrophages and murine macrophages stimulated with acetylated low density lipoprotein (acetyl-LDL), the plasma membrane free cholesterol (FC) became accessible to acyl-CoA:cholesterol acyltransferase (ACAT) as substrate, the result being an accumulation of cholesteryl esters (CE) (Tabas, I., Rosoff, W. J., and Boykow, G. C (1988) J. Biol. Chem. 263, 1266-1272). As the route of delivery of FC to ACAT was not well characterized, we examined this route in the present study. In foam cells derived from rat peritoneal macrophages by preincubation with acetyl-LDL, esterification of the exogenously labeled [3H]FC was low (1.3% of total labeled cholesterol). In contrast, when cells were first labeled with exogenous [3H]FC and then chased with acetyl-LDL, the esterification was more extensive (9.2% of the total labeled cholesterol). During this experiment a significant portion of cellular [3H]FC was released into the medium (up to 33.4% of the total labeled cholesterol). In experiments using a two-compartment chamber in which cells in the lower and upper chambers were separated by filter paper yet the cells in both compartments could communicate without direct contact, [3H]FC released into the medium was biologically active and could serve as an efficient substrate for ACAT. Thus, when acetyl-LDL is not included in culture medium, FC delivery from the macrophage plasma membrane to ACAT is not enhanced, whereas in the presence of acetyl-LDL, plasma membrane FC released and bound to acetyl-LDL may re-enter the cells, possibly through the scavenger receptor. This would provide a significant route for CE synthesis in macrophages.

Purification and properties of an ethylene-forming enzyme from Pseudomonas syringae pv. phaseolicola PK2

A novel ethylene-forming enzyme that catalyses the formation of ethylene from 2-oxoglutarate was purified from a cell-free extract of Pseudomonas syringae pv. phaseolicola PK2. It was purified about 2800-fold with an overall yield of 53% to a single band of protein after SDS-PAGE. The purified enzyme had a specific activity of 660 nmol ethylene min-1 (mg protein)-1. The molecular mass of the enzyme was approximately 36 kDa by gel filtration and 42 kDa by SDS-PAGE. The isoelectric point and optimum pH were 5.9 and ca. 7.0-7.5, respectively. There was no homology between the N-terminal amino acid sequence of the ethylene-forming enzyme of Ps. syringae pv. phaseolicola PK2 and the sequence of the ethylene-forming enzyme of the fungus Penicillium digitatum IFO 9372. However, the two enzymes have the following properties in common. The presence of 2-oxoglutarate, L-arginine, Fe2+ and oxygen is essential for the enzymic reaction. The enzymes are highly specific for 2-oxoglutarate as substrate and L-arginine as cofactor. EDTA, Tiron, DTNB [5,5'-dithio-bis(2-nitrobenzoate)] and hydrogen peroxide are all effective inhibitors.

Differential expression of two types of the neurofibromatosis type 1 (NF1) gene transcripts related to neuronal differentiation

A 360 residue region encoded by the neurofibromatosis type 1 (NF1) gene shows significant homology to the catalytic domains of both mammalian GTPase-activating proteins (GAP) and yeast IRA proteins. This GAP-related domain of the NF1 gene (NF1-GRD), like the GAP and IRA protein, has been reported to mediate hydrolysis of Ras-bound GTP to GDP, resulting in inactivation of Ras protein. In the present study, we identified two different types of NF1-GRD cDNA. One (type I) is identical to the previously reported sequence, and the other (type II) contained an additional 63 bp insertion that encodes for a region of 21 amino acids in the center of the NF1-GRD molecule. Alternative splicing is the most likely mechanism by which these two types of transcripts arise. Our observations reveal that the type I transcript is predominantly expressed in undifferentiated cells, whereas the type II transcript predominates in differentiated cells. Furthermore, the expression pattern of type I and type II NF1-GRD mRNA immediately changed in SH-SY5Y neuroblastoma cells when neuronal differentiation programs were induced by retinoic acid treatment. We propose that the differential expression of type I and type II NF1-GRD transcripts might be an 'on/off' switch that regulates the catalytic activity of the NF1 gene product, which plays an important role in the regulation of neuronal differentiation.

