Relationship between Intake and Rate of Oxidation of Leucine and α-Ketoisocaproate In Vivo in the Rat

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Abstract
Rates of oxidation of leucine by rats fed, ad libitum, diets containing graded amounts (0–2.4%) of leucine together with L-[1-14C]leucine were measured for 12 hours. The amount of leucine oxidized was low until dietary leucine content exceeded that needed for maximum rate of weight gain. Thereafter, the rate of leucine oxidation increased essentially linearly with increasing dietary leucine content. Rates of oxidation of α-ketoisocaproic acid (KIC) by rats fed, ad libitum, diets devoid of leucine to which graded amounts (0–2.05%) of KIC were added together with [1-14C]KIC were also measured for 12 hours. The amount of KIC oxidized increased as dietary KIC content was increased over the entire range of dietary levels tested. At the end of the 12-hour oxidation studies plasma valine and α-ketoisovaleric acid (KIV) and plasma isoleucine and α-keto-β-methylvaleric acid (KMV) concentrations were depressed when dietary levels of leucine were high. Plasma leucine and KIC concentrations were low when dietary levels of leucine were low and increased with increasing dietary leucine content. Leucine and KIC concentrations in plasma of rats fed a diet devoid of leucine did not increase in proportion to increasing dietary KIC content.