Time-dependent Inhibition of Aspartate Aminotransferase Isozymes by dl-Glyceraldehyde 3-Phosphate

Substrate deactivation of phenylalanine-sensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase by erythrose 4-phosphate

3-Deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAH7PS, EC 4.1.2.15) catalyzes the condensation of phosphoenolpyruvate (PEP) with erythrose 4-phosphate (E4P) to give DAH7P via an ordered sequential mechanism. In the absence of PEP (the first substrate to bind), E4P binds covalently to the phenylalanine-sensitive DAH7PS of Escherichia coli, DAH7PS(Phe), deactivating the enzyme. Activity is restored on addition of excess PEP but not if deactivation was carried out in the presence of sodium cyanoborohydride. Electrospray mass spectrometry indicates that a single E4P is bound to the protein. These data are consistent with a slow, reversible Schiff base reaction of the aldehydic functionality of E4P with a buried lysine. Molecular modeling indicates that Lys186, a residue at the base of the substrate-binding cavity involved in hydrogen bonding with PEP, is well placed to react with E4P forming an imine linkage that is substantially protected from solvent water.

Studies on the mechanism of acetaldehyde-mediated inhibition of aspartate aminotransferase (GOT) in human erythrocytes

The mechanism of acetaldehyde-mediated GOT inhibition was studied in human red cells. In the GOT assay mix, acetaldehyde competitively inhibits activation of apoGOT by pyridoxal phosphate and pyridoxamine phosphate. However, Ki values are 100-1000 times greater than Km values for these B6 vitamers. Moreover, incubation of undiluted lysates with acetaldehyde at 37 degrees inhibits GOT activity without increasing apoGOT levels and without altering affinity of apoGOT for either B6 coenzyme. In undiluted lysates, inhibition is not prevented by disulfiram. However, incubation at 4 degrees prevents both acetaldehyde metabolism and GOT inhibition while preincubation with NaF prevents GOT inhibition without affecting acetaldehyde disappearance. The effect of NaF is completely reversed by pyruvate but only partially reversed by NADH. Glyceraldehyde 3-phosphate, the only glycolytic intermediate which directly inhibits GOT, does not reverse the NaF effect. Thus, inhibition of GOT by acetaldehyde (a) requires nonoxidative metabolism of acetaldehyde and (b) is not mediated either by glycolytic substrates or by impaired binding of B6 vitamers to the GOT apoenzyme. Since NaF had no effect on a lysate deficient in glucose 6-phosphate dehydrogenase, the hexose monophosphate shunt may play a role in acetaldehyde-mediated GOT inhibition.

Inhibition of rat liver transaminases by low levels of acetaldehyde and the pharmacologic effects of B6 vitamers

To better define the significance and mechanism of acetaldehyde-mediated transaminase inhibition, acetaldehyde metabolism was studied in rat liver homogenates and cytosols. When either preparation was incubated at 37 degrees with 1.5 mM acetaldehyde for 4 hr, acetaldehyde levels fell rapidly in the first 30 min and little inhibition of aspartate aminotransferase (GOT) or alanine aminotransferase (GPT) resulted. In contrast, incubation with 50 mM ethanol also resulted in a peak acetaldehyde level of 1.0 to 1.5 mM by 2 hr, but this level was then maintained for the next 2 hr and transaminases were inhibited by 20-35%. Sequential addition of low dose (125-250 microM) pulses of acetaldehyde to rat liver preparations resulted in a progressive decrease in the rate of acetaldehyde disappearance. When the pulsing schedule was adjusted accordingly to maintain acetaldehyde levels between 50 and 250 microM for 8 hr, transaminases were again inhibited by 20-40%. Finally, addition of 1-5 mM pyridoxal and pyridoxal 5'-phosphate, aldehydic B6 vitamers, to cytosols 2-4 hr after pulsing with acetaldehyde was begun, almost completely prevented further transaminase inhibition. In contrast, the non-aldehydic B6 vitamers, pyridoxine, pyridoxamine and pyridoxamine 5'-phosphate, did not affect acetaldehyde-mediated transaminase inhibition. These findings suggest that (1) prolonged exposure to low levels of acetaldehyde impairs acetaldehyde metabolism in rat liver homogenates and cytosols; (2) acetaldehyde toxicity may be more dependent on sustained exposure to acetaldehyde than on the peak level of acetaldehyde attained; and (3) aldehydic B6 vitamers can modify on-going acetaldehyde-mediated transaminase inhibition.

Measurement of aminotransferases: Part 1. Aspartate aminotransferase

Aminotransferases are ubiquitous enzymes of mammalian cells and several are of important diagnostic use. The application of aspartate aminotransferase activity measurements in serum from individuals suffering from myocardial infarction brought about a new dimension in clinical laboratory testing in the 1950s. This review focuses on measurement techniques for aspartate aminotransferase and their application (a subsequent article will review other aminotransferases). Assay techniques measuring enzyme activity are direct spectrophotometric measurements, manometric techniques, assays using dye substances, coupled enzyme techniques, and radiometric procedures. Of these procedures, the one employing malate dehydrogenase and NADH is the most important and is covered in particular detail. The estimation of the mitochondrial isoenzyme of aspartate aminotransferase is also of clinical interest, in particular for estimating severity of disease or in specific applications (e.g., chronic alcoholism). Methods reviewed for estimation of this enzyme are electrophoresis, chromatography, differential kinetic behavior, and immunochemical separation. Determination of the enzyme protein by techniques independent of its catalytic activity are also reviewed.

[Aspartate And Alanine Aminotransferase In Fasciola Hepatica]

The activity and distribution of aspartate aminotransferase (EC 2.6.1.1) and alanine aminotransferase (EC 2.6.1.2) in adult Fasciola hepatica have been studied. Fasciola hepatica was fractionated by differential centrifugation into nuclear, mitochondrial and cytosolic fractions. The activity of GOT and GPT was measured by the method of Reitman and Frankel. Isozyme patterns of those enzyme were also examined by DEAE-cellulose column chromatography. The results obtained were as follows: 1. The activity of aspartate and alanine aminotransferase was about 0.55 unit and 0.92 unit per 1 g of Fasciola hepatica, respectively. 2. The activity of those enzymes was relatively low compared with those in mammalian tissues. 3. The distribution of aspartate aminotransferase in the subcellular organelles showed that 71 % of the activity was in cytosolic, 24 % in mitochondrial and 5 % was in nuclear fraction. 4. About 22 % of the total alanine aminotransferase activity was found in the mitochondrial fraction, about 66 percent in the cytosolic fraction. 5. Aspartate aminotransferase from cytosolic fraction was separated into two types of isozymes, whereas alanine aminotransferase from cytosolic fraction gave only one active peak on DEAE-cellulose column chromatography.