Immunochemical approach to characterize advanced glycation end products of the Maillard reaction. Evidence for the presence of a common structure

Reaction of protein amino groups with glucose (the Maillard reaction) leads from early stage products such as Schiff base and Amadori products to advanced glycation end products (AGE), structures implicated in diabetic complications and the aging process. We have prepared the polyclonal anti-AGE antibody and the monoclonal anti-AGE antibody against AGE-bovine serum albumin and made an immunochemical approach to characterize AGE structures. Both polyclonal and monoclonal antibodies reacted with AGE-proteins such as AGE-bovine serum albumin, AGE-human serum albumin, and AGE-hemoglobin but not with unmodified counterparts. Treatments of these AGE-proteins with borohydride had no effect on the immunoreactivity. Moreover, fructosyl-epsilon-caproic acid, a synthetic Amadori compound, did not serve as an antigen, indicating that these antibodies were specific for AGE products but not for early stage products of the Maillard reaction. In addition, these antibodies were also able to recognize AGE products prepared either from alpha-tosyl-1-lysine, alpha-tosyl-1-lysine methyl ester, monoaminocarboxylic acid such as epsilon-aminocaproic acid, gamma-amino-n-butyric acid, and beta-alanine. Thus, these results strongly suggest the presence of a common structure in AGE preparations, regardless of whether AGE products are generated from proteins, amino acids, or monoaminocarboxylic acids.

The role of His143 in the catalytic mechanism of Escherichia coli aspartate aminotransferase

In aspartate aminotransferase (AspAT), His143 is located within a hydrogen-bonding distance to Asp222 that forms a strong ion pair with the ring nitrogen of the coenzyme, pyridoxal 5'-phosphate (PLP) or pyridoxamine 5'-phosphate (PMP). His143 of Escherichia coli AspAT was replaced by Ala or Asn. The mutant enzyme H143A showed a slight increase in the maximum velocity of the overall transamination reaction between aspartate and 2-oxoglutarate, while H143N AspAT showed a decrease to 60% in the maximum rate of the overall reactions in both directions. In all of the half-transamination reactions with four substrates, aspartate, glutamate, oxalacetate, and 2-oxoglutarate, the catalytic competence as defined by kmax/Kd decreased by 3-18-fold upon replacing His143 by either Ala or Asn. The extent of the decrease varied from one substrate to another; it was largely contributed to by the decrease in affinities for all substrates. The equilibrium constants, [PMP-form] [keto acid]/[( PLP-form] [amino acid]), decreased by over 10-fold upon the mutations at position 143. Both H143A and H143N AspATs exhibited a considerably decreased affinity for 2-methylaspartate, an external-aldimine-forming substrate analogue, yet without appreciable alteration in the affinity for succinate and glutarate, which are non-aldimine-forming analogues. All these findings suggest that, although His143 is not essential for catalysis, it might assist the formation of enzyme-substrate complex.

Tyr225 in aspartate aminotransferase: contribution of the hydrogen bond between Tyr225 and coenzyme to the catalytic reaction

