A new enzymic determination of guanidinoacetic acid in urine

We developed and evaluated a colorimetric method for enzymic determination of guanidinoacetic acid (GAA) in urine. Endogenous urinary urea was first eliminated by urease (EC 3.5.1.5), and the added urease was then removed from the sample by centrifugal ultrafiltration. GAA in the ultrafiltrate was subsequently hydrolyzed by guanidinoacetate amidinohydrolase (EC 3.5.3.2) to glycine and urea. The latter substance produced an orange chromogen reacting with o-phthalaldehyde and N-(1-naphthyl)-N'-diethylethylenediamine, the absorbance of which at 465 nm was linearly related to concentrations as high as 200 mg/L for standard solutions of GAA. Analytical recovery of GAA added to urine ranged from 94 to 112% (mean 101%) and the within-run and between-run precision (CVs) of the method for the urinary GAA determination averaged 2.2 and 3.5%, respectively. Results correlated well (r = 0.983) between the present method and a high-performance liquid chromatographic method. The proposed method is accurate and simple. We saw a great decrease in urinary GAA of patients with suspected or proven renal insufficiency as compared with that of healthy volunteers.