An ultraviolet spectrophotometric assay for aspartate transcarbamylase

A procedure for determining the activity of aspartate transcarbamylase, based upon the greater ultraviolet absorbancy of the products of the reaction catalyzed compared to the reactants, was devised. Extinction coefficients were determined at 205, 210, and 215 nm for the compounds carbamoyl aspartate, acetyl aspartate, and aspartate. These values formed the quantitative basis for a spectrophotometric assay in which an enzymatic reaction is monitored at one of these wavelengths. Use of this procedure was illustrated in four kinetic experiments with the allosteric aspartate transcarbamylase from Escherichia coli, and the nonallosteric catalytic subunit of this enzyme: aspartate saturation curve, arsenate saturation curve (reverse reaction), allosteric activation by a transition-state analog employing acetyl phosphate as substrate, and carbamoyl phosphate progress curve (substrate depletion in the presence of excess cosubstrate). Owing to changes in absorbance on the order of 1000 liter mol-1 cm-1 concomitant with the reaction, the sensitivity of the method is comparable to that of many procedures already in the literature.