Current topics in cellular regulation Vol. 6 by Bernard L. Horecker, Earl R. Stadtman

By Bernard L. Horecker, Earl R. Stadtman

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G6PD from Microorganisms Α. Escherichia coli Some lines of evidence indicate the occurrence of metabolic control at a still unidentified site(s) of the pentose phosphate pathway in E, coli {150). Since this regulation does not involve any significant change in steady-state concentration of any of the crucial enzymes in this pathway, namely G6PD, 6-PgD, transketolase, and transaldolase, two possible mechanisms have been suggested to explain the variable contribution of oxidative and nonoxidative segments to pentose synthesis: (a) a fluctuat­ ing demand for NADPH and (b) modulation by some effectors of one or more enzymes involved.

According to their findings, the increased synthesis of fatty acids may rather depend on an increased activity of additional enzymes catalyzing the carbonchain supplying reactions in lipogenesis, such as malic enzyme, citrate cleavage enzyme, and glucokinase (167). Therefore the induction of G6PD by the high carbohydrate diet appears to be secondary to the enhanced lipogenesis and the physiological significance of this induction in the economy of liver metabolism remains obscure. h, Inactivation of G6PD, Although the changes of G6PD activity fol­ lowing a number of nutritional states seem to depend primarily on a modified rate of G6PD synthesis, it has also been demonstrated that 40 Α.

Folia Microbiol. 12, 291 (1967). 66. , Bacteriol. Rev. 33, 48 (1969). 67. , Colloq. Int. Centre Nat. Reck. Sei. (Paris) 124, 553 (1963). 68. , Proc. Nat. Acad. Sei. U. S. 54, 704 (1965). 69. , Spitzer, J. , J. Bacteriol. 106, 739 (1971). 70. Smith, E. , DeLange, R. , and Markland, F. , J. Biol. Chem. 243, 2184 (1968). 71. Smith, E. , Markland, F. , and Glazer, A. , in "Structure-Function Rela­ tionships of Proteolytic Enzymes" (P. Desnuelle, H. Neurath, and M. ), pp. 160-172. Munksgaard, Copenhagen, 1970.

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