An enzymatic method is described for determination of total serum cholesterol by use of a single aqueous reagent. The method requires no prior treatment of sample and the calibration curve is linear to 600 mg/dl. Cholesterol esters are hydrolyzed to free cholesterol by cholesterol ester hydrolase (EC 3.1.1.13). The free cholesterol produced is oxidized by cholesterol oxidase to cholest-4-en-3-one with the simultaneous production of hydrogen peroxide, which oxidatively couples with 4-aminoantipyrine and phenol in the presence of peroxidase to yield a chromogen with maximum absorption at 500 nm. The method is reproducible, and the results correlate well with those obtained by automated Liebermann—Burchard procedures (AA-2 and SMA 12/60) and the method of Abell et al. The present method affords better specificity than those previously reported and has excellent precision.

We report an improved kinetic colorimetric system for measuring lactate dehydrogenase activity in serum. In the system a tetrazolium salt, 2-p-iodophenyl-3-p-nitrophenyl-5-phenyl tetrazolium chloride, is used as the chromogenic indicator of dehydrogenase activity, with diaphorase serving as the electron transfer agent. All ingredients required for an assay are combined in a single dry reagent that is stable at room temperature. The method is 2.5 times as sensitive as the ultraviolet method of Wacker and has a dynamic range three times that of the ultraviolet method. Reducing substances in serum do not affect the results. Precision, range of linearity, and stability of reagent after reconstitution are excellent. Results for fresh sera correlated well with those obtained by the "A-Gent" ultraviolet method (Wacker method at 37°C) and with the SMA 12/60.

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