Difference in transport of leucine in attached and suspended 3T3 cells.

3T3 cells in subconfluent culture take up leucine through a transport system which has a relatively high affinity for leucine (M system). When the culture becomes confluent, the M system is turned off and leucine is transported by another system which has a low affinity for leucine (S system). The M system is reactivated by transferring the cells into subconfluent cultures. In suspension cultures 3T3 cells, initiated from confluent cultures, the M system is not activated and leucine is transported by the S system. In cells suspended from subconfluent culture, the M system continues to operate at a high level for four hours and then is gradually turned off. Tumor virus transformed 3T3 cells (SV3T3 and Py3T3) grow quite well in suspension culture and transport leucine both in monolayer and suspension through a high affinity system, with a high Vmax value. A derivative of 3T3, 3T3/41, which grows in suspension much more slowly than tumor virus transformed 3T3 cells, also takes up leucine through a high affinity transport system both in monolayer and suspension but its Vmax value is lower than that of the transformed cells.