Some effects of tonicity on lupine succinoxidase

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Abstract
Lowering the osmotic concentration of solutions used both for isolationg Lupinus albus mitochondria and enzyme assay decreased [alpha], a measure of the interaction of the malonate and succinate enzyme complexes in the first order reaction of succinoxidase, from values associated with competitive inhibition by malonate to values associated with non-competitive inhibition. Both KS and KI, the apparent Michaelis constants for succinoxidase activation and inhibition by succinate and malonate, were decreased by lowering the osmotic concentration used in preparing the mithochondria. Low assay tonicity, compared with high, decreased Kg and progressive lowering of the preparation tonicity decreased the Ks/KI without affecting in a parallel manner the maximal activity Vm of succinoxidase at infinite succinate concentration. Lowering the preparation and assay tonicities also affected the order of succinoxidase activity with respect to succinate concentration. Most mitochondria isolated in 0.15 [image] solutions showed second order succinoxidase activity while those isolated in higher concentrations less frequently showed the second order reactions. For those preparations which showed the second order reaction, lowering the preparation tonicity decreased the ratio K1/K2, where K1 and K2 are the apparent dissociation constants for succinate in the activating and inhibiting positions, respectively. The convention is used in which first order reactions involve only 1 molecule each of substrate, activator and inhibitor per enzyme site, and 2nd order reactions involve 2 molecules of any 1 reactant per enzyme site.