RESPIRATORY CONTROL AND DEHYDROGENASE ACTIVITY

A comparative study has been made of the stimulation of respiration by ADP or by 2,4-dinitrophenol in liver mitochondria incubated with different substrates of NAD-dependent enzymes.It has been found that the degree of stimulation of respiration by ADP or by 2,4-dinitrophenol increased with the number of dehydrogenases involved in the oxidation of the substrate. The respiratory stimulation was greater with higher substrate concentrations. This suggests that the hydrogen pressure produced by dehydrogenase activity was, in each case, the factor which determined the degree of respiratory stimulation.When the concentration of substrate decreased, the stimulation of respiration diminished proportionally, and at very low substrate concentrations was depressed, the rate of respiration in the presence of ADP or 2,4-dinitrophenol being lower than in their absence. Such inhibition of respiration could be reduced or even abolished by increasing the concentration of substrate in the medium. Therefore, not only the degree of respiratory response to ADP or to 2,4-dinitrophenol but also its stimulatory or inhibitory character depended upon the dehydrogenase activity.The concentration of ADP or 2,4-dinitrophenol necessary to induce a maximal respiratory stimulation also increased with the number of dehydrogenases involved in the oxidation of the substrate.