Nuclear 3,5,3'-triiodothyronine receptors and malic enzyme activity in liver of rats fed fish oil or cocoa butter

Knopp, J.; Klimes, I.; Brtko, J.; Sebokova, E.; Bohov, P.; Hromadova, M.; Langer, P.; Balaz, V.

Nuclear 3,5,3'-triiodothyronine receptors and malic enzyme activity in liver of rats fed fish oil or cocoa butter

1992

The role of different fat supplements (fish oil and cocoa butter), applied intragastrically for 3 weeks, on specific binding of T3 by rat liver nuclear receptors was examined. Fish oil (rich in n-3 polyunsaturated fatty acids) supplementation resulted in a significant (P < 0.05) enhancement of the maximal binding capacity (MBC) for T3 with a diminution of the apparent association constant (Ka). No changes in nuclear receptor MBC or Ka were observed after cocoa butter administration. Moreover, the rate of fatty acid synthesis in the liver was evaluated by measuring malic enzyme (EC.1.1.1.40.) activity in rat liver cytosol, which was found to be decreased in both the cocoa butter- (P < 0.002) and the fish oil- (P < 0.001) treated rats. Interestingly, in fish oil-supplemented animals, not even a higher MBC value for the T3 receptors in liver nuclei and specific binding of T3 to its receptor were able to restore the activity of malic enzyme in liver cytosol. The findings of the same level of thyroid hormones-thyroxine (T4) and triiodothyronine (T3)--in blood of all groups showed that there was apparently no effect of fat supplementation on the thyroid function. However, it may be hypothesized that the decrease of malic enzyme activity in both fat-treated groups may have resulted from a decreased T4 to T3 conversion in liver cells. The data suggest the following: (a) increased dietary fat intake is accompanied by lower lipogenesis in the liver in relation to the degree of fat unsaturation; (b) although polyunsaturated dietary fat increases T3 receptor binding to liver nuclei; (c) thus, an uncoupling between T3 binding and action on malic enzyme activity during raised fish oil intake cannot be ruled out.

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