The synthesis of 2H3-labelled lathosterol is described. This compound was used together with 2H7-labelled cholesterol for simultaneous assay of unesterified lathosterol and cholesterol in serum by isotope dilution-mass spectrometry. After addition of a fixed amount of the two internal standards to a fixed amount of serum (in general 25 μl), the steroids were extracted with chloroform and subjected to Lipidex 5000 chromatography. The fraction containing cholesterol and lathosterol was converted into trimethylsilyl ether and subjected to mass spectrometric analysis with selected monitoring of the ions at m/z 458 (molecular ion of the trimethylsilyl ether derivative of unlabelled cholesterol and lathosterol), m/z 461 (molecular ion of derivative of 2Hrlabelled lathosterol) and m/z 465 (molecular ion of derivative of 2H7-Iabelled cholesterol).

Individual standard curves were used for assay of each steroid. Under the conditions employed, the coefficient of variation of the two assays was less than 6%. In different recovery experiments the maximal difference between expected and found values was less than 7%.

Using a less accurate method for analysis of lathosterol, we have shown previously that there is a high correlation between the hepatic HMG CoA reductase and the relative concentration of unesterified lathosterol in serum (concentration of lathosterol relative to cholesterol). This was confirmed with the present method and a correlation coefficient of about 0.94 was found between the two parameters. It is concluded that the present method may be suitable for detection of cases with accelerated rate of synthesis of cholesterol.