ActiveDirectoryMembershipprovider Get User Guid

Hi,
  I am using the ActiveDirectoryMembershipprovider on our Intranet with the SqlRolesProvider and Personalization. Users authenticate via AD and are also stored by default in the aspnet_Users table. Now I need to get the UserGuid from that table. I though I could use the providerUserKey, however that returns me the SecurityIdentifer. I have already googled around, but I am not able to find anything. Any ideas?
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