Variations in the Response of Mouse Isozymes of Adenylosuccinate
Synthetase to Inhibitors of Physiological Relevance*
Tudor
Borza,
Cristina V.
Iancu,
Evan
Pike,
Richard B.
Honzatko, and
Herbert J.
Fromm
From the Department of Biochemistry, Biophysics, and Molecular
Biology, Iowa State University, Ames, Iowa 5011
Vertebrates have acidic and basic isozymes of
adenylosuccinate synthetase, which participate in the first committed
step of de novo AMP biosynthesis and/or the purine
nucleotide cycle. These isozymes differ in their kinetic properties and
N-leader sequences, and their regulation may vary with tissue type.
Recombinant acidic and basic synthetases from mouse, in the presence of
active site ligands, behave in analytical ultracentrifugation as
dimers. Active site ligands enhance thermal stability of both isozymes.
Truncated forms of both isozymes retain the kinetic parameters and the
oligomerization status of the full-length proteins. AMP potently
inhibits the acidic isozyme competitively with respect to IMP. In
contrast, AMP weakly inhibits the basic isozyme noncompetitively with
respect to all substrates. IMP inhibition of the acidic isozyme is
competitive, and that of the basic isozyme noncompetitive, with respect
to GTP. Fructose 1,6-bisphosphate potently inhibits both isozymes competitively with respect to IMP but becomes noncompetitive at saturating substrate concentrations. The above, coupled with structural information, suggests antagonistic interactions between the active sites of the basic isozyme, whereas active sites of the acidic isozyme
seem functionally independent. Fructose 1,6-bisphosphate and IMP
together may be dynamic regulators of the basic isozyme in muscle,
causing potent inhibition of the synthetase under conditions of high
AMP deaminase activity.
*
This work was supported in part by National Institutes of
Health Research Grant NS 10546 and National Science Foundation Grant MCB-9985565.The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
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