Influence of Ring Substituents on the Action of Phenols on Some Dehydrogenases, Phosphokinases and the Soluble ATPase from Mitochondria

The effects of phenols on the isolated mitochondrial ATP phosphohydrolase (ATPase) have been compared with their effects on several dehydrogenases, phosphokinases, catalase and acid phosphatase. 2,4‐Dinitrophenol and pentachlorophenol inhibit several of these enzymes at phenol concentrations similar to those which inhibit the ATPase. Stimulation of enzyme activity, by certain phenols which have a para‐substituent, is observed with the ATPase but not with any of the other enzymes. The concentrations of phenols required to inhibit lactate dehydrogenase, hexokinase and the ATPase, and to produce maximal stimulation of the ATPase have been measured. These biological effects have been correlated with constants which describe the effect of ring substituents on the hydrophobic bonding and electronic properties of the phenols. The kinetics of the inhibition of lactate dehydrogenase and hexokinase by phenols have been investigated. These inhibitions are competitive with the adenine‐containing substrate and are second order in phenol concentration. When the adenine in NADH is replaced by hypoxanthine the inhibition becomes first order in phenol concentration. When ATP is replaced by ITP as the substrate for the ATPase the inhibition by phenols changes from second to first order. It is concluded that the effects of phenols on the coupling factor ATPase are not related to their ability to uncouple oxidative phosphorylation. The observations suggest that phenols bind to sites in enzymes which normally bind an adenine residue in a substrate.