The effect of fluoride on succinic oxidase system

Abstract

Spectrophotometric methods for measuring the activities of the succinic oxidase system and succinic dehydrogenase are descr. In the man-ometric method of determining succinic dehydrogenase H2O2 is an end product. Thus O2 uptakes obtained by this method should be halved to make them comparable with activities of the complete succinic oxidase system, which yields water as the end product. F and phosphate separately have only very slight effects on succinic dehydrogenase revealed only at low substrate concns. The inhibition by F and phosphate, acting together, is much greater. It is a rapid reaction and is largely reversed by dilution. All 3 inhibitions are completely competitive with respect to succinate. One molecule each of phosphate and of F react with each enzyme molecule. It is concluded that succinic dehydrogenase is the only component of the succinic oxidase system susceptible to F, despite the fact that succinic dehydrogenase, as customarily measured, is inhibited to a smaller degree than the complete system. The reasons for the smaller inhibition in the latter case are: (a) succinic dehydrogenase is not working at its full activity in the test, because the concn. of the H acceptor limits the rate of the reaction; (b) the simplifying assumptions of the Michaelis-Menten theory do not apply to succinic dehydrogenase. Arsenate behaves like phosphate but to a lesser degree. Mn and Mg did not affect the inhibition by F and phosphate. F had the same inhibitory action on a phosphorylating mitochondrial preparation as on a non-phosphorylating prepn. Inhibitory constants, Michaelis constants and rate constants of the uninhibited reaction were calculated. The relation between the Michaelis constant and dissociation constant of the enzyme-substrate compound is discussed.