Effects of salts on DPNH oxidase activity & structure of sweet potato mitochondria

Abstract

The respiratory chain oxidation of external DPNH by particles isolated in sucrose-tris-EDTA medium from sweet potato roots is stimulated as much as tenfold by inorganic salts, at concentrations ranging from 10-4 to 10-1M. There is a specific divalent cation effect, as well as a general ionic strength effect. The DPNH oxidase activity is greatest at pH''s near neutrality, and at alkaline pH''s it depends on the sequence in which buffer and substrate are added. The tonicity of the medium has little effect in the absence of salts, but it does influence the activity when some salts are added. Although DPNH is rapidly oxidized in the absence of salts if ferrlcyanide or a dye is present, the properties of this diaphorase reaction differ from those of the DPNH oxidase. The salts also cause marked increases in the optical density of the particle suspensions. These effects are influenced by the tonicity and pH of the medium, and the evidence indicates that they represent alterations in the particle structure. The DPNH oxidase activity of digitonin fragments of the mitochondria is only slightly stimulated by salts. The results suggest that the salt-stimulation of DPNH oxidase is closely related to changes in particle structure, which may facilitate the access of substrate to the respiratory chain. Some other possible explanations are also considered.