A competitive immunoassay showing the effect of prelabeling TSP with NEM on conversion to the EDTA-conformation. Supernatant TSP was derivatized with [³H]NEM in the presence of 2 mmol/L Ca²⁺. The labeled TSP was purified by heparin-affinity chromatography as described in Materials and Methods. Three hundred fifty micrograms/milliliter Ca-TSP or NEM-TSP was incubated at 37°C for 30 minutes before addition of MoAb D4.6 as described in Fig 5. The results are the means ± SE of 3 separate experiments. (□), 2 mmol/L Ca²⁺; (○), 6 mmol/L EDTA.