Osmotically‐induced modification of ornithine decarboxylase in Physarum

Equilibrium between active and inactive forms of rat liver ornithine decarboxylase mediated by L‐ornithine and salts

“…Experiments using different types of cell culture have shown that ODC activity is greatly affected by ionic content of the media [20,21]. If such equilibrium occurs under physiological conditions, its relative importance in the regulation of ODC activity could be related to the existence of two forms of different metabolic stability rather than to forms of different catalytic activity, since it is known that the structure can play an important role in the process of enzyme degradation, which is an important but unknown event in the regulation of the turnover of this enzyme.…”

Selective inactivation of Physarum A‐form ornithine decarboxylase by α‐difluoromethylornithine

“…h -s, in the presence of 1 mM spermidine. Ratl~er, the induction of DFMO insensitivity appeared to be directly related to the conversion of A-form enzyme to the B-form since the elevation of media osmolarity (following [6]) or the addition of cyclohexirnide [7] also immediately blocked enzyme sensitivity to DFMO (not shown).…”

“…We had shown that very rapid conversion of Bform ornithine decarboxylase to A can be induced by suspending cells in low ionic strength media [6]. As shown in fig.3 the placement ofPhysarum microptasmodia that were pre-exposed to 0.1 mM spermidine and DFMO into distilled water, with neither spermidine nor DFM0, resulted in a sudden increase in the A-form of the enzyme.…”

“…This was centrifuged again, decanted and the pellet frozen in liquid nitrogen. The details of the assay procedure have been reported [6], A-form enzyme activity was determined using 0.01 M Hepps buffer (pH 8.0)/0.5 mM dithiothreitol/0.5 mM EDTA/0.5/aM pyridoxal 5 '-phosphate/0.1 mM L-[ 1-14C] ornithine (0.02/aei). Total assayable enzyme was determined using pH 8.4 Hepps and 100/aM pyridoxal 5'-phosphate along with the EDTA and dithiothreitol.…”

“…In this latter, primitive eukaryotic system, the ornithine decarboxylase-modulating protein is stimulated by polyamines to modify this enzyme such that its affinity for the obligate coenzyme, pyridoxal 5'-phosphate (PLP), is severely diminished. Thus, even though total ornithine decarboxylase protein is rather stable in Physarum, it is rapidly and reversibly converted between the normal (A-form) and modified (B-form) states in response to external polyamines [5], altered media osmolarity [6], protein synthesis inhibitors [7], and normal progression through the mitotic cycle [8].…”