Tyr225 in the active site of Escherichia coli aspartate aminotransferase (AspAT) was replaced by phenylalanine or arginine by site-directed mutagenesis. X-ray crystallographic analysis of Y225F AspAT showed that the benzene ring of Phe225 was situated at the same position as the phenol ring of Tyr225 in wild-type AspAT. The mutations resulted in a great decrease in the rate of the transamination reaction, suggesting that Tyr225 is important for efficient catalysis. The kinetic analysis of half-transamination reactions of Y225F AspAT with four substrates (aspartate, glutamate, oxalacetate, and 2-oxoglutarate) and some analogues (2-methylaspartate, succinate, and glutarate) revealed a considerable increase in the affinities for all these compounds. In contrast, affinity for the amino acid substrates was decreased by mutation to arginine, but affinities for the keto acid substrates and the two dicarboxylates (succinate and glutarate) were increased. The electrostatic interaction between O(3') of the coenzyme [pyridoxal 5'-phosphate (PLP)] and the residue at position 225 affected the pKa value of the Schiff base, which is formed between the epsilon-amino group of Lys258 and the aldehyde group of PLP; based on the spectrophotometric titration the pKa values were determined to be 6.8 for wild-type AspAT, 8.5 for Y225F AspAT, and 6.1 for Y225R AspAT in the absence of substrate. The absorption spectra of the three AspATs were almost identical in the acidic pH region, but the spectrum of Y225F AspAT differed from that of wild-type or Y225R AspAT in the alkaline pH region.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical cross-linking alters high-density lipoprotein to be recognized by a scavenger receptor in rat peritoneal macrophages

Rat peritoneal macrophages possess a surface receptor for high-density lipoprotein (HDL). To obtain the functional aspect of the HDL receptor, the present study was undertaken to modify HDL with three different cross-linkers; dimethylsuberimidate, disuccinimidylsuberate and dithiobissuccinimidylpropionate (DSP) and determine their effect on the ligand activity for the HDL receptor. Upon modification at a low reagent concentration, DSP was found to be most effective in cross-linking of HDL apolipoproteins. The ligand activity of DSP-HDL for the HDL receptor was reduced by greater than 60%. Experiments with these macrophages at 37 degrees C showed; (i) the amounts of the cell-associated [125I]DSP-HDL as 3.5-fold higher than [125I]HDL; (ii) the cell-association of [125I]DSP-HDL was effectively (greater than 70%) inhibited by unlabeled DSP-HDL, whereas HDL showed a partial inhibition (30%); (iii) [125I]DSP-HDL underwent chloroquine-sensitive intracellular degradation; and (iv) DSP-HDL induced a 3-fold increase in the incorporation of [14C]oleic acid into cholesteryl oleate when compared with unmodified HDL. Experiments at 0 degrees C showed that the cellular binding of [125I]DSP-HDL was competed by acetylated low-density lipoprotein and dextran sulfate. These findings indicate that DSP-HDL is recognized as a ligand by a scavenger receptor of rat peritoneal macrophages, a notion consistent with HDL modified with tetranitromethane (Kleinherenbrink-Stins, M.F. et al. (1989) J. Lipid Res. 39, 511-520).

Biochemical demonstration of endocytosis and subsequent resecretion of high-density lipoprotein by rat peritoneal macrophages

To investigate the post-binding events of high-density lipoprotein (HDL), rat peritoneal macrophages were enriched by collagen gel-coated plates and incubated in a cell-suspension system with fluorescein isothiocyanate-labeled HDL (FITC-HDL), followed by fluorescence spectroscopic analyses. Upon incubation with FITC-HDL at 37 degrees C for 30 min, the microenvironmental pH of the cell-associated FITC-HDL was 6.50, whereas a 0 degree C-incubation gave a corresponding pH of 7.15. Carbonyl cyanide m-chlorophenylhydrazone, a protonophore known to dissipate the proton gradient, restored the former acidic pH (pH 6.40) to pH 7.20, but had no effect on the latter. This indicates that cell surface-bound HDL is internalized and exposed to an acidic compartment. When cells were incubated with FITC-HDL at 37 degrees C for 30 min and the cell-associated FITC-HDL was chased at 37 degrees C, the fluorescence intensity at 490 nm showed a time-dependent increase. This increase was explained by a release of endocytosed FITC-HDL into the extracellular medium but not by a simple outward dissociation of the cell-associated FITC-HDL. The microenvironmental acidic pH of the cell-associated FITC-HDL changed to a less acidic pH during the chase whereas that of FITC-HDL became constant after released into the medium, indicating that endocytosed HDL was resected back into the extracellular medium. This resection process was temperature-dependent and accelerated by HDL itself. These results provide the biochemical evidence for the presence of an endocytic-exocytic pathway for HDL in rat peritoneal macrophages.

