The regulation of ornithine decarboxylase activity in intact normal and transformed cells maintained with a minimal salts/glucose medium

Abstract

Ornithine decarboxylase activity could be enhanced 40 to 50 fold when confluent (G o) intact Chinese hamster ovary cells were incubated for 7–8 h in a simple buffered salts/glucose solution. The apparent minimal buffer requirements established in this study for optimal enzyme activation were: Hepes buffer pH 7.2 (50 mmol/l), NaCl (100 mmol/l), KCl (5 mmol/l), CaCl 2 (2 mmol/l), glucose (5 mmol/l), L-asparagine (10 mmol/l), dibutyryl cAMP (0.5 mmol/l), and l-methyl, 3-isobutylxanthine (0.1 mmol/l). Little to no enzyme activation resulted by individually omitting from the activating solution CaCl 2, KCl, L-asparagine, or agents which elevate cellular cAMP levels. The asparagine activation of ornithine decarboxylase was antagonized by the addition of L-ornithine, D,L-β-Aspartyl hydroxamate, lanthanum chloride, and sodium EGTA. The antagonism by the latter two agents reinforces the notion that calcium ions are involved in the activation of ornithine decarboxylase. Several other amino acids including qlutamine, serine, alanine, and glycine were equally as effective as asparagine in activating the enzyme and in permitting the cAMP mediated activation of ornithine decarboxylase. The enhancement of ornithine decarboxylase by asparagine and cAMP was also studied in two normal (Syrian hamster embryo cells, Balb 3T3) and in two transformed (Ni 3S 2 transformed Syrian hamster embryo cell, SV-40 transformed Balb 3T3) cell culture systems. Asparagine and cAMP enhanced ornithine decarboxylase activity (30–50 fold) in transformed cells maintained in Earle salts medium. Little enzyme activation was detected under similar conditions in the respective normal cells.