Hepatic delta 6-desaturase activity is primarily located in the mitochondrial fraction in mice. Both delta 6- and delta 5-desaturase activities are increased in the liver of young (6-week-old) obese mice. The increase in hepatic delta 6-desaturase activity in obese mice does not occur until weaning. Neither restriction of food intake nor hyperinsulinaemia normalize hepatic delta 6-desaturase activity of obese mice. Both cold acclimation and tri-iodothyronine (30 micrograms/day per kg) decreased hepatic delta 6-desaturase activity of obese mice to levels observed in lean mice, whereas the increase in activity in obese mice was still maintained after the induction of hypothyroidism.

The activity of hepatic [Na+ + K+]ATP-ase showed a gene-dosage relationship in 6 week old mice. Before weaning hepatic [Na+ + K+]ATP-ase activity was normal in preobese mice but fell within 7 days of weaning to the low levels observed in older ob/ob mice. Brain [Na+ + K+]ATP-ase activity was unchanged in ob/ob mice although [3H]-ouabain binding was reduced. Arrhenius plots of [Na+ + K+]ATP-ase activity in liver and brain and of [3H]-ouabain binding to brain preparations showed breakpoints at lower temperatures in ob/ob than lean mice. These breakpoints were altered by pretreatment of tissue with deoxycholate. It is suggested that changes in membrane lipid composition might be an important factor regulating [Na+ + K+]ATP-ase in ob/ob mice.

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