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The specific activities of the fatty acid synthetases in the cytosolic fraction of livers and epididymal fat pads from fed, fasted or refed rats were determined. Refed rats received diets which provided, as the primary energy sources, sucrose or starch (75%) or both (59%) with beef tallow or safflower oil (16%) or the monosaccharide components (75%). From these determinations of fatty acid synthetase activity, the relative contribution of each tissue to the rat's overall lipogenic capacity was estimated. In rats fed a cereal-based stock diet ad libitum the adipose tissue accounted for 58% of total activity. During a 2-day fast the hepatic activity decreased from 14 units/100 g body weight to 2.7 units whereas the adipose tissue activity fell from 19 to 13.4 units/100 g body weight. At this time, 82.8% of the activity was in the latter tissue. After refeeding the sucrose diet for 2 days, the hepatic activity had increased 45-fold to 122.3 units/100 g body weight while the adipose tissue activity increased only 2.2-fold to 29.1 units, 19% of the total activity. We estimate that these adaptive responses deteriorate slowly (12.3 days, adipose tissue: 15.4 days, liver) with continued refeeding to the levels present in rats fed the cereal-based stock diet. Activation of residual or constituative enzyme was the major factor in the adipose tissue response whereas activation and induction played roles in the hepatic response. Responses to the refeeding of the other diets were of lesser magnitude. Generally our estimates based on fatty acids synthetase activities are consistent with other estimates based on radiolabel incorporation, NADPH generation and rate-limiting enzyme activities.

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