A new enzymatic method to determine creatine.

A new enzymatic method is described for the determination of creatine in serum and urine using creatine amidinohydrolase (EC 3.5.3.3), sarcosine oxidase (EC 1.5.99.1) and formaldehyde dehydrogenase (EC 1.2.1.1). The principle of the method is as follows. Creatine is degraded to sarcosine and urea, and the sarcosine formed is measured with sarcosine oxidase in the presence of formaldehyde dehydrogenase and NAD+. The NADH + H+ produced is measured at 340 nm. Creatine concentration can be calculated directly from the absorptivity of NADH + H+ generated in the reaction or from creatine standard solutions. The assay takes less than 20 min. The standard curve is linear up to 50 mg creatine/1 (serum) and 800 mg creatine/1 (urine). Fifty random samples were assayed by this method (y) and simultaneously by the Folin method (x). The correlation coefficients were 0.995 for serum samples, 0.994 for urine samples, and the regression equations were y = 0.979x - 0.01 (serum) and y = 0.978x - 0.01 (urine).