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PubMed1977

On the determination of isozyme levels in preparations containing cytoplasmic and mitochondrial aspartate aminotransferase.

A spectrophotometric assay has been developed for the determination of the content of each isozyme of aspartate transaminase (L-aspartate:2-oxoglutarate aminotransferase, EC 2.6.1.1) in physiological fluids or tissue extracts. The methods relies on the ability of adipate, at low pH and ionic strength to inhibit the cytoplasmic isozyme but not the one from mitochondria. Two assays are necessary, one at pH 8.0 which measures the content of both isozymes and another at low pH which measures primarily the amount of mitochondrial isozyme. Results obtained by this simple procedure match those in which each isozyme is inhibited by its antibody. The validity of the results obtained by the new method was tested at different ratios of cytoplasmic:mitochondrial isozyme and with tissue extracts. Since the amounts of each isozyme determined by radial immunodiffusion match those values gathered by following enzymatic activity, it is concluded that the quantity of each isozyme obtained from its respective catalytic activity must represent the total protein content of each isozyme in a given sample.

Animals↗
PubMed1994

Dependence of the folding and import of the precursor to mitochondrial aspartate aminotransferase on the nature of the cell-free translation system.

The precursor to mitochondrial aspartate amino-transferase (pmAspAT), when newly synthesized in vitro using either rabbit reticulocyte lysate (RRL) or wheat germ extract (WGE), is highly susceptible to proteolysis. Treatment of these translation products with trypsin generates a characteristic pattern of proteolytic fragments which differs between WGE and RRL. pmAspAT synthesized in RRL acquires over time the trypsin resistance characteristic of purified recombinant pmAspAT in a process that reflects folding of the nascent protein in a cytosolic-like environment and results in the loss of its ability to be imported into mitochondria (Mattingly, J.R., Jr., Youssef, J., Iriarte, A., and Martinez-Carrion, M. (1993) J. Biol. Chem. 268, 3925-3937). Folding in RRL is temperature-dependent. By contrast, pmAspAT synthesized in WGE does not acquire trypsin resistance at any temperature and is unable to be imported into isolated mitochondria. Yet, addition of RRL to the protein synthesized in WGE restores both the folding and import processes. This effect is temperature and N-ethylmaleimide-sensitive and requires ATP. We have also detected that newly synthesized pmAspAT, in both the mammalian (RRL) and plant (WGE) environments, associates at an early stage, possibly cotranslationally, with cytosolic hsp70. This complex is transient and short-lived in RRL but is stable for protein synthesized in WGE. The mature form of the protein is also found associated with hsp70 early on during its synthesis. Yet, in RRL, it dissociates from the complex more easily than its precursor form. This suggests that the interaction between the precursor and hsp70 is at least partially dependent on the signal peptide. The dissociation of the precursor/hsp70 complex seems to precede ATP-requiring subsequent folding steps. Furthermore, release of precursor from this complex, either to be imported into mitochondria or to resume folding when in the absence of mitochondria, appears to be influenced by other factor(s) present in RRL but absent or inhibited in WGE.

Animals↗