Do branched chain keto amino acids regulate their own metabolism?

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Abstract
The effects of branched-chain α-ketoacids on flux through and activity state of the branched-chain α-ketoacid dehydrogenase complex were studied in hepatocytes prepared from chow-fed, starved, and low-protein-diet-fed rats. Very low concentrations of α-ketoisocaproate caused a dramatic stimulation (50% activation at 20 μM) of α-ketoisovalerate decarboxylation in hepatocytes from low-protein-fed rats. α-Keto-β-methylvalerate was also effective, but less so than α-ketoisocaproate. α-Ketoisocaproate did not stimulate α-ketoisovalerate decarboxylation by hepatocytes from chow-fed or starved rats. To a smaller degree, α-keto-β-methylvalerate and α-ketoisovalerate stimulated α-ketoisocaproate decarboxylation by hepatocytes from low-protein-fed rats. The implied order of potency of stimulation of flux through branched-chain α-ketoacid dehydrogenase was α-ketoisocaproate > α-keto-β-methylvalerate > α-ketosiovalerate, i.e., the same order of potency of these compounds as branched-chain α-ketoacid dehydrogenase kinase inhibitors. Fluoride, known to inhibit branched-chain α-ketoacid dehydrogenase phosphatase, largely prevented α-ketoisocaproate and α-chloroisocaproate activation of flux through the branched-chain α-ketoacid dehydrogenase. Assay of the branched-chain α-ketoacid complex in cell-free extracts of hepatocytes isolated from low-protein-diet-fed rats confirmed that α-ketoacids affected the activity state of the complex. Branched-chain α-ketoacids failed to activate flux in hepatocytes prepared from chow-fed and starved rats because essentially all of the complex was already in the dephosphorylated, active state. These findings indicate that inhibition of branched-chain α-ketoacid dehydrogenase kinase activity by branched-chain α-ketoacids is important for regulation of the activity state of hepatic branched-chain α-ketoacid dehydrogenase.
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