Automated EnzymaticAssayfor PlasmaAmmonia

An enzymatic method for determining plasma ammonia with the Du Pont Automatic Clinical Analyzer (aca) is described. The assay requires a sample volume of 500 pL for a kinetic ammonia measurement. The reaction is initiated with glutamate dehydrogenase and the rate of depletion of NADPH is monitored with two measurements, 17 s apart, at 340 nm. Reaction conditions have been optimized for maximum sensitivity through both one-factor-at-a-time and multiple variable response surface optimization techniques. Linearity to 1000 tmol of ammonia per liter of plasma has been achieved. No significant interferences were observed from anticoagulants or endogenousblood components, including pyruvate and oxalacetate.Useof the coenzymeNADPH(insteadof NADH) in this aca procedureeliminatesthe lengthypre-incubation otherwise required for endogenousdehydrogenasereactions.