The metabolic route by which oleate is converted into cholesterol in rat hepatocytes

The effect of (-)-hydroxycitrate on the conversion of [1-14C]oleate into cholesterol was dependent on the time of day at which the cells were prepared and on the extracellular oleate concentration. In hepatocytes prepared during the light phase of the diurnal cycle (L2-hepatocytes), (-)-hydroxycitrate inhibited the conversion of L-[U-14C]lactate (2 mM) and of 0.13 mM-[1-14C]oleate into cholesterol. However, when [1-14C]oleate was present at 1.3 mM, most of the sterol carbon was derived from this source, and under these conditions (-)-hydroxycitrate had no inhibitory effect on [14C]cholesterol formation. In these cells, non-radioactive acetoacetate blocked the conversion of 1.3 mM-[1-14C]oleate, but not of 0.13 mM-[1-14C]oleate, into cholesterol. In cells prepared during the dark phase of the diurnal cycle (D6-hepatocytes), irrespective of the concentration of [1-14C]oleate, (-)-hydroxycitrate decreased its conversion into cholesterol. In both types of cell preparation, the inhibitory effect of (-)-hydroxycitrate on the conversion of L-[U-14C]lactate into cholesterol was greater than that on the overall rate of cholesterol production from all endogenous sources. These results provide evidence for the following. (1) The major metabolic route by which oleate is converted into cholesterol is dependent on its extracellular concentration. (2) When oleate is the major source of hepatic sterol carbon, the flux of substrate through citrate into cholesterol is dependent on the nutritional state of the animal. (3) When endogenous substrates are the sole source of sterol carbon, a substantial proportion of the carbon enters the cholesterol pathway through routes not involving citrate cleavage.