Evaluation of enzyme activities by gas chromatography-mass spectrometry : HMG-CoA reductase and cholesterol 7-hydroxylase

Methods are described for the evaluation of HMGCoA reductase and cholesterol 7 alpha-hydroxylase activities. The methods are based on the measurement by selected-ion monitoring of mevalonate, formed from HMGCoA and of 7 alpha-hydroxycholesterol formed from cholesterol, respectively. The methods are as sensitive as those based on the use of radioisotopes but less time-consuming because quantitation is carried out on total lipid extracts. In the case of cholesterol 7 alpha-hydroxylase, the procedure does not require the use of exogenous cholesterol as the substrate, so problems related to its equilibration with endogenous microsomal cholesterol are avoided.