Effect of putrescine on S-adenosylmethionine decarboxylase in a small intestinal crypt cell line.

Two key enzymes in polyamine biosynthesis are ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (SAMDC). SAMDC decarboxylates S-adenosylmethionine, which then donates aminopropyl groups for spermidine and spermine synthesis. The purpose of our study was to determine whether putrescine, taken up from medium or synthesized endogenously by ODC, alters SAMDC activity. Studies were conducted in the IEC-6 cell line derived from rat small intestinal crypt cells. Cells were grown in Dulbecco's minimal essential medium containing 5% dialyzed fetal bovine serum (dFBS). They were deprived of serum for 24 h before experiments. Basal SAMDC activity was increased significantly by > or = 10(-4) M of putrescine. Lower doses had no significant effect. The same doses of putrescine decreased ODC activity to near zero. Asparagine at 10 mM or 5% dFBS not only stimulated ODC activity and the intracellular putrescine levels but also increased significantly SAMDC activity as well. ODC activity peaked at 3 h, and the maximum level of SAMDC occurred 3-4 h after exposure to asparagine or serum. Treatment with DL-alpha-difluoromethylornithine (DFMO), a specific ODC inhibitor, prevented the increases in both cellular putrescine levels and SAMDC activity in asparagine- and serum-treated cells. In the presence of DFMO, exogenous putrescine returned SAMDC activity toward control levels but had no effect on ODC. A very slight increase of SAMDC half-life in IEC-6 cells grown in the presence of putrescine was not statistically significant.(ABSTRACT TRUNCATED AT 250 WORDS)