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Limited proteolysis as a probe of conformational changes in aspartate aminotransferase from Sulfolobus solfataricus.

The analysis of conformational transitions using limited proteolysis was carried out on a hyperthermophilic aspartate aminotransferase isolated from the archaebacterium Sulfolobus solfataricus, in comparison with pig cytosolic aspartate aminotransferase, a thoroughly studied mesophilic aminotransferase which shares about 15% similarity with the archaebacterial protein. Aspartate aminotransferase from S. solfataricus is cleaved at residue 28 by thermolysin and residues 32 and 33 by trypsin; analogously, pig heart cytosolic aspartate aminotransferase is cleaved at residues 19 and 25 [Iriarte, A., Hubert, E., Kraft, K. & Martinez-Carrion, M. (1984) J. Biol. Chem. 259, 723-728] by trypsin. In the case of aspartate aminotransferase from S. solfataricus, proteolytic cleavages also result in transaminase inactivation thus indicating that both enzymes, although evolutionarily distinct, possess a region involved in catalysis and well exposed to proteases which is similarly positioned in their primary structure. It has been reported that the binding of substrates induces a conformational transition in aspartate aminotransferases and protects the enzymes against proteolysis [Gehring, H. (1985) in Transaminases (Christen, P. & Metzler, D. E., eds) pp. 323-326, John Wiley & Sons, New York]. Aspartate aminotransferase from S. solfataricus is protected against proteolysis by substrates, but only at high temperatures (greater than 60 degrees C). To explain this behaviour, the kinetics of inactivation caused by thermolysin were measured in the temperature range 25-75 degrees C. The Arrhenius plot of the proteolytic kinetic constants measured in the absence of substrates is not rectilinear, while the same plot of the constants measured in the presence of substrates is a straight line. Limited proteolysis experiments suggest that aspartate aminotransferase from S. solfataricus undergoes a conformational transition induced by the binding of substrates. Another conformational transition which depends on temperature and occurs in the absence of substrates could explain the non-linear Arrhenius plot of the proteolytic kinetic constants. The latter conformational transition might also be related to the functioning of the archaebacterial aminotransferase since the Arrhenius plot of kcat is non-linear as well.

Amino Acid Sequence↗

Glycation and inactivation of aspartate aminotransferase in diabetic rat tissues.

Glycated cytosolic aspartate aminotransferase was detected in the liver and kidney of streptozotocin diabetic rats using a boronate affinity column for adjacent cis-hydroxyl groups and an immunoblotting technique. The enzymatic activity and amount of immunoreactive substance were determined in the liver, kidney, and erythrocytes of diabetic and control rats. The ratio of enzymatic activity to the amount of enzyme was lower in diabetic rat tissue than in that in the control rats. It has been suggested that there is an inactive aspartate aminotransferase molecule in the tissues of diabetic rats. We therefore suggest that the cytosolic aspartate aminotransferase was inactivated in the diabetic rat tissues by a glycation reaction, accompanied by an impairment in glucose utilization.

Alkaline Phosphatase↗

Apoenzyme of aspartate aminotransferase isozymes in serum and its diagnostic usefullness for hepatic diseases.

Aspartate aminotransferase in the sera of normal subjects and of patients with hepatic diseases has been immunologically separated into two isoenzymes, cytosolic aspartate aminotransferase and mitochondrial aspartate aminotransferase. The activity of the isoenzymes was measured in three different buffer solutions with or without pyridoxal 5'-phosphate. To attain maximal activation, the apoenzyme of mitochondrial fraction must be preincubated with pyridoxal 5'-phosphate longer than that of the cytosolic fraction in either of the three reaction mixtures. In most sera the activity of both isoenzymes increased substantially in the presence of pyridoxal 5'-phosphate regardless of the type of buffer solutions. Both the apoenzymatic activity and the ratio of apo- to holo-enzymatic activity of each of the isoenzymes varied among samples from the patients with hepatic diseases. However, significantly high ratios of apo- to holo-enzymatic activity of both isoenzymes were observed in the patients with hepatoma in contrast with those with other hepatic diseases. These findings suggest that the simultaneous measurement of both apo- and holo-enzyme activities of aspartate aminotransferase isoenzymes may be useful in the clinical assessment of hepatic diseases.

