Volume 9, number 1
MOLECULAR8Z CELLULARBIOCHEMISTRY
October 31, 1975
REGULATION OF MAMMALIAN PYRUVATE DEHYDROGENASE* Richard M. DENTON, Philip J. RANDLE, Barbara J. BRIDGES, Ronald H. COOPER, Alan L. KERBEY, Helen T. PASK, David L. SEVERSON,t David STANSBIE, and Susan WHITEHOUSE Department of Biochemistry, University of Bristol BS8 l TD . England
(Received November 15, 1974)
Summary In mammalian tissues, two types of regulation of the pyruvate dehydrogenase complex have been described: end product inhibition by acetyl CoA and NADH; and the interconversion of an inactive phosphorylated form and an active nonphosphorylated form by an ATP requiring kinase and a specific phosphatase. This article is largely concerned with the latter type of regulation of the complex in adipose tissue by insulin (and other hormones) and in heart muscle by lipid fuels. Effectors of the two interconverting enzymes include pyruvate and ADP which inhibit the kinase, acetoin which activates the kinase and Ca 2 + and Mg 2+ which both activate the phosphatase and inhibit the kinase. Evidence is presented that all components of the pyruvate dehydrogenase complex including the phosphatase and kinase are located within the inner mitochondrial membrane. Direct measurements of the matrix concentration of substrates and effectors is not possible by techniques presently available. This is the key problem in the identification of the mechanisms involved in the alterations in pyruvate dehydrogenase activity observed in adipose tissue and muscle. A number of indirect approaches have been used and these are reviewed. Most hopeful is the recent finding in this laboratory that in both adipose tissue and heart muscle, differences in activity of pyruvate An invited article. t Present address, Department of Medicine, University of California, San Diego, U.S.A.
dehydrogenase in the intact tissue persist during preparation and subsequent incubation of mitochondria.
Introduction The conversion of pyruvate to acetyl CoA catalysed by the pyruvate dehydrogenase complex serves both bioenergetic and biosynthetic roles. In many tissues including heart muscle, brain and kidney, the acetyl CoA is almost exclusively metabolised through the citrate cycle whilst in other tissues such as adipose tissue, mammary gland and liver a significant proportion is utilised in the synthesis of fatty acids and sterols. In animals, acetyl CoA cannot be used as a source of carbohydrate and thus metabolism of pyruvate through this essentially irreversible step represents net loss of body carbohydrate reserves. Regulation of this step is of critical importance to the general energy balance and fuel economy of animals. In heart and diaphragm muscles, lipid fuels (fatty acids, ketone bodies and acetate) are oxidised in preference to carbohydrate fuels (glucose, glycogen and pyruvate) 1-5. When rat hearts are perfused with medium containing glucose and insulin the further addition of palmitate, acetoacetate or acetate results in clear inhibition of glycolysis but even more marked inhibition of pyruvate oxidation (Table 1). A search for the mechanism of this latter inhibition led to the finding in 1964 by GARLANDand RANDLE 6 that the pyruvate dehydrogenase
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Table 1 The effects of palmitate, acetoacetate and acetate on glucose and pyruvate metabolism in the perfused rat heart
Experimental as ~ control Additions to medium (experimental)
glycolysis
lactate plus pyruvate output
pyruvate oxidation
42 58 55 36
105 125 83 100
11 36 25