Assay of unesterified cholesterol-5,6-epoxide in human serum by isotope dilution mass spectrometry. Levels in the healthy state and in hyperlipoproteinemia.

Accurate methods based on isotope dilution-mass spectrometry were developed for assay of free cholesterol-5,6-epoxide (sum of 5 alpha,6 alpha- and 5 beta,6 beta-epimer) and cholestane-3 beta,5 alpha,6 beta-triol in human serum. In all serum samples tested, the level of cholesterol epoxides was well above the detection limit (about 10 ng/ml) whereas the level of cholestane-3 beta,5 alpha,6 beta-triol was below or near the detection limit in most cases. Immediate addition of antioxidant was found to be necessary in order to obtain reproducible results in the serum analyses, and prolonged storage of frozen samples had to be avoided. The level of cholesterol epoxide in healthy subjects 23-35 years of age ranged from 67 ng/ml to 293 ng/ml (mean 131 ng/ml, n = 9). There was a tendency to higher levels with increasing age, but there was no correlation to serum cholesterol. In marked contrast to results previously reported with a less accurate method, patients with various forms of hyperlipoproteinemia did not have increased levels of cholesterol epoxide. On the contrary, many of these patients had levels lower than normal.