Apoenzymes↗

Turnover of cytoplasmic and mitochondrial aspartate aminotransferase isozymes in mouse liver and transplantable hepatomas.

Activities of the cytoplasmic and mitochondrial isozymes of aspartate aminotransferase (aspartate:2-oxoglutarate aminotransferase, EC 2.6.1.1, AAT) in transplantable mouse hepatomas BW7756 and H-4 are reduced when compared to normal adult liver. Both proteins have been purified to homogeneity from a single preparation of mouse liver and monospecific antibodies raised to each isozyme. By quantitative immunotitration analysis, the activity of each isozyme in liver and hepatoma has been shown to correlate with levels of immunoprecipitable protein. Furthermore, for each isozyme, the liver versus hepatoma species is indistinguishable by heat inactivation kinetics, Km's for substrates, and molecular weights. Thus, the reduction of mitochondrial and cytoplasmic AAT activities in hepatoma tissue is due not to alterations in the catalytic activity of the enzyme molecules, but to a decrease in the number of enzyme molecules present. Turnover of the isozymes was studied in liver and hepatoma tissue using in vivo radiolabeling and specific immunoprecipitation techniques. The cytoplasmic isozyme has a similar rate of degradation in liver and hepatoma, while the rate of synthesis of this isozyme in hepatoma is approximately tenfold less than in liver. The mitochondrial isozyme is also degraded at a similar rate in both tissues, but the rate of synthesis is sixfold greater in normal liver tissue than in hepatoma. It is concluded that decreased amounts of both isozymes in hepatoma as compared to liver are the result of a reduction in the rate of synthesis of each isozyme without any change in the rate of degradation.

Animals↗

Red cell aspartate aminotransferase saturation with oral pyridoxine intake.

CONTEXT AND OBJECTIVE: The coenzyme of aspartate aminotransferase is pyridoxal phosphate, generated from fresh vegetables containing pyridoxine. Vitamin B6-responsive sideroblastic anemia, myelofibrosis and Peyronies syndrome respond to high pyridoxine doses. The objective was to investigate the oral pyridoxine oral dose that would lead to maximized pyridoxal phosphate saturation of red cell aspartate aminotransferase. DESIGN AND SETTING: Controlled trial, in Hematology Division of Instituto Adolfo Lutz. METHODS: Red cell aspartate aminotransferase activity was assayed (before and after) in normal volunteers who were given oral pyridoxine for 15-18 days (30 mg, 100 mg and 200 mg daily). In vitro study of blood from seven normal volunteers was also performed, with before and after assaying of aspartate aminotransferase activity. RESULTS: The in vivo study showed increasing aspartate aminotransferase saturation with increasing pyridoxine doses. 83% saturation was reached with 30 mg daily, 88% with 100 mg, and 93% with 200 mg after 20 days of oral supplementation. The in vitro study did not reach 100% saturation. CONCLUSIONS: Neither in vivo nor in vitro study demonstrated thorough aspartate aminotransferase saturation with its coenzyme pyridoxal phosphate in red cells, from increasing pyridoxine supplementation. However, the 200-mg dose could be employed safely in vitamin B6-responsive sideroblastic anemia, myelofibrosis and Peyronies syndrome treatment. Although maximum saturation in circulating red cells is not achieved, erythroblasts and other nucleated and cytoplasmic organelles containing cells certainly will reach thorough saturation, which possibly explains the results obtained in these diseases.

Adult↗

Purification and characterization of aspartate aminotransferase from the thermoacidophilic archaebacterium Sulfolobus solfataricus.

