The developmental regulation of L-ornithine decarboxylase in Dictyostelium discoideum and its induction by cAMP.

By the use of an in vivo assay, ornithine decarboxylase (L-ornithine carboxy-lyase, EC 4.1.1.17) is shown to be developmentally regulated in Dictyostelium discoideum. High levels of cAMP can induce ornithine decarboxylase activity in preaggregative cells kept in shaking suspension, under similar conditions as where other markers for development can also be induced. This induction by cAMP is solely dependent on the total amount of cAMP to which the cells have been exposed, and not on the manner of cAMP addition. Induction of ornithine decarboxylase activity, when measured in vitro, is caused by both an increase in total enzyme activity and by a proportional increase in activity of the high-affinity form for the cofactor pyridoxal phosphate. When measured in vivo, an additional regulatory mechanism seems to be involved. Kinetic studies with the competitive inhibitor putrescine suggest that in cAMP-stimulated cells the low affinity form of the enzyme may also be active in vivo.