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Factors Influencing the in vivo and in vitro Rates of Lipogenesis in Rat Liver

Department of Biochemical Nutrition, Hoffmann-La Roche Inc., Nutley, New Jersey 07110

In vivo and in vitro rates of lipogenesis were studied in rat liver. Several factors were established as important for the production of reproducibly consistent and maximal rates of lipogenesis in vivo: 1) a 48-hour prefast, 2) a single 3-hour meal-feeding daily, 3) a diet high in monosaccharides or disaccharides, 4) a body weight of 120 to 160 g. The rate of lipogenesis was found to be inversely related to body weight. Under the described conditions, the maximal rate was reached 5 to 9 days after institution of meal-feeding. Subsequently, the rate dropped to approximately one-half maximum and this level was maintained for at least 34 days. The kinetics of in vivo and in vitro lipogenesis were studied on day 7 of meal-feeding. Lipogenesis reached a maximum 5 hours after meal initiation and a minimum at 24 hours, when the in vivo rate had declined to 3% of the 5-hour level while the in vitro rate had declined to only 18 to 25% of this value. The ratio of the in vivo to the in vitro rate was a relative index used to compare the lipogenic capacity 5 and 24 hours after feeding. Five hours after feeding, the in vivo rate of lipogenesis was maximal, but apparently only 55 to 59% of the available lipogenic enzyme pool, as determined by the in vitro rate, was required to produce this rate. Twenty-four hours after feeding, only 7 to 8% of the lipogenic enzyme pool was used to produce the in vivo rate, the in vitro rate of lipogenesis being 13 to 15 times the observed in vivo rate. This suggests that in addition to lipogenic enzyme activities other regulators are functioning in the control of lipogenesis.