Aspartate aminotransferase from the archaebacterium Sulfolobus solfataricus, a thermoacidophilic organism isolated from an acidic hot spring (optimal growth conditions: 87 degrees C, pH 3.5) was purified to homogeneity. The enzyme is a dimer (Mr subunit = 53,000) showing microheterogeneity when submitted to chromatofocusing and/or isoelectric focusing analysis (two main bands having pI = 6.8 and 6.3 were observed). The N-terminal sequence (22 residues) does not show any homology with any stretch of known sequence of aspartate aminotransferases from animal and bacterial sources. The apoenzyme can be reconstituted with pyridoxamine 5'-phosphate and/or pyridoxal 5'-phosphate, each subunit binding 1 mol of coenzyme. The absorption maxima of the pyridoxamine and pyridoxal form are centered at 325 and 335 nm, respectively; the shape of the pyridoxal form band does not change with pH. The enzyme has an optimum temperature higher than 95 degrees C, and at 100 degrees C shows a half-inactivation time of 2 h. The above properties seem to be unique even for enzymes from extreme thermophiles (Daniel, R. M. (1986) in Protein Structure, Folding, and Design (Oxender, D. L., ed) pp. 291-296, Alan R. Liss, Inc., New York) and lead to the conclusion that aspartate aminotransferase from S. solfataricus is one of the most thermophilic and thermostable enzymes so far known.

Amino Acid Sequence↗

Irreversible inactivation of aspartate aminotransferase by 2-oxoglutaconic acid and its dimethyl ester.

Incubation of pig heart cytosolic aspartate aminotransferase (pyridoxal 5'-phosphate form) with 10 mM 2-oxoglutaconic acid dimethyl ester for 2 h at 25 degrees C (pH 7.0) results in slight inactivation (approximately 15%). However, incubation of the enzyme with glutamate, or prior conversion of the enzyme to the pyridoxamine 5'-phosphate form, results in more extensive inactivation. The inactivation of the enzyme by 2-oxoglutaconic acid dimethyl ester is most pronounced in the presence of both glutamate and alpha-ketoglutarate. N-Ethylmaleimide was previously shown to alkylate two surface cysteine residues (I and II) and to react syncatalytically with a third cysteine residue (III) of cytosolic pig heart aspartate aminotransferase [Birchmeier et al. (1973) J. Biol. Chem. 248, 1751-1759]. Alkylation of cysteine III results in inactivation of the enzyme, despite the fact that this residue is not essential for catalysis. The present results suggest that 2-oxoglutaconic acid dimethyl ester reacts with the enzyme in a similar fashion to that exhibited by N-ethylmaleimide. Some inactivation by alkylation of a susceptible group at the active site cannot be ruled out. However, the rate of inactivation of cytosolic pig heart aspartate aminotransferase is proportional to the concentration of 2-oxoglutaconic acid dimethyl ester up to a concentration of at least 40 mM, suggesting that the compound binds very poorly to the active site or that alkylation at the active site is slow compared with syncatalytic alkylation of cysteine III. The t 1/2 for inactivation of pig heart cytosolic aspartate aminotransferase by 40 mM 2-oxoglutaconic acid dimethyl ester (in the presence of 10 mM L-glutamate, pH 7.2, 25 degrees C) is 9 min. Incubation of cytosolic pig heart aspartate aminotransferase with 10 mM 2-oxoglutaconate for 2 h (25 degrees C, pH 7.2) results in significant inactivation (approximately 30%). The enzyme is protected against inactivation by the presence of alpha-ketoglutarate, but glutamate enhances the inactivation. These findings suggest that 2-oxoglutaconate is an active site-directed inhibitor. The binding of 2-oxoglutaconate to the enzyme exhibits saturation kinetics (K1 approximately 2 mM), but the rate of inactivation is slow (limiting rate constant for inactivation in the presence of L-glutamate approximately 0.01 min-1; pH 6.0, 25 degrees C; t 1/2 max approximately 70 min). This finding suggests that 2-oxoglutaconate does not readily react in a syncatalytic fashion with cysteine III. Possibly, the two negative charges of 2-oxoglutaconate do not allow ready approach to cysteine III. Rather, the findings suggest that 2-oxoglutaconate binds at the active site of the pyridoxal 5'-phosphate form of the enzyme as an affinity labeling reagent. However, the increased rate of 2-oxoglutaconate-induced inactivation in the presence of glutamate suggests that this unsaturated alpha-keto acid also exhibits the properties of a kcat inhibitor. 2-Oxoglutaconate inactivates aspartate aminotransferase in cytosolic and mitochondrial fractions of rat kidney and purified pig heart alanine aminotransferase. Injection of 2-oxoglutaconate into mice results in inhibition of kidney aspartate aminotransferase. 2-Oxoglutaconate is a substrate of glutamate dehydrogenase. The kinetic constants are similar to those obtained with alpha-ketoglutarate. The results suggest that unsaturated alpha-keto acids and their esters may be useful probes for the study of alpha-keto acid-utilizing enzymes.

