A double-isotope method for the measurement of ketone-body turnover in the rat. Effect of l-alanine

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Abstract
The synthesis of 4-3H-labeled ketone bodies and their use along with 14C-labeled ketone-body precursors is employed using an in vivo rat infusion model to measure ketone-body turnover. The use of 2 isotopes is necessary to measure ketone-body turnover when ketogenesis may occur from > 1 precursor such as glucose and fatty or amino acids. Requirements of isotopic equivalence in terms of metabolic similarity, valid stoichiometry and the lack of differences in the kinetics of relevant enzymes is demonstrated for the 4-3H- and 14C-labeled ketone bodies. The hypoketonemic effect of L-alanine is shown by 2 distinct phases after the administration of L-alanine. During the first 12 min after alanine administration there was a 50% decrease in acetoacetate and a 30% decrease in 3-hydroxybutyrate production, with no significant change in the utilization of either compound. The hypoketonemic action of alanine during the following 16 min was primarily associated with an uptake of 3-hydroxybutyrate that was somewhat greater than the increase in its production. There were essentially equivalent decreases in production and utilization of acetoacetate, resulting in no significant net change in the level of this ketone body in the blood.