[¹³C]cholesterol as a tracer for studies of cholesterol metabolism in humans

The investigation of cholesterol metabolism in humans would be facilitated by the availability of a tracer that is not radioactive. However, to be useful such a tracer must be detectable in the large pool of body cholesterol over an extended time. To meet these requirements we used synthetic [23,24,25,26,27-¹³C₅]cholesterol with detection by isotope ratio mass spectrometry (IRMS), a technique in which the ratio of ¹³CO₂/¹²CO₂ can be determined with high precision in the combustion products of a biological sample. [¹³C]cholesterol (65 mg) and [¹⁴C]cholesterol (15 μCi) were solubilized in Intralipid and injected simultaneously (IV) into three normal subjects, and plasma samples were obtained over the ensuing 10 weeks. Cholesterol was isolated from plasma and either counted for radioactivity or combusted to CO₂ and analyzed by IRMS. [¹⁴C]cholesterol and [¹³C]cholesterol tracer concentrations in plasma were very similar. [¹³C]cholesterol kinetic parameters calculated using a standard two- compartment cholesterol turnover model and a new three-compartment minimal model were 103 ± 10.5 SD % of those computed from corresponding [¹⁴C]cholesterol data. These results show that [¹³C₅]cholesterol can be used as a practical tracer for cholesterol metabolic studies lasting many weeks.