TABLE 1.

Substrate specificity of PLX for RFC, PCFT and FRα.

	RFC	PCFT	FR-α
PLX	2.8 ± 0.1	14.8 ± 2.7	0.0035 ± 0.0005
l-LV	5.5 ± 0.7	4.0 ± 0.1	0.073 ± 0.0010
MTX	12.8 ± 1.3
PMX		0.4 ± 0.1
FA			1

The units are defined as follows: For RFC: Concentration (μM) required to inhibit RFC mediated uptake of [³H]-l-LV (5 μM) in CCRF-CEM/7A cells via RFC by 50%. For PCFT: Concentration (μM) required to inhibit [³H]-l-LV (2.5 (μM) uptake by 50% in CHO-C5-PCFT cells at pH 5.5. The uptake of l-LV at pH 7.4. was 11% of that at pH 5.5. For FR-(α: the inverse ratio of compound to displace 50% of [³H] folic acid (FA) from FR-positive cells, with the relative affinity of FA set at 1 in each experiment. Values are means ± SEM of three separate experiments.