Abstract
Leucine was oxidized by rat adipose tissue at a rate which was not limited by the activity of branched chain amino acid transaminase since high concentrations (10 mM) of [1-14C]leucine and its transamination product, alpha-keto[1-14C]isocaproate, were oxidized at similar rates. Despite the apparent abundance of transaminase activity, however, [1-14C]valine was oxidized at only 10 to 25% of the rate of its transamination product, alpha-keto[1-14C]isovalerate. The net rate at which [1-14C] valine was transaminated by intact tissues was estimated as the sum of the rates of 14CO2 production and alpha-ketoiso[1-14C]valerate release into the medium. Transamination did not limit the rate of valine oxidation since valine was transaminated 3 times as fast as it was oxidized. The rate of valine transamination increased 18-fold when its concentration was raised 100-fold, but the fraction of [1-14C]valine oxidized to 14CO2 remained constant over the range of incubation conditions studied. The oxidation/transamination ratio for leucine was also constant and exceeded the oxidation/transamination ratio for valine unless valine oxidation was stimulated, either by the addition of glucose or leucine. Stimulation of valine oxidation did not increase its transamination but reduced the rate at which alpha-ketoisovalerate was released from the tissue. The faster oxidation of alpha-ketoisocaproate than of alpha-ketoisovalerate may be due to the activation of branched chain alpha-keto acid dehydrogenase by alpha-ketoisocaproate, but the alpha-keto acid oxidation rates do not fully account for the faster transamination of leucine than of valine.
Highlights
Leucine was oxidized by rat adipose tissue at a rate limit the rate of valine catabolism [9]
The finding which was not limited by the activityof branched chain that addition of leucine increases valine oxidation by adipose amino acid transaminase sincehigh concentrations (10 tissue [3] is inconsistent with this suggestion unless leucine mM)of [1-"Clleucine and its transamination product, increases the transamination of valine
Leucine is thought to increase the formation of a-ketoglutarate from glutamate by stimulation of the enzyme glutamate dehydrogenase [17, 18].To determine whether transamination might limit the rate of valine catabolism, we estimated the rate of valine transamination in situ as the sum of the rates of a-keto acid oxidation and efflux
Summary
Leucine was oxidized by rat adipose tissue at a rate limit the rate of valine catabolism [9]. The finding which was not limited by the activityof branched chain that addition of leucine increases valine oxidation by adipose amino acid transaminase sincehigh concentrations (10 tissue [3] is inconsistent with this suggestion unless leucine mM)of [1-"Clleucine and its transamination product, increases the transamination of valine. Despite the apparenatbundance of transaminase of BCDH in adipose tissue [3]. This unique effect of leucine activity, [1-"Clvaline was oxidized at only appears to be due to theproperties of the kinase that catalyzes. The properties of BCDH kinase do not account for the transamination of leucine at a rate fastetrhan that of valine [9]
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