A ^sup 14^C-leucine absorption, distribution, metabolism and excretion (ADME) study in adult Sprague-Dawley rat reveals [beta]-hydroxy-[beta]-methylbutyrate as a metabolite

• Published in: Amino acids Vol. 47; no. 5; p. 917
• Main Authors: Lee, Anthony J, Beno, David W; A, Zhang, Xiaolin, Shapiro, Robin, Mason, Mark, Mason-bright, Tanita, Surber, Bruce, Edens, Neilé K
• Format: Journal Article
• Language: English
• Published: Vienna Springer Nature B.V 01.05.2015
• ISSN: 0939-4451; 1438-2199
• DOI: 10.1007/s00726-015-1920-6

Leucine is an essential branched-chain amino acid that acts as a substrate for protein synthesis and as a signaling molecule. Leucine not incorporated into muscle protein is ultimately oxidized through intermediates such as [beta]-hydroxy-[beta]-methylbutyrate (HMB) which itself is reported to enhance muscle mass and function in rats and humans. HMB has been reported in the plasma following oral leucine administration in sheep and pigs but not in Sprague-Dawley rats, the standard preclinical model. Therefore, we conducted radiolabeled absorption, distribution, metabolism and excretion (ADME) studies in rats using a low (3 mg/kg) or high dose (1,000 mg/kg) of ^sup 14^C-leucine. Blood, tissue, and urine samples were analyzed for ^sup 14^C-leucine and its metabolites by HPLC-MS. Our results show for the first time that ^sup 14^C-HMB appears in plasma and urine of rats following an oral dose of ^sup 14^C-leucine. ^sup 14^C-leucine appears in plasma as ^sup 14^C-[alpha]-ketoisocaproic acid (KIC) with a slower time course than ^sup 14^C-HMB, a putative product of KIC. Further, two novel metabolites of leucine were detected in urine, N-acetyl leucine and glycyl leucine. Mass balance studies demonstrate that excretory routes accounted for no more than 0.9 % of the radiolabel and approximately 61 % of the dose was recovered in the carcass. Approximately 65 % of the dose was recovered in total, suggesting that approximately one-third of the leucine dose is oxidized to CO2. In conclusion, this study demonstrates endogenous production of HMB from leucine in adult rats, a standard preclinical model used to guide design of clinical trials in nutrition.