Porcine cytosolic aspartate aminotransferase reconstituted with [4'-13C]pyridoxal phosphate. pH- and ligand-induced changes of the coenzyme observed by 13C NMR spectroscopy

Apoenzyme samples of aspartate aminotransferase (AspAT) purified from the cytosolic fraction of pig heart were reconstituted with [4'-13C]pyridoxal 5'-phosphate (pyridoxal-P). The 13C NMR spectra of AspAT samples thus generated established the chemical shift of 165.3 ppm for C4' of the coenzyme bound as an internal aldimine with lysine 258 of the enzyme at pH 5. In the absence of ligands the chemical shift of C4' was shown to be pH dependent, shifting 5 ppm upfield to a constant value of 160.2 ppm above pH 8, the resulting pKa of 6.3 in agreement with spectrophotometric titrations. The addition of the competitive inhibitor succinate to the internal aldimine raises the pKa of the imine to 7.8, consistent with the theory of charge neutralization in the active site. In the presence of saturating concentrations of 2-methylaspartic acid the C4' signal of the coenzyme was shown to be invariant with pH and located at 162.7 ppm, midway between the observed chemical shifts of the protonated and unprotonated forms of the internal aldimine. The intermediate chemical shift of the external aldimine complex is thought to reflect the observation of an equilibrium mixture composed of roughly equal populations of the protonated ketoenamine and a dipolar anion species, corresponding to their respective spectral bands at 430 and 360-370 nm. Conversion to the pyridoxamine form was accomplished via reaction of the internal aldimine with L-cysteinesulfinate or by reduction with sodium borohydride, and the resulting C4' chemical shifts were identified by difference spectroscopy. Finally, the line widths of the C4' resonance under the various conditions were measured and qualitatively compared. The results are discussed in terms of the current mechanism and molecular models of the active site of AspAT.

Structural and functional role of the amino-terminal region of porcine cytosolic aspartate aminotransferase. Catalytic and structural properties of enzyme derivatives truncated on the amino-terminal side

In porcine cytosolic aspartate aminotransferase, a dimeric enzyme, the amino-terminal region anchoring onto the neighboring subunit is linked to the adjoining floppy peptide segment (residues 12-47), an integral part of the small domain whose facile movement upon substrate binding is a striking "induced fit" feature of this enzyme. To assess the contribution by the amino-terminal region to small domain movement and protein stability, a series of enzyme derivatives truncated on the amino-terminal side (residues 1-9) was prepared by using oligonucleotide-directed in vitro mutagenesis. Deletion of residues 1-3 showed no effect on catalytic activity and heat stability. Del 1-5 mutant enzyme with an extra methionine at position 5 showed only 43% of the kappa cat value (in the overall transamination) of the wild-type enzyme. Further deletion up to residue 9 resulted in a slight decrease in kappa cat values. Del 1-9 mutant enzyme still retained a kappa cat value of 33% that of wild-type enzyme. Km values for aspartate and 2-oxoglutarate increased sharply upon deletion of residues 1-9. Accordingly, Del 1-9 mutant enzyme showed a striking decrease in the kappa cat/Km value, to only 2% of that for the wild-type enzyme. Deletion of amino-terminal residues 1-9 resulted also in a large decrease in thermostability and in an enhanced susceptibility to limited proteolysis by protease 401, which is known to cleave at Leu20 of the wild-type enzyme. These findings indicate that an increase in the conformational freedom of the floppy segment (residues 12-47) would occur upon the loss of most of the anchorage region, thereby presenting an entropic barrier to conformational changes that facilitate substrate binding with high affinity.

