Preincubation with DF protects bFGF from degradation by trypsin and chymotrypsin. bFGF (A,B) or VEGF165 (C; ∼0.5 μg in 20 μL, pH 7.4) with tracer amounts of ¹²⁵I-bFGF or ¹²⁵I-VEGF165 were exposed to 2% trypsin (w/w) in the absence or presence of 25 μmol/L DF. Trypsin digestion was carried out at 37°C for 4 hours, and the digestion products were mixed with sample buffer, boiled for 5 minutes, and analyzed by 12% SDS-PAGE. Gel bands were quantitated by laser densitometry. Data are presented as the average plus or minus SD, n = 3. *P (vs no oligo) less than .04; **P (vs no oligo) less than .02. (D) DF protects bFGF from inactivation at 37°C. bFGF was incubated at 37°C for 3.5 hours with or without 25 μmol/L DF, added to the C11 FGFR1 IIIc–overexpressing cells (2.2 × 10⁴ cells/well in 48-well plate) in media containing DF (25 μmol/L) but without IL-3. Cell proliferation was measured after 3 days by SRB. bFGF (1 nmol/L) without preincubation at 37°C for 3.5 hours was used as a positive control. Data are presented as the average plus or minus SD, n = 3. *P (vs bFGF + preincubation) less than .03.