Chick muscle trophic factor previously reported by us and chick transferrin were studied comparatively in order to clarify their identity. On the basis of physicochemical analyses such as electrophoresis, isoelectric focusing, peptide mapping, absorption spectrum, and iron binding, they were indistinguishable. Also, they were immunologically identical and similarly myogenesis-promoting in vitro. Their trophic activity was likely to be directly dependent on the iron bound. From these results, taken together with those obtained from the comparative study with ovotransferrin, we conclude that muscle trophic factor is most likely identical to transferrin.
Removal of transferrin (Tf) by immunoprecipitation and Fe removal by dialysis led to a loss of muscle trophic activity of chick embryo extract (EE). The activity was restored by the addition of Tf or Fe3+. These results show that holoTf is an essential component of EE for avian myogenic cell growth in vitro.