The movie I’m looking for must be from the early eighties. It is about some (FBI?) agent that has to protect a scientist who is threatened to be killed by criminals because of what he knows. The agent has a lot of work with protecting the scientist all through the movie, but at the end he hears on the news that the man has died from a heart attack or something.
One scene I remember pretty well: a hand grenade is thrown to kill the scientist and someone (another agent?) throws himself on it, to save the scientist.