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Iodothyronine 5'-Deiodination in Rats Fed Low Protein Diets: Lack of Correlation with Energy Balance¹

Department of Food Science and Human Nutrition, Michigan State University, East Lansing, MI 48824-1224

Rats fed low protein diets often exhibit an elevation in adaptive thermogenesis that may result in part from activation of thyroid hormone action at the level of peripheral deiodination of thyroxine (T₄) to 3,5,3'-trilodothyronine (T₃). This possibility was examined by comparing the kinetic parameters of iodothyronine 5'-deiodination in hepatic and renal microsomes and the efficiency of energy retention in weanling rats fed diets containing 5% (LP-5), 8% (LP-8) or 22% (NP) casein. Energy balance measurements revealed that efficiency of energy retention was reduced in rats fed LP-5, whereas no significant change in adaptive thermogenesis was observed in rats fed LP-8. In the LP-5 group the maximal activity of hepatic 5'-deiodinase per milligram of microsomal protein was not different from that of the NP group; however, the V_max of the renal enzyme was 21% higher. In both tissues, the K_m of the reaction was not affected by dietary treatment. Hepatic and renal 5'-deiodination in rats fed LP-8 was similar to that in rats fed LP-5. When compared to corresponding NP controls, each of the low protein dietary groups (LP-5, LP-8) had a higher serum total T₃ concentration, whereas serum free T₃ and total T₄ concentrations were unaffected by the dietary treatments. The findings demonstrate that iodothyronine 5'-deiodination and circulating thyroid hormone concentrations vary in a similar manner in LP-5 and LP-8 animals even though these two groups exhibit different responses in energy efficiency relative to NP controls. This indicates that the above parameters regulating thyroid hormone action do not directly mediate reduced energy efficiency resulting from consumption of a low protein diet.

KEY WORDS: • thyroid hormone action • adaptive thermogenesis • reverse triiodothyronine 5'-deiodinase • Lineweaver-Burk analysis • liver • kidney • energetic efficiency

¹ Supported in part by National Institutes of Health Grants AM 15847 and AM 26851. Michigan Agricultural Experiment Station journal article 11637.

² Present address: Department of Pharmacology, Case Western Reserve University, Cleveland, OH 44106.

³ Author to whom reprint requests should be addressed.

Manuscript received 10 April 1986. Revision accepted 2 October 1986.