A STEP FORWARD IN ENZYMATIC MEASUREMENT OF CREATININE

We describe an improved enzymatic ultraviolet absorbance method for as saying creatinine in serum, plasma, and urine. Creatinine is hydrolyze d by creatinine iminohydrolase (EC 3.5.4.21) to ammonia and N-methylhy dantoin. The ammonia produced combines with 2-oxoglutarate and NADPH i n the presence of glutamate dehydrogenase to yield glutamate and NADP( +). The consumption of NADPH, measured by a two-point fixed-time assay , is proportional to the amount of creatinine in the sample. The assay is carried out in two steps: The first step eliminates background abs orbance in hyperlipemic samples and endogenous ammonia through a ''cle aring system'' and an isocitrate dehydrogenase-based ''ammonia scaveng er system''; the second step starts creatinine measurement. The method affords a simple, rapid, and sensitive assay with good precision and extended linearity; it employs working solutions stable at least 4 mon ths. Test results compare closely with those of the isotope dilution-m ass spectrometry Definitive Method, the HPLC procedure, and the fuller 's earth method. The proposed method is not subject to interference fr om several metabolites or from the 72 drugs tested. Because it is easi ly automated, the method is suitable for routine work in clinical labo ratories.