Import of mutant forms of mitochondrial aspartate aminotransferase into isolated mitochondria

Kinetic properties and thermal stabilities of mutant forms of mitochondrial aspartate aminotransferase

Kinetic properties and thermal stabilities of the precursor form of mitochondrial aspartate aminotransferase, the mature form lacking 9 amino acids from the N-terminus, and forms of the mature protein in which cysteine-166 had been mutated to serine or alanine were compared with those of the mature enzyme. The precursor and the cysteine mutants showed moderately impaired catalytic properties consistent with decreased ability to undergo transition from the open to the closed conformation which is an integral part of the mechanism of action of the enzyme. The deletion mutant had a kcat only 2% of that of the mature enzyme but also much reduced Km values for both substrates. In addition it showed enhanced reactivity of cysteine-166 with 5,5'-dithiobis(2-nitrobenzoate), which is characteristic of the closed form of the enzyme, with no enhancement of reactivity in the presence of substrates. This is taken to show that the deletion mutant adopts a conformation that is significantly different from that of the mature enzyme particularly in respect of the small domain. The deletion mutant was found to be more resistant to thermal inactivation over a range of temperatures than were the other forms of the enzyme consistent with its having a more tightly packed small domain.

Ethanol up-regulates fatty acid uptake and plasma membrane expression and export of mitochondrial aspartate aminotransferase in HepG2 cells

To explain the increased plasma mitochondrial aspartate aminotransferase (mAspAT) observed in alcoholics, we cultured HepG2 hepatoma cells in ethanol. Acute (24 hour) exposure to 0, 20, 40, or 80 mmol/L ethanol produced a dose-dependent (r = .98) increase in mAspAT messenger RNA (mRNA) of < or = thirteen-fold, with no significant change in the cellular content of mAspAT or of several other enzymes. The recovery of mAspAT in the medium over 24 hours of ethanol exposure correlated with both ethanol concentration and with mAspAT mRNA (r = .90), reaching 808% of cellular enzyme content/24 hours at 80 mmol/L. Recovery of all other enzymes studied was < or = 20% of cellular content and unaffected by ethanol. Plasma membrane mAspAT content also correlated with mAspAT mRNA (r = .96) and mitochondrial levels were unchanged. No mitochondrial morphologic abnormalities were observed at any ethanol concentration studied. In cells cultured chronically at 0 to 80 mmol/L ethanol, fatty acid uptake Vmax increased in parallel with plasma membrane expression of mAspAT (r = .98). Cellular triglyceride content was highly correlated with Vmax. Thus, the data suggest that: 1) the increased plasma mAspAT observed in alcoholics may reflect pharmacologic upregulation of mAspAT mRNA and of mAspAT synthesis by ethanol; and 2) increased mAspAT-mediated fatty acid uptake may contribute to alcoholic fatty liver.

Biogenesis of the covalently flavinylated mitochondrial enzyme dimethylglycine dehydrogenase

Rat dimethylglycine dehydrogenase (Me2GlyDH) was used as model protein to study the biogenesis of a covalently flavinylated mitochondrial enzyme. Here we show that: 1) enzymatically active holoenzyme correlated with trypsin resistance of the protein; 2) folding of the reticulocyte lysate-translated protein into the trypsin-resistant, holoenzyme form was a slow process that was stimulated by the presence of the flavin cofactor and was more efficient at 15 degrees C than at 30 degrees C; 3) the mitochondrial presequence reduced the extent but did not prevent holoenzyme formation; 4) covalent attachment of FAD to the Me2GlyDH apoenzyme proceeded spontaneously and did not require a mitochondrial protein factor; 5) in vitro only the precursor, but not the mature form, of the protein was imported into isolated rat liver mitochondria; in vivo, in stably transfected HepG2 cells, both the precursor and the mature form were imported into the organelle; 6) holoenzyme formation in the cytoplasm did not prevent the translocation of the proteins into the mitochondria in vivo; and 7) lack of vitamin B2 in the tissue culture medium resulted in a reduced recovery of the precursor and the mature form of Me2GlyDH from cell mitochondria, suggesting a decreased efficiency of mitochondrial protein import.