The Estimation of Desmosterol and Cholesterol in Plasma.

Desmosterol (24-dehydrocholesterol) is the immediate precursor of cholesterol in the biosynthesis of the latter compound. The structures of these two sterols are very similar, and desmosterol interferes with many of the colorimetric methods used for estimating plasma cholesterol levels. Normally, desmosterol is not present in plasma in significant amounts, but in patients treated with triparanol, which inhibits the conversion of desmosterol to cholesterol, considerable quantities of desmosterol may be detected (Frantz, Mobberley, and Schroepfer, 1960; Avigan et al., 1960a, b; Steinberg, Avigan, and Feigelson, 1960, 1961). Triparanol was used therapeutically because of its ability to reduce plasma cholesterol levels, but, when studying the effects of treatment with this drug, it was shown (Frantz et al., 1960; Avigan et al., 1960a; Steinberg et al., 1960) that misleading results were obtained if the only sterol estimation consisted of the measurement, at a single wavelength (in the region 620-660 mu), of the colour developed in the Liebermann-Burchard reaction. Methods were described (Frantz et al., 1960; Avigan et al., 1960b; Hollander, Chobanian, and Wilkins, 1960) that could be used for estimating the content of cholesterol and desmosterol in specimens of plasma containing both these sterols, but, for various reasons which will be discussed, these methods failed to gain general acceptance, or were open to criticism. In the absence of a convenient method for estimating desmosterol and cholesterol in blood, reports continued to appear (Oille, 1961; Jepson, 1962) about patients in whom the laboratory control of triparanol treatment had consisted solely of the measurement of the Liebermann-Burchard colour in the region 620-660 mu; the results of such studies cannot be accurately interpreted, since cholesterol and desmosterol both show marked but different chromogenicity under these conditions; this means that the single colorimetric reading in this wavelength region cannot be used to measure either the total plasma sterol level or the plasma cholesterol concentration when significant amounts of desmosterol are present in the sample. Triparanol was withdrawn by the manufacturers in 1962 because several serious side-effects had been reported (Laughlin and Carey, 1962; Perry et al., 1962; Kirby et al., 1962). It is, however, possible that other drugs will be submitted for clinical trial with a mode of action similar to triparanol and, for the proper assessment of such drugs, it will be necessary to measure plasma levels of both desmosterol and cholesterol. This paper describes a simple colorimetric method for the separate determination of these two sterols in single samples of plasma. The method was used to investigate the response of 32 patients to treatment with triparanol, and it will be shown that, because patients did not respond uniformly to such treatment, measurement of the individual plasma sterol levels