Limits for the use of (18O) cholesterol and (18O)sitosterol in studies of cholesterol metabolism in humans

Abstract

The present study was undertaken in order to determine whether 18O-labeled sterols could be used in place of 14C-sterols in clinical studies of cholesterol metabolism. (3 beta-18OH)Cholesterol and (3 beta-18OH)sitosterol were simply and inexpensively synthesized and precisely and accurately quantified by gas chromatography/mass spectrometry. 18O-Sterols added to fecal homogenate and saponified were completely recovered. However, in a series of validation studies in humans, the fecal recoveries of orally administered (18O)cholesterol and (18O)sitosterol were significantly lower than the recoveries of 14C-sterols given simultaneously. We found that the losses were largely limited to the coprostanol and ethylcoprostanol fecal metabolites. In vitro fecal incubations of 18O-sterols and unlabeled water or of unlabeled sterols with H2(18)O indicated that the losses occurred during fecal bacterial metabolism and were likely due to 3 beta-oxygen exchange with the oxygen of water, possibly via a 3-ketosteroid intermediate. These data indicate that (18O)cholesterol and (18O)sitosterol are invalid tracers for the measurement of human cholesterol metabolism by methods based on fecal sterol recovery.

Stable Isotopes in Human Nutrition—Laboratory Methods and Research Applications

“…IRMS is an alternative method with several advantages with respect to cholesterol metabolism. In this technique, enrichment in circulating cholesterol is determined on purified cholesterol after combustion to CO 2 or reduction water of combustion to hydrogen gas (Hudgins et al, 1988;Jones et al, 1993). The principal positive feature is that the isotope ratios can be measured to a very high precision, sometimes as much as 10 -5 , or 20-fold that of GC-MS.…”

Supported Membrane Composition Analysis by Secondary Ion Mass Spectrometry with High Lateral Resolution

“…Besides deuterium, there are three more potential isotopic labels for lipids: 13 C, 18 O, and 15 N. 15 N can be selectively substituted into the headgroup of a phospholipid, but this generates only a single unique nuclide per lipid molecule. Although there is not a simple method to replace the six oxygen atoms within a phospholipid with 18 O, an 18 O label can be easily incorporated into cholesterol (Hudgins et al, 1988). Preliminary results indicate that the background signal from the natural abundance of 18 O (0.2%) in the oxidized silicon substrate overwhelms the 18 O ÿ secondary ion signal collected from the sample, obscuring visualization of the 18 O-cholesterol (substrates prepared with 18 O-depleted oxygen may curtail this background signal).…”