Hydroxy-A5-C27-steroid Oxidoreductase

The second step in the synthesis of bile acids, according to Hylemon et al. (1991), is the conversion of 7a-hydroxycholesterol to 7a-hydroxy-4-cholesten-3-one by NAD+-dependent 3/3-hydroxy-A5-C27-steroid oxidoreductase. This enzyme is located in the endoplasmic reticulum of liver, and its catalysis of the 3/3-hydroxy group also results in isomerization of the double bond from A5 to A4. 

The steroid products are separated by reversed-phase chromatography on a Beckman Ultrasphere ODS (4.6 mm x 25 cm, 5 fiM) column equlibrated with 70 30 (v/v) acetonitrile-methanol. The absorbance is monitored at 240 nm and the amount of product is determined from a calibration curve. 

One milligram of microsomal protein is added to 0.1 M potassium phosphate buffer (pH 7.4) containing 50 mM NaF, 10 mM dithiothreitol, 1 mM EDTA, 20% glycerol (v/v), 150 iM 5-cholestene-3/3, 7a-diol, and 0.915% CHAPS. The reaction is initiated by 1 mM NAD+ to give a final reaction volume of 1.0 mL. After incubation at 37°C for 5 minutes, the reaction is terminated by adding 2 mL of 95% ethanol. An internal recovery standard, 4-cholesten-3-one (3 fig in methanol) is also added. The steroid products are extracted into 5 mL of petroleum ether (repeated twice). After the ether has been removed at 40°C under a stream of nitrogen, the products are dissolved in 100 fxL of mobile phase and 20 ju.L is injected into the column. The amount of product formed is linear with protein (to 1.5 mg) and with time (up to 10 min, 1 mg protein). The assay is much more sensitive than the direct spectrophotometric assay, and it avoids the use of thin-layer chromatography and radioisotopes described in other methods. 

The source of enzyme is rat liver microsomes prepared by standard techniques. 

The side chain cleavage of cholesterol, producing pregnenolone, is catalyzed by cytochrome P450JCC. This is the initial step in the biosynthesis of several steroid hormones. In this assay, the initial product, pregnenolone, is quantitatively converted to progesterone by treatment with cholesterol oxidase, which increases by about 10-fold the sensitivity of the assay. 

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