Physiological covalent regulation of rat liver branched-chain α-ketoacid dehydrogenase

• Published in: Archives of biochemistry and biophysics Vol. 243; no. 2; pp. 542 - 555
• Main Authors: Harris, Robert A., Powell, Steven M., Paxton, Ralph, Gillim, Sarah E., Nagae, Hidetoshi
• Format: Journal Article
• Language: English
• Published: San Diego, CA Elsevier Inc 01.12.1985; Elsevier
• Subjects: 3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide); Animals; Applied sciences; Dietary Proteins - administration & dosage; Dietary Proteins - pharmacology; Exact sciences and technology; Female; Ketone Oxidoreductases - metabolism; Leucine - pharmacology; Liver - enzymology; Male; Multienzyme Complexes - metabolism; Obesity - enzymology; Other techniques and industries; Radiochemistry; Rats; Rats, Inbred Strains; Rats, Zucker; Starvation - enzymology; Temperature
• ISSN: 0003-9861; 1096-0384
• DOI: 10.1016/0003-9861(85)90531-4

A radiochemical assay was developed for measuring branched-chain α-ketoacid dehydrogenase activity of Triton X-100 extracts of freeze-clamped rat liver. The proportion of active (dephosphorylated) enzyme was determined by measuring enzyme activities before and after activation of the complex with a broad-specificity phosphoprotein phosphatase. Hepatic branched-chain α-ketoacid dehydrogenase activity in normal male Wistar rats was 97% active but decreased to 33% active after 2 days on low-protein (8%) diet and to 13% active after 4 days on the same diet. Restricting protein intake of lean and obese female Zucker rats also caused inactivation of hepatic branched-chain α-ketoacid dehydrogenase complex. Essentially all of the enzyme was in the active state in rats maintained for 14 days on either 30 or 50% protein diets. This was also the case for rats maintained on a commercial chow diet (minimum 23% protein). However, maintaining rats on 20, 8, and 0% protein diets decreased the percentage of the active form of the enzyme to 58, 10, and 7% of the total, respectively. Fasting of chow-fed rats for 48 h had no effect on the activity state of hepatic branched-chain α-ketoacid dehydrogenase, i.e., 93% of the enzyme remained in the active state compared to 97% for chow-fed rats. However, hepatic enzyme of rats maintained on 8% protein diet was 10% active before starvation and 83% active after 2 days of starvation. Thus, dietary protein deficiency results in inactivation of hepatic branched-chain α-ketoacid dehydrogenase complex, presumably as a consequence of low hepatic levels of branched-chain α-ketoacids, established inhibitors of branched-chain α-ketoacid dehydrogenase kinase. With rats fed a low-protein diet and subsequently starved, inhibition of branched-chain α-ketoacid dehydrogenase kinase by branched-chain α-ketoacids generated as a consequence of endogenous proteolysis most likely promotes the greater branched-chain α-ketoacid dehydrogenase activity state.