Abstract
To compare endogenous cholesterol biosynthesis measured by deuterium incorporation (DI) and mass isotopomer distribution analysis (MIDA), cholesterol fractional and absolute synthetic rates were measured simultaneously by both techniques under identical physiological conditions. Twelve subjects (22 to 39 years of age) underwent a dual stable isotope protocol, involving oral deuterium oxide administration and measurement of incorporation of deuterium into cholesterol coincident with constant infusion of sodium [1-(13)C]acetate and measurement of the mass isotopomer distribution pattern of newly synthesized cholesterol. Synthesis was determined over 24 h with a 7-h feeding period. Both methods yielded similar measurements of fractional cholesterol synthesis (7.8 +/- 2.5% day(-)(1) for DI vs. 6.9 +/- 2.2% day(-)(1) for MIDA). Correlation of fractional synthesis across techniques was strong (r = 0.84, P = 0.0007). Absolute synthesis rates were also not different at 24 h (13.4 +/- 4.3 mg kg(-)(1) day(-)(1) for DI vs. 11.9 +/- 3.6 mg kg(-)(1) day(-)(1) for MIDA, r = 0.79, P < 0.002). We conclude that despite different assumptions and analytical requirements, deuterium incorporation and MIDA yield similar rates of cholesterogenesis in humans when measurements are made over 24 h. The decision as to which method to adopt depends on available clinical and analytical facilities
Highlights
To compare endogenous cholesterol biosynthesis measured by deuterium incorporation (DI) and mass isotopomer distribution analysis (MIDA), cholesterol fractional and absolute synthetic rates were measured simultaneously by both techniques under identical physiological conditions
The fundamental outcome of both methods is the fractional synthesis rate (FSR), which is defined as the fraction of the rapidly turning over cholesterol pool that is newly synthesized from a precursor over a 24-h period
We have shown previously that the FSR obtained from erythrocyte membrane cholesterol corresponds to the FSR obtained from plasma free cholesterol by the DI method [5, 20]
Summary
To compare endogenous cholesterol biosynthesis measured by deuterium incorporation (DI) and mass isotopomer distribution analysis (MIDA), cholesterol fractional and absolute synthetic rates were measured simultaneously by both techniques under identical physiological conditions. Prior to the use of stable isotopes, endogenous cholesterol synthesis could be measured by sterol balance, 3-hydroxy-3-methylglutaryl-CoA reductase activity, cholesterol turnover with [14C]cholesterol, and quantitation of plasma cholesterol precursor levels [4, 5] These methods each possess various limitations, including the requirement for extended measurement periods and/or indirect or overly invasive approaches [4, 5]. Stable isotope methods including deuterium incorporation (DI) and mass isotopomer distribution analysis (MIDA) meet the demand for immediate, direct, and not overly invasive measurement of cholesterol biosynthesis. The deuterated water tracer equilibrates in total body water and NADPH, that is, a precursor pool from which 22 of the 46 hydrogens in cholesterol derive Deuterium enrichment of this precursor pool, accessible as plasma water, is analyzed by isotope ratio mass spectrometry (IRMS) [4, 5].
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