Animals↗

Cloning and sequence analysis of A cDNA encoding bovine cytosolic aspartate aminotransferase.

1. Complementary DNA encoding cytosolic aspartate aminotransferase was isolated from an adult bovine heart library. 2. The amino acid sequence deduced for the protein (412 amino acids) is extremely similar (> 94% identity) to that of porcine cytosolic aspartate aminotransferase but interesting differences were noticed comparing the position of cysteine residues.

Amino Acid Sequence↗

Aspartate aminotransferase activity in fiber tracts of the rat brain.

Activity of aspartate aminotransferase, an enzyme which catalyzes the interconversion of the excitatory transmitter candidates, glutamate and aspartate, has been measured in fiber tracts of rat, with an emphasis on sensory and motor systems of the brain. Most tracts had significantly higher activities than the cholinergic facial nerve root, consistent with the possibility that a component of aspartate aminotransferase activity might serve as a marker for neurons using glutamate and/or aspartate as neurotransmitter. Highest activity was in the auditory nerve root. On the other hand, a close correlation was found between aspartate aminotransferase and malate dehydrogenase activities in the fiber tracts, raising the question whether aspartate aminotransferase activity may be more closely related to energy metabolism than to transmitter metabolism.

Animals↗

Population distribution profiles of the activities of blood alanine and aspartate aminotransferase in the normal F344 inbred rat by age and sex.

Data on the blood enzyme activities of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were collected from control male and female F344 rats participating in toxicity studies at 17, 30, 56, 80 and 106 weeks of age. The data were skewed to the right with significant deviation from normality. Applying the Box-Cox transformation, it was concluded that approximately normal distributions could be obtained by using the reciprocal transformation. In general, the males showed a greater degree of heterogeneity for both enzymes. Enzyme activities at 17 weeks were lowest for both ALT and AST. There was a high correlation between the activities of blood ALT and AST, with some animals showing dramatic transient increases. Significant differences among studies with respect to the enzyme activities in rats of the same age were demonstrated.

Aging↗

Usefulness of alanine and aspartate aminotransferases in the diagnosis of microlithiasis in idiopathic acute pancreatitis.

CONCLUSION: Serum increases of aminotransferases, especially alanine aminotransferase (ALT), were suggestive of microlithiasis in idiopathic acute pancreatitis, particularly when assessed early after the onset of abdominal pain. BACKGROUND: It has been shown that biochemical laboratory values only are useful parameters in distinguishing gallstone from nongallstone acute pancreatitis. We assessed the diagnostic usefulness of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) for identification of occult microlithiasis in idiopathic acute pancreatitis. METHODS: Ninety-one patients with idiopathic acute pancreatitis who underwent microscopic examination of stimulated duodenal bile sediments were retrospectively studied. According to earliness of ALT and AST assay after the onset of abdominal pain, patients were divided into two groups: group A, within the first 24 h (n = 56) and group B, between 24 and 72 h (n = 35). RESULTS: ALT and AST values expressed as number of elevations of the upper limits of normal were higher in group A patients with positive biliary drainage than in group B. Median (range) ALT and AST values were 2.5 (0.1-18.1) vs 0.4 (0.1-8.6) and 3 (0.3-17.4) vs 0.5 (0.3-11.9), respectively. In the univariate analysis and receiver operating characteristic (ROC) curves, ALT within the first 24 h showed a sensitivity of 73%, specificity of 86%, and positive predictive value of 92% for a cutoff of 1.2 elevations of the upper limit of normal. These values were slightly higher, although without statistically significant differences, than those of AST (73, 80, and 89%, respectively).