Inhibition of carrageenin-induced paw edema by a superoxide dismutase derivative that circulates bound to albumin

Although the possible involvement of superoxide radical and its metabolite(s) in the pathogenesis of various types of edema have been suggested, direct evidence supporting this concept is lacking. Since intravenously administered Cu2+Zn2(+)-type superoxide dismutase (SOD) rapidly disappeared from the circulation with a half-life of 4 min, the enzyme could not be used to test whether superoxide radicals played a critical role in the modulation of vascular permeability. We previously synthesized a SOD derivative (SM-SOD) by linking poly(styrene co-maleic acid butyl ester) (SM) to the enzyme (Ogino, T., Inoue, M., Ando, Y., Awai, M., Maeda, H. and Morino Y. (1988) Int. J. Pept. Protein Res. 32, 1583-1588); SM-SOD circulates bound to albumin with a half-life of 6 h. To test whether superoxide radicals play an important role in the regulation of vascular permeability, the effect of SM-SOD on experimental paw edema was studied in the rat. Subcutaneous injections of carrageenin to the paw rapidly induced local edema by increasing vascular permeability. Intravenous administration of SM-SOD markedly inhibited the carrageenin-induced increase in vascular permeability and suppressed the development of paw edema. In contrast, the same dose of SOD showed no such inhibitory effect. These results suggest that superoxide radical and/or its metabolite(s) might play a critical role in the pathogenesis of carrageenin-induced vasogenic edema.

The significance of serum mitochondrial aspartate aminotransferase activity in the surgical field

A study was carried out on the clinical significance of changes in the serum level of mitochondrial aspartate aminotransferase before and after surgery on patients suffering from various hepato-biliary diseases. The patients included those whose livers were impaired with cirrhosis, jaundice, or direct surgical intervention such as hepatectomy. It was found that the postoperative liver function of patients whose preoperative values of serum mitochondrial aspartate aminotransferase had been less than 10 Karmen units recovered with a favorable course, whereas that of patients whose values had exceeded 20 units deteriorated with persistent jaundice, ascites or hepatic insufficiency. Both the sensitivity and specificity of respective liver function tests were examined by the preoperative values of routine liver function tests and the postoperative liver functions. The results revealed the serum value of mitochondrial aspartate aminotransferase to be more sensitive than the other tests, while specificity was not significantly different. A safety limit for a favorable postoperative course in terms of liver function was observed in patients showing a preoperative serum mitochondrial aspartate aminotransferase activity of below 10 units. Thus, serum mitochondrial aspartate aminotransferase activity could be applied as a useful marker for hepato-biliary surgery.

Cholesterol uptake by human glioma cells via receptor-mediated endocytosis of low-density lipoprotein

Low-density lipoprotein (LDL) is a carrier of the cholesterol found in human plasma. Cells utilize cholesterol for membrane synthesis by taking up LDL via receptor-mediated endocytosis. In the present study, interactions of LDL with human malignant glioma cell lines (U-251 MG and KMG-5) were investigated biochemically and morphologically. The LDL, labeled with the fluorescent dyes 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine (DiI) and fluorescein isothiocyanate (FITC), was internalized by both cell processes and cell bodies. Reductive methylation of DiI-labeled LDL, which abolishes the ability of the cell to bind to the LDL receptor, prevented the internalization of the cholesterol moiety of LDL. Cellular binding of 125I-LDL to U-251 MG cells at 4 degrees C revealed the presence of a specific saturable-associated receptor (dissociation constant (Kd) approximately 38 micrograms/ml). Endocytic uptake of 125I-LDL or 3H-cholesterol oleate-labeled LDL (3H-LDL) at 37 degrees C demonstrated the cell-associated 125I-LDL and 3H-LDL increase. The intracellular degradation of protein moiety increased linearly with time. Reductive methylation of 3H-LDL led to a remarkable decrease in the cell-associated cholesterol moiety of LDL. The difference in uptake of the cholesterol moiety of LDL between U-251MG cells and KMG-5 cells showed that the U-251MG cells, which proliferate more actively than KMG-5 cells, take up more of the cholesterol moiety of LDL than do the KMG-5 cells. Thus, LDL cholesterol seems to be endocytosed predominantly via the LDL receptor present on the plasma membrane of malignant glioma cells. In addition, for growth, these cells may require large amounts of the cholesterol moiety of LDL.

