The activity of the intramitochondrial branched-chain 2-oxo acid dehydrogenase (BCDH), like that of pyruvate dehydrogenase, is regulated, at least in part, by interconversion between the active dephosphorylated enzyme and its inactive phosphorylated form. The stimulatory effect of insulin on BCDH activity was compared with its effect on phosphorylation of the enzyme. Intact tissues were incubated in the presence or the absence of insulin, and then mitochondria were isolated and disrupted before assaying for enzyme activity or estimating the extent of enzyme phosphorylation. Tissues were incubated in either the presence or the absence of leucine, which also stimulated BCDH activity up to 10-fold. Insulin (1 munit/ml) doubled the activity of BCDH in the absence and in the presence of leucine. Together, 1 mM-leucine and insulin appeared to stimulate BCDH activity fully. Phosphorylation of BCDH was estimated indirectly by measuring the incorporation of 32P into phosphorylation sites that remained unesterified after preparing mitochondrial extracts under conditions that preserved the effect of insulin on BCDH activity. Increased incorporation of 32P in these experiments implies decreased phosphorylation in situ when tissues were incubated with insulin and leucine. In the absence of leucine, little incorporation of 32P into BCDH was detected. In the presence of leucine, however, incorporation of 32P into BCDH was markedly increased, and insulin increased 32P incorporation still further. The results support the hypothesis that leucine and insulin both stimulate the activity of BCDH by promoting its dephosphorylation.

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