Photometry of Inulin and Polyfructosan by Use of a Cysteine/ Tryptophan Reaction

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Abstract
Described are a macromethod for measuring 2 to 30 pg of polyfructoside in sample volumes of 0.5 ml and a mi¬cromethod for measuring 40 to 400 ng in 10-RI samples. The absorptivity of the cysteine/tryptophan color product is 2.5- to 10-fold those found with use of five other colorimetric methods for inulin. The method is highly specific for alkali-stable polyfructoside. Color, measured at 515 nm, was maximal on development at 56 °C for 25 min in dilute (57-58% by vol) sulfuric acid, after the addition of the combined cysteine hydrochloride/tryptophan reagent. Analytical recoveries of inulin and polyfructosan from plasma and urine were complete. With initial alkaline heating, the endogenous blank inulinoid values averaged 0.6 ± 0.47 (SD) mg/liter for plasma and 0.05 ± 0.024 mg/min for urine. The coefficient of variation of the macromethod was <3%; that for the micromethod ranged from 1.3 to 6.3%. The micromethod appears adaptable to the determination of nanogram quantities of polyfruc¬toside in nanoliter volumes of biological fluids.