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On the Structure and Catalytic Function of Mammary Glucose 6-Phosphate Dehydrogenase

From the ¹ From The Biological Research Laboratories, Department of Bacteriology and Botany, Syracuse University, Syracuse, New York 13210

Experiments were conducted in order to elucidate the relationship between the structure of mammary glucose 6-phosphate dehydrogenase and its catalytic activities with nicotinamide adenine dinucleotide phosphate and nicotinamide adenine dinucleotide. A model is proposed to account for the results which depicts the enzyme as existing in the form of two monomers, X and Y, in rapid, mobile equilibrium with each other, and in which X can be reversibly dissociated into inactive subunits under suitable conditions and Y can be dimerized at high protein concentration. The formation of Y from X is promoted by the addition of NADP⁺ or NADPH, whereas glycerol or NAD⁺ promote the formation of X from Y. The NADP-linked activity of Y is greater than that of X, but X possesses greater NAD-linked activity than Y.

The experiments here presented in support of the proposed model deal with the interconversion of X and Y. The kinetics of NADPH inhibition of NAD-linked glucose 6-phosphate dehydrogenase is compared in aqueous solution and in 40% glycerol. Sigmoid kinetics is seen in aqueous solution, but normal kinetics obtains in 40% glycerol. Experiments are reported showing that, under the conditions used in the kinetic studies, NADPH does not cause the enzyme to dimerize. It is also shown that the rate and extent of reactivation of glucose 6-phosphate dehydrogenase from inactive subunits are promoted in the presence of NADPH and hindered in the presence of glycerol. Finally, the effects of various reagents upon the NADP-linked and NAD-linked activities of glucose 6-phosphate dehydrogenase are seen to be compatible with their action on the equilibrium between X and Y. Interconversion of X and Y may be of importance in the regulation of mammary glucose 6-phosphate dehydrogenase activity.

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