Inhibition of corneal inflammation by an acylated superoxide dismutase derivative

Superoxide radicals and their metabolite(s) have been postulated to play an important role in the pathogenesis of inflammation. Hence, superoxide dismutase (SOD) has been used to reduce tissue injury caused by reactive oxygens. However, protection of the cornea and other ocular tissues from oxygen toxicity could not be achieved by administering SOD presumably due to its unfavorable in vivo behavior. To scavenge superoxide radicals on the outer surface of corneal epithelial cells, the authors synthesized an acylated SOD derivative (AC-SOD) by linking capric acid. When instilled into rabbit eyes, a significant amount of AC-SOD remained bound to the corneal surface for a fairly long time. Intracorneal injection of lipopolysaccharide (LPS) triggered infiltration of polymorphonuclear leukocytes (PMNs) to the cornea and induced severe keratitis. Topical administration of AC-SOD to the LPS-treated cornea markedly inhibited the infiltration of PMNs and suppressed the occurrence of keratitis. Under identical conditions, topically administered SOD was rapidly removed by tears and, hence, did not inhibit LPS-induced keratitis. When the number of PMNs in the systemic circulation was reduced by intravenous administration of hydroxyurea, LPS-induced keratitis was inhibited markedly. These results indicate that superoxide radicals and circulating PMNs might play a critical role in LPS-induced keratitis.

Inhibition of oxygen toxicity by targeting superoxide dismutase to endothelial cell surface

Since enzymes that degrade reactive oxygens, such as superoxide dismutase (SOD), are significantly lower in plasma than in intracellular compartments, cell surface membranes should be protected against hazardous oxygens particularly when animals are challenged with oxidative stress. To minimize oxygen toxicity on endothelial cell surface, a fusion gene consisting of cDNA coding human Cu2+/Zn2(+)-SOD and heparin-binding peptide was constructed and expressed in yeast. The resulting enzyme (HB-SOD) bound to a heparin-Sepharose column and cultured endothelial cells; binding was inhibited either by high NaCl concentrations or heparin. When injected intravenously, HB-SOD predominantly bound to vascular endothelial cell surface. Carrageenin-induced paw edema and cold-induced brain edema of the rat were markedly inhibited by a single dose of HB-SOD. These results suggest that superoxide radical and/or its metabolite(s) occurring at or near the outer surface of vascular endothelial cells might play a critical role in the pathogenesis of vasogenic edema.

Inhibition of stress-induced gastric mucosal injury by a long acting superoxide dismutase that circulates bound to albumin

To determine whether oxygen-derived free radicals play an important role in the pathogenesis of stress-induced tissue injury, the effect of a superoxide dismutase derivative, which binds to albumin and circulates with a half-life of 6 h in intact rats, on acute gastric mucosal lesion was observed in rats which were given water-immersion-restraint. This enzyme derivative also circulated bound to albumin with a half-life of 8 h in rats which were challenged with water-immersion-restraint. This treatment significantly perturbed systemic circulation of animals by decreasing the effective volume of circulating blood, increased vascular permeability of the gastric mucosa, and induced acute gastric mucosal lesion. Intravenous administration of this enzyme derivative normalized both systemic circulation and vascular permeability of the gastric mucosa and prevented the occurrence of stress-induced gastric injury. These findings suggest that the superoxide radical and/or its metabolite(s) plays an important role in the pathogenesis of stress-induced acute gastric mucosal lesion.

