Enzymatic determination of ammonia in blood plasma

A kinetic method is described for the determination of the ammonia content of blood plasma using glutamate dehydrogenase and giving satisfactory accuracy and precision. The reaction mixture is composed of 2.2 ml of 0.05 M diethanolamine buffer pH 8.4, 0.1 ml of 4.2 mM reduced nicotinamide adenine dinucleotide, 0.1 ml of 0.1 M 2-oxoglutarate and 0.5 ml of fresh blood plasma. One tenth ml of glutamate dehydrogenase suspension containing 400 U of the enzyme is added after preincubation for 25 min at 20° and the initial velocity of the enzyme reaction measured by reading or recording the decrease in absorbance at 340 nm due to the oxidation of reduced nicotinamide adenine dinucleotide during glutamate formation from the ammonia present. The ammonia concentration in plasma is obtained from a calibration curve or by calculation. Possible effects of endogenous enzymes are discussed.