1. The synthesis of long-chain fatty acids de novo was measured in the liver and in regions of adipose tissue in intact normal and genetically obses mice throughout the daily 24h cycle. 2. The total rate of synthesis, as measured by the rate of incorporation of 3H from 3H2O into fatty acid, was highest during the dark period, in liver and adipose tissue of lean or obese mice. 3. The rate of incorporation of 14C from [U-14C]glucose into fatty acid was also followed (in the same mice). The 14C/3H ratios were higher by a factor of 5-20 in parametrial and scapular fat than that in liver. This difference was less marked during the dark period (of maximum fatty acid synthesis). 4. In normal mice, the total rate of fatty acid synthesis in the liver was about twofold greater than that in all adipose tissue regions combined. 5. In obese mice, the rate of fatty acid synthesis was more rapid than in lean mice, in both liver and adipose tissue. Most of the extra lipogenesis occurred in adipose tissue. The extra hepatic fatty acids synthesized in obese mice were located in triglyceride rather than phospholipid. 6. In adipose tissue of normal mice, the rate of fatty acid synthesis was most rapid in the intra-abdominal areas and in brown fat. In obese mice, all regions exhibited rapid rates of fatty acid synthesis. 7. These results shed light on the relative significance of liver and adipose tissue (i.e. the adipose 'organ') in fatty acid synthesis in mice, on the mino importance of glucose in hepatic lipogenesis, and on the alterations in the rate of fatty acid synthesis in genetically obese mice.

A non-invasive method has been developed for measuring milk intake of suckling mice under physiological conditions. This method was used to determine whether genetically obese (ob/ob) mice are hyperphagic at 10 and 15 days of age. Lactating dams were injected with tritiated water (3H2O), which equilibrated in body water within 30 minutes. A constant specific activity of 3H2O was maintained over a 24-hour period by provision of 3H2O in drinking water. Tritium accumulation in body water of pups was proportional to their milk intake. After 24 hours, pups were removed from the dam, weighed, and blood samples (less than 10 microliters) obtained for assay of plasma 3H content by liquid scintillation counting. Body water content was computed from body weight. The composition of mouse milk taken from dams on days 10 and 15 of lactation was analyzed both volumetrically and gravimetrically. Water content was 68 to 69%; lipid content was 20% on day 10, 17% on day 15. At 10 days, mean milk intake was 0.96 ml, independent of litter size. At 15 days, intake per pup tended to decrease, from 1.4 to 0.8 ml, with increasing litter size. Using this method we have established that under physiological conditions ob/ob mice, which were identified as such at 4 to 5 weeks of age, do not have increased milk consumption at either 10 or 15 days of age.

1. The effect of nutritional status on fatty acid synthesis in brown adipose tissue was compared with the effect of cold-exposure. Fatty acid synthesis was measured in vivo by 3H2O incorporation into tissue lipids. The activities of acetyl-CoA carboxylase and fatty acid synthetase and the tissue concentrations of malonyl-CoA and citrate were assayed. 2. In brown adipose tissue of control mice, the tissue content of malonyl-CoA was 13 nmol/g wet wt., higher than values reported in other tissues. From the total tissue water content, the minimum possible concentration was estimated to be 30 microM 3. There were parallel changes in fatty acid synthesis, malonyl-CoA content and acetyl-CoA carboxylase activity in response to starvation and re-feeding. 4. There was no correlation between measured rates of fatty acid synthesis and malonyl-CoA content and acetyl-CoA carboxylase activity in acute cold-exposure. The results suggest there is simultaneous fatty acid synthesis and oxidation in brown adipose tissue of cold-exposed mice. This is probably effected not by decreases in the malonyl-CoA content, but by increases in the concentration of free long-chain fatty acyl-CoA or enhanced peroxisomal oxidation, allowing shorter-chain fatty acids to enter the mitochondria independent of carnitine acyltransferase (overt form) activity.

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