INACTIVATION OF ALCOHOL-DEHYDROGENASE IN RICE SEEDLINGS IS RELATED TO A MICROSOMAL FATTY-ACID ALPHA-OXIDATION SYSTEM

The selective inactivation of alcohol dehydrogenase by the inactivator found in the microsomal fraction of rice (Oryza sativa) seedlings growing in air (Shimomura, S. and Beevers, H. (1983) Plant Physiol. 71, 736-741; 742-746) was further studied. This inactivation was found to be essentially dependent on the presence of free fatty acids. The specificity for fatty acids and the inhibitory effects of imidazole, 2-hydroxyfatty acids and dithiothreitol on the inactivation were all consistent with the properties of the fatty acid .alpha.-oxidation system in plants. Both O2 consumption and decarboxylation of fatty acid due to .alpha.-oxidation were also demonstrated in rice microsomes. When purified rice alcohol dehydrogenase was added to the .alpha.-oxidation system in rice microsomes, the decarboxylation of fatty acid was inhibited, and the cysteinyl residues of alcohol dehydrogenase were oxidized. The oxidation of two cysteinyl residues per monomer resulted in the complete inactivation of the enzyme. The activity of the inactivator in the isolated microsomes was gradually lost during storage and was rapidly lost upon heating. The inactivation of alcohol dehydrogenase was observed even when the enzyme was separated from microsomes by a dialysis membrane. These results indicate that the inactivation of alcohol dehydrogenase is closely related to fatty acid .alpha.-oxidation. We postulate that an intermediate of .alpha.-oxidation is the inactivator.