Automated enzymatic analysis of inulin

We have developed an automated enzymatic assay for quantitation of inulin in plasma and urine that can be performed on the Cobas FARA II. In the assay, inulinase hydrolyzes inulin to fructose, and sorbitol dehydrogenase converts fructose to sorbitol with consumption of NADH, which is detected by spectrophotometry. The method incorporates a sample blank (inactivated inulinase) for each specimen to subtract contributions of endogenous fructose. Recovery of fructose or inulin was near 100%, with linearity to 300 mg/L. The enzymatic assay (y) agreed well with an anthrone comparison method (x) for analysis of inulin in both urine specimens (y = 1.00x - 138; Sy/x = 714) and plasma specimens (y = 1.00x - 3.5; Sy/x = 5.5). Glucose at 300 mg/L yielded an apparent inulin value of 1.3 mg/L in the enzymatic assay, but reacted at nearly 10% equivalency in the anthrone assay. Interferences from sorbitol, mannitol, and xylitol were negligible. CVs for day-to-day precision studies were 1-4%. The automated enzymatic assay of inulin is faster and avoids the use of caustic reagents required by the classical anthrone method.