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Leucine-Induced Amino Acid Antagonism in Rats: Muscle Valine Metabolism and Growth Impairment¹

Renal Division, Emory University School of Medicine, Atlanta, GA 30322 ^* Department of Medicine, Harvard Medical School, Brigham and Women's Hospital, Boston, MA 02115

The deleterious effects of branched-chain amino acid (BCAA) antagonism caused by excess dietary leucine include growth depression and subnormal valine and isoleucine pools. To investigate mechanisms causing these changes, rats were gavage-fed low-protein (9%) diets with or without BCAA supplements, and the metabolism of another BCAA (valine) was measured in incubated rat epitrochlearis muscles. A 10% leucine supplement (HL-10) inhibited growth; growth remained subnormal even when 2.6% isoleucine and 2.4% valine (HLIV-10) were added to the diet. Valine decarboxylation in muscle increased 170–270% in rats fed the HL-10 or HLIV-10 diets, but was still markedly lower than we previously found in muscle of rats fed a 14% protein diet. Valine incorporation into muscle protein as an estimate of protein synthesis was unaffected by any of the BCAA supplements. When a lower (4%) concentration of leucine (without or with 0.16% isoleucine and 0.16% valine) was studied, growth was also suppressed but only if rats had not been preconditioned to 9% protein. Although increased BCAA decarboxylation in muscle caused by excess dietary leucine contributes to low valine and isoleucine pools, abnormal growth appears to be independent of low valine and isoleucine levels and is not reflected in suppression of valine incorporation into muscle protein.

KEY WORDS: • branched-chain amino acids • valine metabolism • muscle metabolism • rats

¹ Supported by Grant DK 37175 from the National Institutes of Health.

² Current address: Kanonierstrasse 8, 4400 Munster, Germany.

³ To whom correspondence should be addressed.

Manuscript received 10 October 1989. Revision accepted 20 September 1990.