Warren Buffett’s Berkshire Hathaway lost its top credit grade from Fitch Ratings this week on concern over the holding company’s exposure to volatile markets and the daunting task of finding an eventual successor for its legendary chairman.

The action came hours after another colossus, General Electric, lost its triple-A rating from Standard & Poor’s on Thursday, and underscores Wall Street’s fears that even corporate America’s most unassailable institutions will not make it through the deepening financial crisis unscathed.