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Measurement of cholesterol synthesis in kinetically defined pools using fecal steroid analysis and double labeling
Abstract:
The purpose of the study was to develop a kinetic method for measurement of different parameters of cholesterol metabolism in man using labeled cholesterol precursors that could initially be incorporated even into the slowly exchangeable cholesterol pool. For this purpose, tritiated water and [2-14C]mevalonate were given to five normocholesterolemic subjects and the activities for serum cholesterol and body water were measured serially for up to eight weeks. Elimination of cholesterol was measured by fecal analysis of neutral and acidic steroids. For comparison, two subjects received a mixture of [4-14C]cholesterol and [2-3H]mevalonate. The data were subjected to multicompartmental analysis by computer, with the assumption that synthesis occurred in two compartments. The rapidly exchangeable cholesterol (pool 1) and the fractional hydrogon transport constant from body water to cholesterol could not be measured directly; therefore, the influence of two different mass transport values was tested. The best fit was obtained with the smaller mass of cholesterol in pool 1 associated with a hydrogen transport constant of 0.700 (32 out of 46 hydrogens originate from water). Kinetic analysis of the data allows estimates of the exchangeable cholesterol mass, flux rates of cholesterol between pools 1 and 2, and synthesis of cholesterol separately in the two pools. The results of computer analysis suggested that, in contrast to what has been assumed earlier on the basis of studies with radiolabelled cholesterol, 22-53% of endogenous cholesterol synthesis took place in pool 2 from body water and that this synthesis tended to correlate with the total body fat mass. The study with [2-3H]mevalonate and [4-14C]cholesterol indicated synthesis in pool 2 to be 20-22% of the total. Up to 50% of the DL[2-14C]mevalonate dose was incorporated into cholesterol. The fractional incorporation of DL-mevalonate into pool 2 was correlated with that of tritiated water, indicating that both precursors studied yielded essentially the same kinetic result.