Acute Disease↗

The enantiomeric error frequency of aspartate aminotransferase.

The enantiomeric error frequency of aspartate aminotransferase (mitochondrial isoenzyme from chicken) was assessed by adding the enzyme in high concentration (0.89 mM) to a mixture of L-glutamate and 2-oxoglutarate (12 and 1.2 mM, respectively, at pH 7.5 and 25 degrees C). The substrates continuously undergo the transamination cycle under these conditions. Thereby, L-glutamate is progressively racemized, a 1:1 ratio of two enantiomers being reached within 240 h. The enantiomeric error frequency, i.e. the ratio of the rate of D-glutamate production and the rate of the transamination reaction with glutamate and 2-oxoglutarate as substrates, is 1.5 x 10(-7). D-Glutamate is also converted to a 1:1 racemic mixture. The racemizing activity of a mixture of free pyridoxal 5'-phosphate and pyridoxamine 5'-phosphate is about two orders of magnitude lower than that of aspartate aminotransferase. The error frequency of the enzyme in the case of the C4 substrate pair aspartate and oxalacetate is 3.4 x 10(-8), i.e. 4 times lower than that with the C5 substrate pair.

Animals↗

Possible mechanism for the decrease of mitochondrial aspartate aminotransferase activity in ischemic and hypoxic rat retinas.

Glutamate is believed to be an excitatory amino acid neurotransmitter in the retina. Enzymes for glutamate metabolism, such as glutamate dehydrogenase, ornithine aminotransferase, glutaminase, and aspartate aminotransferase (AAT), exist mainly in the mitochondria. The abnormal increase of intracellular calcium ions in ischemic retinal cells may cause an influx of calcium ions into the mitochondria, subsequently affecting various mitochondrial enzyme activities through the activity of mitochondrial calpain. As AAT has the highest level of activity among enzymes involved in glutamate metabolism, we investigated the change of AAT activity in ischemic and hypoxic rat retinas and the protection against such activity by calpain inhibitors. We used normal RCS (rdy+/rdy+) rats. For the in vivo studies, we clamped the optic nerve of anesthetized rats to induce ischemia. In the in vitro studies, the eye cups were incubated with Locke's solution saturated with 95% N2/5% CO2. The activity of cytosolic AAT (cAAT) was about 20% of total activity, whereas mitochondrial AAT (mAAT) was about 75% in rat retina. Ninety minutes of ischemia or hypoxia caused a 20% decrease in mAAT activity, whereas cAAT activity remained unchanged. To examine the contribution of intracellular calcium ions to the degradation of mAAT, we used Ca2+-free Locke's solution containing 1 mM EGTA, ryanodine (Ca2+ channel blocker), and thapsigargin (Ca2+-ATPase inhibitor). In the present study, thapsigargin in Ca2+-free Locke's solution, but not ryanodine in this solution, was found to prevent AAT degradation. AAT degradation was also prevented by calpain inhibitors (Ca2+-dependent protease inhibitor) such as calpeptin at 1 nM, 10 nM, 0.1 microM, 1 microM and 10 microM, and by calpain inhibitor peptide, but not by other protease inhibitors (10 microM leupeptin, pepstatin, chymostatin). Additionally, we determined the subcellular localization of calpain activity and examined the change of calpain activity in ischemic rat retinas. Our results suggest that decreased activity of mAAT in ischemic and hypoxic rat retinas might be evoked by the degradation by calpain-catalyzed proteolysis in mitochondria.

