TABLE 2.

Relationship between putrescine transport activity and spermidine transport activity, expressed as the percentage of radiolabeled polyamine taken up in the presence of excess substrate^a

Nonradiolabeled polyamine(s) added	Radiolabel uptake (%)
	Putrescine	Spermidine
No competitor	2.7 ± 0.71	2.9 ± 0.32
15 μM putrescine	2.4 ± 0.41	2.3 ± 1.1
15 μM spermidine	2.8 ± 0.53	0.32 ± 0.11
15 μM putrescine + 15 μM spermidine	0.87 ± 0.26	NDb
100 μM putrescine	0.25 ± 0.044	2.9 ± 0.22
100 μM spermidine	0.54 ± 0.21	0.15 ± 0.041
100 μM putrescine + 100 μM spermidine	0.022 ± 0.010	ND

^aUnlabeled putrescine and spermidine were added at concentrations of 0, 15, and 100 μM, either singly or together. Radiolabeled putrescine or spermidine (10⁵ dpm) was added to the reaction mixture, and uptake was measured. Excess spermidine inhibits both spermidine uptake and putrescine uptake, whereas excess putrescine does not appear to affect spermidine uptake. Significant differences among means were detected with the nonparametric Kruskall-Wallis test (n = 3).

^bND, not detected.