The significance of serum mitochondrial aspartate aminotransferase activity in obstructive jaundice: experimental and clinical studies

The serum level of mitochondrial aspartate aminotransferase was determined in experimental and clinical obstructive jaundice, using an immunoabsorbance method which allowed the differential determination of cytosolic and mitochondrial isozymes in the serum. In experimental obstructive jaundice using dogs, the serum mitochondrial aspartate aminotransferase value rapidly decreased to normal after biliary decompression following a period of biliary obstruction of within 3 weeks. On the other hand, when the period of jaundice was prolonged for 5 weeks, the activity of the enzyme after biliary drainage still continued to show high values, being 14.2 +/- 1.8 Karmen units at 4 weeks following biliary decompression. Determination of aspartate aminotransferase activity in tissue from such organs as the liver, heart, kidney, skeletal muscle and brain, as well as serum samples withdrawn from local veins, confirmed that high serum values of the enzyme in experimental obstructive jaundice were mostly attributable to hepatic impairment induced by biliary obstruction not by secondarily damaged tissues of other organs. Mitochondrial aspartate aminotransferase proved to be a more useful marker than other routine tests in icteric dogs. In 13 clinical patients with obstructive jaundice, decreasing rates of serum mitochondrial aspartate aminotransferase on the 7th and 14th postoperative days could be applied to evaluate the viability of the icteric liver. The decreasing rates were more advantageous than the preoperative activity itself in predicting the postoperative function of the liver. Thus, mitochondrial aspartate aminotransferase appears to serve as a useful marker for assessing the liver function in obstructive jaundice.

Pre-steady-state kinetics of Escherichia coli aspartate aminotransferase catalyzed reactions and thermodynamic aspects of its substrate specificity

The four half-transamination reactions [the pyridoxal form of Escherichia coli aspartate aminotransferase (AspAT) with aspartate or glutamate and the pyridoxamine form of the enzyme with oxalacetate or 2-oxoglutarate] were followed in a stopped-flow spectrometer by monitoring the absorbance change at either 333 or 358 nm. The reaction progress curves in all cases gave fits to a monophasic exponential process. Kinetic analyses of these reactions showed that each half-reaction is composed of the following three processes: (1) the rapid binding of an amino acid substrate to the pyridoxal form of the enzyme; (2) the rapid binding of the corresponding keto acid to the pyridoxamine form of the enzyme; (3) the rate-determining interconversion between the two complexes. This mechanism was supported by the findings that the equilibrium constants for half- and overall-transamination reactions and the steady-state kinetic constants (Km and kcat) agreed well with the predicted values on the basis of the above mechanism using pre-steady-state kinetic parameters. The significant primary kinetic isotope effect observed in the reaction with deuterated amino acid suggests that the withdrawal of the alpha-proton of the substrates is rate determining. The pyridoxal form of E. coli AspAT reacted with a variety of amino acids as substrates. The Gibbs free energy difference between the transition state and the unbound state (unbound enzyme plus free substrate), as calculated from the pre-steady-state kinetic parameters, showed a linear relationship with the accessible surface area of amino acid substrate bearing an uncharged side chain.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of replacement of tryptophan-140 by phenylalanine or glycine on the function of Escherichia coli aspartate aminotransferase

Trp140 of E. coli aspartate aminotransferase has been converted to Phe or Gly by site-directed mutagenesis. As compared to the wild-type enzyme, either of the mutant enzymes showed 10- to 100-fold increase in Km's for natural dicarboxylic substrates, but did not show appreciable changes in Km's for aromatic substrates. Teh kcat values for dicarboxylic and aromatic substrates were greatly decreased by [Trp140----Gly] mutation, but were decreased to lesser extents by [Trp140----Phe] mutation. These findings suggested that N(1) of Trp140 may not be essential for catalysis, but may be partly involved in the binding of the distal carboxylate group of the dicarboxylic substrates.