Ornithine decarboxylase in Phycomyces: in vitro and in vivo properties.

The properties of ornithine decarboxylase from Phycomyces blakesleeanus were examined. Enzyme from mycelial cultures was extracted and purified approximately 70-fold. The apparent molecular weight is 96K. The Michaelis constants with respect to ornithine and pyridoxal 5'-phosphate are 90 and 0.37 microM, respectively. Putrescine is a potent competitive inhibitor with a Ki of 75 microM. Exposure of ornithine decarboxylase to sulfhydryl-modifying reagents resulted in a rapid inhibition of activity. In vivo addition of putrescine produced characteristic decreases in cellular ornithine decarboxylase activity. Light stimulation of dark-adapted mycelial cultures also decreased cellular ornithine decarboxylase activity.