Theoretical analysis of cotransport: its use in alanine uptake in plasma membrane vesicles

A theoretical model of a cotransport system in plasma membrane vesicles has been utilized for the analysis of the Na(+)-dependent L-alanine transport into plasma membrane vesicles purified from Yoshida ascites hepatoma (AH 130) cells in the exponential and stationary phases of growth. The analysis was performed by comparing the experimental curves with computer simulations. In particular we considered the differences in alanine uptake observed in the two preparations and we tried to ascribe them to changes of some parameters of the transport model. The simulations indicate that sodium, alanine or water passive permeability changes cannot explain the experimental data which are consistent, on the contrary, with a relevant enhancement of the Vmax of the transport agency. The involvement of the membrane electrical potential difference is also discussed.