The unfolding and refolding of cytoplasmic aspartate aminotransferase from pig heart.

The unfolding of cytoplasmic aspartate aminotransferase from pig heart in solutions of guanidinium chloride (GdnHCl) was studied. Data from protein fluorescence, c.d. and thiol-group reactivity indicated that the enzyme was unfolded in 6 M-GdnHCl. Spectroscopic studies showed that this unfolding was accompanied by dissociation of the pyridoxal 5'-phosphate cofactor. On dilution of the GdnHCl, re-activation of the enzyme occurred in reasonable yield, provided that dithiothreitol and pyridoxal 5'-phosphate were present. The regain of activity obeyed second-order kinetics. In the absence of added dithiothreitol and pyridoxal 5'-phosphate, substantial formation of high-Mr aggregates occurred.     

The nature of the multiple forms of cytoplasmic aspartate aminotransferase from pig and sheep heart

Starch-gel electrophoresis of sheep heart aspartate aminotransferase was carried out over the range pH7.0-8.5. The enzyme separates into three subforms in the same way as the pig heart enzyme. As the pH was increased the distance migrated by each subform increased by the same amount, so that they remained the same distance apart. Titration of the enzyme over the appropriate pH range was used to calculate the difference in charge between the subforms and it was concluded that they differ by one charged group per dimer from their nearest neighbour on the electrophoretogram over the whole pH range studied. It was also shown that the pig-heart alpha and beta subforms differ by almost one charged group per dimer in the range pH5.5-5.7 and that the spacing between the subforms on starch-gel electrophoresis at pH8.0 is the same as that for the sheep enzyme. Since the charge difference between the subforms is maintained over such a wide range of pH, it is concluded that they probably differ from each other in covalent structure, because of the improbability that conformational differences can give rise to such behaviour. The relationship between the subforms and inactive binding of the coenzyme is also examined.     

The binding of 8-anilinonaphthalene-1-sulphonate to cytoplasmic aspartate aminotransferase from pig heart.

Anilinonaphthalenesulphonate binds to cytoplasmic aspartate aminotransferase with high affinity (Kd about 10 muM) and with a stoicheiometry of one molecule per dimer. It is not displaced by aliphatic or aromatic dicarboxylate substrate analogues. The enzyme is believed to be a symmetrical dimer with identical subunits; it can evidently function asymmetrically in binding anilinonaphthalenesulphonate.     

Primary structure of cytoplasmic aspartate aminotransferase from pig heart muscle. Amino acid sequence of peptides from cyanogen bromide hydrolysate]

Amino acid sequences were determined for the six peptides from cyanogen bromide hydrolysis of cytoplasmic aspartate aminotransferase. These peptides accounted for 177 amino acid residues of the enzyme. Partial sequence of N-terminal peptide accounting for 212 amino acid residues of enzyme was also determined.     

Phosphorus-31 nuclear magnetic resonance study on cytoplasmic aspartate aminotransferase from pig heart. A reinvestigation

An average target size of 251 kDa has been obtained for the (Ca2+ + Mg2+)-ATPase of calmodulin-depleted erythrocyte ghosts by radiation inactivation with 16 MeV electrons. This is close to twice the size of the purified calcium-pump polypeptide. When calmodulin was included during the ATPase assay, a component of about 1 MDa appeared in addition to the activated dimer.     

The primary structure of mitochondrial aspartate aminotransferase from human heart

The complete amino acid sequence of the mitochondrial aspartate aminotransferase (L-aspartate:2-oxoglutarate aminotransferase, EC 2.6.1.1) from human heart has been determined based mainly on analysis of peptides obtained by digestion with trypsin and by chemical cleavage with cyanogen bromide. Comparison of the sequence with those of the isotopic isoenzymes from pig, rat and chicken showed 27, 29 and 55 differences, respectively, out of a total of 401 amino acid residues. Evidence for structural microheterogeneity at position 317 has also been obtained.     

Primary structure of cytoplasmic aspartate aminotransferase from pit heart muscle. Amino acid sequence of thermolytic peptides]

Amino acid sequences of 128 thermolytic peptides from carboxymethylated aspartate aminotransferase were determined. These peptides contain a total of 515 amino acid residues and account for a sequence of 384 amino acid residues in the aspartate aminotransferase.     

The unfolding and attempted refolding of mitochondrial aspartate aminotransferase from pig heart.

The role of prostaglandins in the regulation of muscle protein breakdown is controversial. We examined the influence of arachidonic acid (5 microM), prostaglandin E2 (PGE2) (2.8 microM) and the prostaglandin-synthesis inhibitor indomethacin (3 microM) on total and myofibrillar protein breakdown in rat extensor digitorum longus and soleus muscles incubated under different conditions in vitro. In other experiments, the effects of indomethacin, administered in vivo to septic rats (3 mg/kg, injected subcutaneously twice after induction of sepsis by caecal ligation and puncture) on plasma levels and muscle release of PGE2 and on total and myofibrillar protein breakdown rates were determined. Total and myofibrillar proteolysis was assessed by measuring production by incubated muscles of tyrosine and 3-methylhistidine respectively. Arachidonic acid or PGE2 added during incubation of muscles from normal rats did not affect total or myofibrillar protein degradation under a variety of different conditions in vitro. Indomethacin inhibited muscle PGE2 production by incubated muscles from septic rats, but did not lower proteolytic rates. Administration in vivo of indomethacin did not affect total or myofibrillar muscle protein breakdown, despite effective plasma levels of indomethacin with decreased plasma PGE2 levels and inhibition of muscle PGE2 release. The present results suggest that protein breakdown in skeletal muscle of normal or septic rats is not regulated by PGE2 or other prostaglandins.     

Chemical structure of the active site of pig heart mitochondrial aspartate aminotransferase labeled with beta-chloro-l-alanine

Formate-induced inactivation of pig heart mitochondrial aspartate aminotransferase by beta-chloro-L-alanine resulted in the modification of the epsilon-amino group of the lysyl residue which is involved in the formation of an aldimine bond with 4-formyl group of the coenzyme, pyridoxal 5'-phosphate. The tryptic peptide isolated from the labeled site of the enzyme was composed of 25 residues and exhibited positive circular dichroism at 325 and 254 nm where the pyridoxyl chromophore of the labeled site peptide absorbs, while the phosphopyridoxyl peptide isolated from the boro-hydride-reduced enzyme did not show any ellipticity in this spectral region. Its comparison with the analogous tryptic peptide from the labeled site of the cytosolic isoenzyme revealed a high degree of homology in their primary structures as well as in spectral properties. Structural analysis of the labeled site peptide and mechanistic consideration of the labeling process indicated that with both isoenzymes the phosphopyridoxyl group is covalently bound to the alpha amino group of the alanyl moiety derived from beta-chloro-L-alanine, the beta carbon of which is covalently linked to the epsilon-amino group of the lysyl residue.