The activity of receptors specific for human transcobalamin 11-Cobalamin (TC I I-Cbl) were measured in virus-transformed lymphoblasts, hepatocytes (hepatoma) and diploid fibrolasts. In all three types of human cells the receptor activity increased as cells went from a resting phase to the most actively dividing phase. Receptor activity declined as cell division slowed. The changes in activity of lymphoblasts and hepatocytes were produced by changes in receptor number and not by changes in affinity between receptors and TC II-Cbl. The basis of the change in fibroblasts was not clear. The Cbl-dependent methionine synthetase activity of fibroblasts, in contrast, tended to be greatest when t h e cultures were confluent and replication had slowed. As the fibroblasts became senescent the receptor activity for TC 11-Cbl declined and the fluctuations with the phase of the cell were blunted. However, the release of apo TC II from t h e cells was maintained. These observations must be taken into consideration when the respective cells are used as models. Even more important are the implications of t h e observations of t h e changes in receptor activity for TC 11-Cbl for the regulation of the entry of Cbl into cells.
Previously (Hall, 1984), we showed that the activity of