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Spergualin (SG) and 15-deoxyspergualin (DSG) were more slowly incorporated into L5178Y cells than spermidine. SG and DSG inhibited carrier-mediated transport of [3H]spermidine competitively with inhibition constants of 0.67 mM and 0.45 mM, respectively. Addition of calf serum stimulated uptake of [3H]spermidine into the cells in a serum concentration-dependent manner. The effect was not observed when horse serum was used in place of calf serum. Preincubation of spermidine in calf serum for 1 hour before addition to cells remarkably decreased cellular incorporation of tritium. Three amine oxidase inhibitors, aminoguanidine, 3-hydroxybenzyloxyamine, and semicarbazide, inhibited stimulation of uptake of [3H]spermidine by calf serum and the decrease of it by preincubation in calf serum. So we propose that cellular incorporation or binding of products generated by oxidation of spermidine by amine oxidase in calf serum was much faster than that of spermidine itself and they were unstable and transformed quickly to unincorporable or non-binding substances if cellular targets were not present. Effect of amine oxidase inhibitors on cytotoxic activity of SG and DSG were determined in low and high concentrations of calf serum. In the presence of 10% calf serum in the basal medium, cytotoxicity to L5178Y cells by SG and DSG was suppressed at high drug concentrations (above 10 micrograms/ml) and enhanced at low drug concentrations (below 2.5 micrograms/ml) by amine oxidase inhibitors. In the presence of 0.5% calf serum suppression of cytotoxicity at high drug concentrations by amine oxidase inhibitors was also observed, but enhancement at low drug concentrations was obscure. These data may suggest the existence of two kinds of cytotoxic mechanism of SG and DSG, one dependent on and one independent of amine oxidase in serum.

Serum effect on cellular uptake of spermidine, spergualin, 15-deoxyspergualin, and their metabolites by L5178Y cells.