TABLE 1.

IC₅₀ values for inhibition of radiolabel substrate uptake by purine and pyrimidine substrates

IC₅₀ values are means ± S.D. for ≥3 experiments. Pyrimidines were competed with 50 nM [³H]adenosine; purines were competed with 250 nM [³H]uridine.

Uptake Inhibition IC50 Values
Pyrimidine	IC50 Versus [3H]Adenosine	Purine	IC50 Versus [3H]Uridine
	µM		µM
Uracil	2833 ± 243	Adenine	306 ± 19
Uridine	400 ± 152	Adenosine	190 ± 2.5
Thymine	1142 ± 185	Hypoxanthine	53 ± 1.65
Thymidine	91.3 ± 38	Inosine	3.7 ± 0.1
Cytosine	9697 ± 455	Guanine	1142 ± 44
Cytidine	26726 ± 4116	Guanosine	21.3 ± 0.6
		Xanthine	979 ± 13
		Xanthosine	626 ± 17