The influence of deuterium oxide and organic solvents on the interaction of respiratory chain components.

1. The influence of D20 and organic solvents on the rate of oxidation of added substrates by heart muscle particles and tightly coupled rat liver mitochondria has been investigated. Solvents examined include glycerol, sucrose, dimethyl sulfoxide, dimethyl formamide, and ethylene glycol. Each of these solvents was found to induce a marked inhibition of respiratory chain activity. 2. The relative constancy in the steady state reduction level of the respiratory carriers at each increment of solvent concentration prevents the assignment of a single specific site of respiratory inhibition, and favors the hypothesis that the solvent effects are due to a “nonspecific” inhibition which operates in a similar manner at several sites in the respiratory chain. 3. No correlation was found between the inhibition of respiratory activity by solvents and the viscosity of the solvent reaction mixtures. 4. The organic solvent inhibition of the DPNH oxidase activity of heart muscle particles was a linear function of the water content of the solvent reaction mixtures in which the particles were suspended, and independent of the concentration, tonicity, or chemical nature of the solvent used. 5. It is suggested that several protonolytic reactions may be operative during the transfer of reducing equivalents from substrate to oxygen by way of the respiratory chain. The respiratory chain components mediating these reactions are in some manner accessible to, and markedly influenced by, the aqueous phase surrounding the particles, and therefore do not function entirely in a lipid nonaqueous milieu.