Metabolism of polyamines by cultured glioma cells. Effect of asparagine on γ-aminobutyric acid concentrations

The activity of ornithine decarboxylase (EC 4.1.1.17) increased in confluent cultures of glioma C₆BU-1 cells 3 h after adding a complete serum-containing medium, and was maximal 5 h later. The activity of S-adenosyl-L-methionine decarboxylase (EC 4.1.1.50) increased soon after addition of the complete medium to the cells, and reached its peak after 11 h. The activity of diamine oxidase (EC 1.4.3.6) also increased soon after adding complete medium and was maximal 8h later, when the activity of ornithine decarboxylase reached its peak. The increase in the activity of S-adenosyl-L-methionine decarboxylase was accompanied by changes in cellular spermidine and spermine concentrations, whereas the increase in the activity of diamine oxidase was followed by the accumulation of γ-aminobutyric acid, which was detected both in the cells and in the medium. Asparagine enhanced the utilization of radioactive putrescine by glioma cells suspended in buffered-salt/glucose solution and increased intracellular and extracellular γ-aminobutyric acid concentrations. Radioactive putrescine was converted into spermidine and spermine by glioma cells after addition of a serum-containing medium, but not after adding buffered-salt/glucose solutions, in the presence or absence of asparagine. The kinetics of ornithine decarboxylase 'induction' and the half-life of the enzyme differed in cells incubated with buffered asparagine solutions and serum-containing media.