Purification and properties of acetyl-CoA:l-glutamate N-acetyltransferase from human liver

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Abstract
Acetyl-CoA:L-glutamate N-acetyltransferase (amino acid acetyltransferase, EC 2.3.1.1) was isolated from human liver mitochondria by precipitation with (NH4)2SO4 and chromatography on hydroxyapatite, DEAE-cellulose and Sephacryl 300. This gave a 360-fold purification. The molecular weight was estimated to be approx. 190 000. The kinetic properties in the absence of arginine are compatible with a rapid-equilibrium random Bi Bi mechanism. The estimated constants are: for the substrates Km, acetyl-CoA 4.4 mM, Ki, acetyl-CoA 4.7 mM, Km, glutamate 8.1 mM, Ki, glutamate 8.8 mM; for the products, Ki, acetylglutamate 0.28 mM, Ki, CoA 5.6 mM. The rate constant for the forward direction is 1.24s-1. If in vivo the constants are of the same order of magnitude as in vitro, the synthesis of N-acetylglutamate, an obligate activator of the first step of urea synthesis, can be expected to occur in the mitochondrion under conditions where the amino acid acetyltransferase is not saturated by its substrates. The regulation of the first step of urea synthesis could thus depend mainly on the intramitochondrial substrate and perhaps product concentrations of amino acid acetyltransferase.