I'm a dentist graduated 2005 currently doing my masters in endodontics. What are the requirements to practice in united states or united kingdom?

Do 3rd and 4th year. In the us in most states, you must redo 3 and 4th year of dental school. There are a few states like california that allow you to practice after passing the state/regional boards.
Adv. standing cert. . For most states you must do an 'advanced standing' program which is essentially the third and fourth year of dental school. After this you are eligible to write the dental board exams. There are different board exams depending on the region you want to practice in the usa. There are a few instances where you do no need to meet these requirements. I would contact the ada for these exclusions.
Clinical exam . And application for a state license are the main requirements in the us. You need to take and pass one of the five regional licensing exams recognized by the state where you want to practice. Not sure about the uk.