N-acetylglutamate-independent activity of carbamyl phosphate synthetase (ammonia): implications for the kinetic assay of acetylglutamate

Archives of Biochemistry and Biophysics
N S Cohen


In the presence of Mn2+, carbamyl phosphate synthetase (ammonia) catalyzes considerable carbamyl phosphate synthesis in the absence of the allosteric activator, N-acetylglutamate. Under standard conditions, the acetylglutamate-independent activity of a purified carbamyl phosphate synthetase preparation was 8 to 10% of the Vmax observed at saturating (1 mM) acetylglutamate. The product formed in the reaction was identified unequivocally as carbamyl phosphate. Standard conditions included 5 mM ATPMn and 1.5 mM excess Mn2+. The highest rate of acetylglutamate-independent activity was observed at [excess Mn2+] of 1.5 mM; increasing the [ATPMn] from 5 to 20 mM doubled the acetylglutamate-independent activity, to 18% of Vmax. Only 1/20 as much acetylglutamate-independent activity was observed when Mg2+ was substituted for Mn2+. When both Mn2+ and Mg2+ were present, the acetylglutamate-independent activity was less than when Mn2+ alone was present. Measurement of acetylglutamate-dependent activity of carbamyl phosphate synthetase (ammonia) revealed that one-half Vmax with Mn2+ was achieved at 17 microM acetylglutamate (about one-fifth of the value reported with Mg2+), and the Vmax with Mn2+ under standard conditions was only 60% of th...Continue Reading


Feb 15, 1976·The Biochemical Journal·J D McGivanJ Mendes-Mourão
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