Sources of Error in Spectrophotometric Measurement of Aspartate Aminotransferase and Alanine Aminotransferase Activities in Serum

We examined the measurement of serum aspartate and alanine aminotransferase activities with a short-interval enzyme-activity analyzer. Double-beam spectrophotometry was used to elucidate the source of errors in such measurements. For both enzymes the principal error sources are: (a) the presence of endogenous substrate for lactate dehydrogenase, and (b) reaction of 2-oxoglutarate with serum glutamate dehydrogenase and ammonium ions added to the reaction mixture in admixture with the secondary enzymes of the coupled reactions. In the case of serum aspartate aminotransferase, a less important source of error is the reaction of serum alanine aminotransferase with endogenous substrate. Use of 2-oxoglutarate as a reaction initiator in conventional methods causes errors. Suitable blank reagent mixtures are described that permit accurate, rapid measurement of these activities by double-beam spectrophotometry in a short-interval enzyme-activity analyzer.