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Comparative Amino Acid and Protein Metabolism in Obese and Non-Obese Zucker Rats^1,2,

Nutrition Laboratories, Department of Dairy and Animal Science, The Pennsylvania State University, University Park, PA 16802

A mixture of [U-¹⁴C]amino acids was injected intraperitoneally into fed obese and non-obese Zucker rats that were either growing (8 weeks of age) or at maintenance (17 weeks of age). The metabolic fate of the dietary amino acid pool was determined from the distribution of ¹⁴C into carcass lean tissue, carcass lipid, CO₂ and urine. In a second experiment, urinary and skeletal muscle 3-methylhistidine content was used to compare the rate of muscle protein breakdown between phenotypes at 8 weeks of age. The obese rat deposited a smaller proportion of its dietary amino acid pool in lean tissue compared with its non-obese control during growth and at maintenance. Obese rats incorporated a greater proportion of dietary amino acids into body lipid at both ages and metabolized a greater proportion of dietary amino acid carbon to CO₂ during growth. At 8 weeks of age, the obese rat had a higher fractional rate of muscle protein breakdown and was less efficient at retaining amino acids that had been incorporated into muscle. These latter differences were major factors in producing the variation in dietary amino acid utilization and protein accretion between growing obese and non-obese rats. At maintenance, the variation in dietary amino acid utilization between phenotypes was due principally to the smaller body protein mass of the obese animals.

KEY WORDS: • genetic obesity • Zucker rats • amino acid utilization • protein degradation • 3-methylhistidine

¹ Authorized for publication 3 January 1980 as journal series no. 5889 of the Pennsylvania Agricultural Experiment Station.

² From a thesis submitted by M. A. Dunn in partial fulfillment of the requirements for the M. S. degree. A preliminary report of some aspects of this work was presented at the 62nd Annual FASEB meeting, Atlantic City, NJ, 9–14 April 1978. Fed. Proc. 37, 675 (abs. 2431).

³ Deceased.

Manuscript received 22 January 1980.