Regulation by progesterone and pregnenolone of dimeric aldehyde dehydrogenase from rat testis cytoplasm

1. Aldehyde dehydrogenase from rat testis cytosol has been purified to electrophoretic homogeneity. With an isoelectric point of 9.5, the enzyme appears a dimer with a subunit molecular weight of 52,500. 2. The influence of pregnenolone and progesterone on the kinetic behaviour has been investigated using valeraldehyde as substrate. 3. The kinetic data were fitted to a modified version of the Monod-Wyman-Changeux model and the fitting procedure resulted in a good correspondence between theoretical and experimental reaction rates over a wide range of valeraldehyde concentrations. 4. According to the model, the dimeric enzyme is in equilibrium between two conformational states R and T. The R state displays higher affinity for valeraldehyde, but lower catalytic power. In the absence of substrates and effectors the [T]/[R] ratio is near to 1. 5. Pregnenolone and progesterone activate the enzyme by stabilizing the more active state T and by increasing the catalytic power of the R state. The increase of activity is counteracted by the inhibition exerted by both steroids on the T state.