Evidence for inhibitory activity against cell proliferation in bovine liver

Bovine calf liver was homogenized in 0.1M NH₄Ac, pH 7.4, and centrifuged. The supernatant was lyophilized and resuspended in 44 mM NaHCO₃. After size exclusion chromatography on sephacryl S-300, a fraction of approximately 100 to 160 kDa was shown to inhibit the proliferation of rat L6 myoblasts in culture. The inhibitory activity was abolished when the resuspended preparation was heated at 70 C for 30 min before testing in L6 cell proliferation assay. Addition of 10-⁹M IGF-I did not influence the inhibitory response. Two IGF-I-binding proteins, 30 and 36 kDa, were identifiable in this fraction. These two proteins were more evident in other fractions in which no inhibitory activity was found. Inhibitory activity was not associated with IGF-I binding proteins.