I need to acces Partner server url global variable from apex code in order to pass it into web service.

In visualforce i can write the followin statement in page layout  {!$Api.Partner_Server_URL_120} which renderes something like that https://na5.salesforce.com/services/Soap/u/12.0/461200D70000000IfeF

How can i access {!$Api.Partner_Server_URL_120}  variable from apex code?

Thanks in advance