CRM 2011 Odata endpoint

hello,

i'm working on crm 2011 using Odata jquery library to update  systemuser field  and it's working fine using this url: http://xxxxxxx/xxxxx/XRMServices/2011/OrganizationData.svc/SystemUserSet(guid'xxxxx-xxxx-xxxx-xxxxx') . but i have a problem when i need to update a customized field that exists in the systemuser extension base table in this case i don't know which entity set i have to use instead of SystemUserSet

Any help please?

Thank You