Animals↗

Blood level of mitochondrial aspartate aminotransferase as an indicator of the extent of ischemic necrosis of the rat liver.

To assess the severity of ischemic liver injury, we examined release of mitochondrial aspartate aminotransferase (EC 2.6.1.1) and its cytoplasmic isozyme from the ischemic rat liver into the circulation. Their patterns of leakage were quite different: the level of cytoplasmic aspartate aminotransferase reached a peak soon after the circulation to the ischemic liver was restored, while that of mitochondrial aspartate aminotransferase increased slowly, reaching a maximum after more than 10 hr. On anoxic incubation of mitochondria isolated from the normal liver, oxidative phosphorylation capacity was lost within 2 hr, at which time no leakage of matrix enzymes was observed: more than 10 hr after-loss-of-oxidative phosphorylation were needed for the matrix enzymes to leak out of the mitochondrial membrane. Since the viability of cells is considered to depend on the capacity of oxidative phosphorylation, it is highly likely that the delayed appearance of mitochondrial aspartate aminotransferase in blood indicates the postmortem changes of injured cells. In fact, the cumulative activity of mitochondrial aspartate aminotransferase but not cytoplasmic aspartate aminotransferase in circulation after ischemic liver injury correlated fairly well with the decrease of total adenine nucleotides which were monitored to measure viable cells. The difference between mitochondrial aspartate aminotransferase and cytoplasmic aspartate aminotransferase as quantitative indices of hepatic necrosis may be due to the relative stability of the former and significant inactivation of the latter during hepatic ischemia. Therefore, the determination of mitochondrial aspartate aminotransferase in blood may be useful in the assessment of liver necrosis after ischemic injury.

Adenine Nucleotides↗

Molecular cloning and sequence analysis of the human cytosolic aspartate aminotransferase gene.

Structural organization of the human cytosolic aspartate aminotransferase gene was determined by analyzing the phage clones obtained from two kinds of genomic DNA libraries, using mouse cytosolic aspartate aminotransferase cDNA as a probe. The gene is more than 32 kb long and is split into 9 exons by 8 introns of various sizes. The 5' and 3'-flanking regions and the exact sizes and boundaries of the exon blocks were determined. The 5' end of the gene lacks the TATA and CAAT boxes, but contains G+C rich sequences and one potential binding site for the transcription factor, Sp1. Comparison of the nucleotide sequence of 250 bp upstream from the translation-initiation site revealed that the sequences of binding sites for the nuclear proteins, previously identified in the mouse, are highly conserved between human and mouse cytosolic aspartate aminotransferase genes.

Amino Acid Sequence↗

Cooperative effects in the binding of pyridoxal 5'-phosphate to mitochondrial apo-aspartate aminotransferase.

Titrations of mitochondrial apo-aspartate aminotransferase with pyridoxal 5'-phosphate in the presence of AMP, contrary to what has been observed in the case of the cytosolic isoenzyme [(1983) FEBS Lett. 153, 98-102], show sigmoidal isotherms, with Hill coefficients ranging from nH = 1.4, in the absence of AMP, to nH = 1.8, in the presence of 5.9 mM AMP. The experimental data were successfully fitted by the Monod-Wyman- Changeaux model. The best fit, in the absence of AMP, was obtained with L = 30, KR = 4.72 X 10(-7) M and KT = 1.18 X 10(-5) M. Binding curves in the presence of AMP fit the model by keeping KR as a constant. This implies that AMP could bind to the apoenzyme only in the T state. In contrast, binding curves in the presence of phosphate ion (Pi) showed a less pronounced cooperativity, the Hill coefficient dropping to nH = 1.0 in the presence of 0.1 mM Pi. The above results suggest a regulatory role of AMP and Pi in the reconstitution of aspartate aminotransferase.

Adenosine Monophosphate↗

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.

Adolescent↗