Regulation of putrescine metabolism in Ehrlich ascites carcinoma cells exposed to hypotonic medium.

When exposed to hypotonic growth medium, Ehrlich ascites carcinoma cells showed a rapid stimulation of ornithine decarboxylase (EC 4.1.1.17) activity in 4 h, followed by a rise in their putrescine content. This effect was totally abolished by addition of a slightly hypertonic concentration of sodium chloride or sucrose to the medium. The general protein synthesis was unaffected by the hypotonic treatment. The uptake of putrescine and, to a lesser extent, spermidine was enhanced, the conversion of the radioactive putrescine into spermidine appeared partially inhibited during later stages of the hypotonic treatment. As a result, the half-life of putrescine increased from 2.8 h under isoosmotic conditions to 7.3 h in hypoosmotic medium. Both exogenous ([14C]-putrescine-derived) and endogenous ([14C]ornithine-derived) putrescine degraded at similar rates in control and hypotonic cells, yet the putrescine taken from the medium degraded preferably to nonpolyamine products, while the putrescine synthetized in the cell was converted evenly to spermidine and to other metabolites. Adenosyl-methionine decarboxylase activity (EC 4.1.1.50), which provides the second precursor for spermidine and spermine synthesis, was distinctly inhibited in the hypotonic medium. Inhibition was likewise observed in spermidine synthase activity, while spermine synthase was marginally stimulated. It appears that the hypotonic treatment serves a special condition under which not only the formation of putrescine is enhanced dramatically but the cells also attempt to converse the diamine by preventing its further metabolism to higher polyamines.