Rapid exchange of subunits of mammalian ornithine decarboxylase.

The subunit structure of mouse L-ornithine decarboxylase (ODC) was investigated using mutants involving single amino acid changes that greatly reduced the catalytic activity. Studies were carried out both by expressing the enzyme protein in a coupled transcription/translation system and mixing the various purified mutant ODCs and wild type enzyme together. The results confirm that ODC activity requires the formation of a dimer and that this dimer contains two active sites, each made up from part of one subunit that contains amino acids lysine 69, lysine 169, and histidine 197 and a part of the other subunit that contains cysteine 360. Mixing of the purified ODC mutant enzymes with each other and with the wild type enzyme indicated that there was a very rapid exchange of subunits between the enzyme dimers even under physiological conditions without addition of chaotropic agents. This rapid exchange may facilitate the binding of antizyme and the rapid turnover of ODC in vivo.