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The basal activity of neutral cholesterol esterase (CEase) in isolated hepatocytes from fed rats showed a characteristic variation during incubation with an initial rise at 30 min and subsequent significant decrease at 2 h. Addition of estradiol (10(-4)M) not only prevented this decrease but also caused an early increase in the activity of the enzyme (125% above basal value) after 60 min of incubation. The simultaneous addition of dibutyryl cyclic AMP (db-cAMP) (10(-5)M) blocked the induction, whereas the cyclic nucleotide alone inhibited significantly the esterase activity. When hepatocyte suspensions were incubated in the presence of different fatty acids (oleate, myristate and laurate, initial concentration 1 mM) for a period of 1 h, a reduction in the CEase activity was observed. The present results suggest that the activity of rat liver neutral CEase may be under both direct and indirect hormonal control.

5'-Nucleotidase was measured in isolated fat cells from normal, hypothyroid and hyperthyroid rats. This was done to find out whether thyroid hormones had an effect on the production of adenosine by the fat cell. The results showed that 5'-nucleotidase is modified when the rats received injections of 3,3',5-triiodo-L-thyronine (T(3)). There was no change in the enzyme in hypothyroidism or when T(3) was added to incubation of cells.

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