Isolation and characteristics of a leukemia-growth-promoting factor from calf thymus

Leukemia-growth-promoting factor (LGPF) as we previously reported stimulates the growth of a murine leukemia subline (L17R) extensively. LGPF was isolated and 10(4) fold purified from calf thymuses by a combination of ammonium sulfate precipitation, Sephadex G-100 chromatography, hydroxylapatite chromatography, and Mono S-fast protein liquid chromatography (Mono S-FPLC). The mol. wt of LGPF was estimated to be approximately 25,000 a.m.u. from the elution pattern of Sephadex G-100 chromatography. The activity had high affinity for Mono S beads which are cation exchangers. Mono S fractions of LGPF are effective at a low concentration of 5 ng/ml. The activity was inactivated by heat (56 degrees C, 30 min), 1 mg/ml trypsin (37 degrees C, 1h), and 50 mM dithiothreitol (20 degrees C, 1h). The growth L17R leukemia cells are not only stimulated by LGPF, but also by pituitary and brain fibroblast growth factor (FGF). These data strongly suggest that LGPF is a heat sensitive cationic protein(s) acting as a member of FGF family.