R12 Payment Processing

I have  one global instance but  a processing centers for each region. One for Americas, One for Europe and one for Asia.  Each processing center handles multiple  operating units ( which is  tied to a company code) . For example, in the processing for Americas, it comprised of multipe  operating units.

How can I restrict each processing centers to  to access their respective operating units?  As an example, I don't want  processing team from Asia  to have access  to operating units that belongs to Americas.