TRIS and PBS buffer with ERK (enterokinase) - ERK (enterokinase) (Feb/25/2008 )

Dear Friends,
Can any one explain me why i have to use enterokinase with TRIS - buffer. I want to use it in PBS - buffer.
If anyone have experience with it please help me.
greetings
sreeni

Tris makes better buffer than PBS for invitro assays.