Relationship between Ca²⁺ and lasparagine in the induction of ornithine decarboxylase in H-35 rat hepatoma cells

The activity of ornithine decarboxylase (ODC) in H-35 hepatoma cells depleted of Ca²⁺ by washing with 2 mM EGTA increased 35-fold after incubating for 4h in a simple salt-glucose solution containing 10 mM L-asparagine and only if Ca²⁺ was replenished. Actinomycin D (5 μg/ml) and cycloheximide (20 μM) reduced the stimulatory effect by 84 and 100% respectively. Increase of active enzyme protein was also demonstrated by a 3-fold increase in α-difluoromethylornithine binding. Asparagine prolonged the half-life of induced ODC by 20% whereas Ca²⁺ reduced it by 32%. The observed inductive effects are not accounted for entirely by a direct influence of Ca²⁺ and asparagine on the turnover of ODC protein. These factors are likely to be parts of a signalling pathway leading to amplification of cellular ODC.