Stability of S1P. (A) Time course of effects of LPA, nocodazole, and S1P on binding of 70-kD fragment to MG63 cells. Binding of ¹²⁵I-70-kD fragment of fibronectin in the absence (□) or presence of 200 nmol/L LPA (◊), 10 μmol/L nocodazole (○), or 100 nmol/L S1P (▵) was determined at different time points. Specific binding was calculated by subtraction of nonspecific binding (in the presence of 500 mg/L unlabeled fibronectin) from total binding and expressed as nanograms of 70-kD fragment bound per milligram of cellular protein. Data represent the average of duplicates that varied by less than 10%. (B) 200 nmol/L LPA (LPA) or S1P (S1P) was preincubated with MG63 cells for 20 hours at 37°C (▩) or left at 37°C for 20 hours (▩) and was transferred to naive cells for a 45-minute ¹²⁵I-70-kD fragment binding assay. Specific binding was calculated by subtraction of nonspecific binding (in the presence of 500 mg/L unlabeled fibronectin) from total binding and expressed as nanograms of 70-kD fragment bound per milligram of cellular protein. Data represent the mean ± SD (n = 3).