The effects of x-irradiation in solution on the kinetic, sedimentation and fluorescent properties of glutamic acid dehydrogenase.

Abstract

An explicit mathematical statement for the effects of irradiation on the kinetic parameters K (Michaelis constant) and V (maximum velocity) of enzymes is provided. Irradiation of glutamic acid dehydrogenase, in solution, increases K and decreases V for the substrates glutamate and alpha -oxoglutarate. V is decreased and K is unchanged for the coenzymes diphosphopyridine nucleotide and reduced diphosphopyridine nucleotide (DPNH). The increase in K for glutamste is not associated with a change in the relative inhibitory power of dicarbexylic analogues of glutamate. Irradiation reduces the main protein pesk seen in the ultracentrifuge and produces slower sedimenting material. This is consistent with a splitting of the large kinetic molecular unit into sub-units. At very high doses rapidly sedimenting material, presumed to be aggregates, is produced. The decrease in the main component is coincident with a decrease in enzymic activity. Irradiation of the enzyme reduces its ability to enhance the fluorescence of DPNH. This is observed whether wavelength of the exciting light is 365 or 280 m mu . The change observed parallels a decrease in the maximum velocity, V. The increase in K for glutamate is not accompanied by a decrease in the augmented DPNH fluorescence observed on the addition ofmore » glutamate to enzyme -- DPNH mixtures. The decrease in maximum enzyme velocity is not paralleled by a marked fall in the intrinsic protein fluorescence. It is suggested that the primary action of the aqueous radicals formed on irradiation leads to modification in the tertiary structure of the enzyme with resulting loss in maximunn velocity. (auth)« less