Pressure variation of enzymatic reaction rates: yeast and liver alcohol dehydrogenase.

The kinetics of yeast and liver alcohol dehydrogenase (YADH and LADH) have been investigated by spectrophotometry at pressures between 1 and 2000 bar. For YADH the common random two substrate mechanism has been used as a model for evaluation of the pressure variation of five kinetic constants in the ethanol-NAD reaction. The dissociation volume associated with each constant is estimated and it is found that the dissociation of binary complexes is followed by large volume decreases, while the dissociation of ternary complexes is followed by smaller volume increases. There is a volume increase following formation of the activated complex in the rate determining step, and the over-all reaction rate decreases with pressure, going to zero at 2000 bar. LADH shows a complicated behaviour at high pressure. This is believed to be due to the substrate inhibition phenomenon occurring at ethanol concentrations above 10 mM. At such concentrations the reaction rate increases with pressure, reaching a maximum at about 1200 bar and goes to zero at 2500 bar. At ethanol concentrations lower than 10 mM there is a small decrease of reaction rate with pressure. To relate the volume Changes of the over-all process to those of the intermediate complexes, the partial molal volume of ethanol, acetaldehyde, NAD⁺ and NADH are determined by density measuraments.