Help needed with Ca-Transport assays

[HERNDZA]

Hi
I am working on Ca-transport across plasma membrane in fungi. I have, so far, 
found a 'P'-Type Ca-ATPase of which I should like to determine its kinetic 
parameters (Km, Vmax, Ki's). However, I am encountering some problems when I 
vary the conc. of Ca or ATP in the assay, although I think I am taking into 
account the changes in ionic strength etc. and I vary the Ca/EGTA buffer 
accordingly.
Is there anyone in the net that have done such experiments in other systems?. 
Any help would be much appreciated.

Agustin Hernandez

e-mail: Agustin.Hernandez at BBSRC.ac.uk