Nippon Life Insurance Co. and Harris Associates L.P., two major shareholders of Olympus Corp., asked the Japanese firm to respond to investor concerns about takeover payments that its fired CEO revealed, sending its stock price plunging 47 percent.

Nippon Life, the largest shareholder of Olympus, asked the company to take "prompt" action to address investor distrust, Akira Tsuzuki, a spokesman for the life insurer, said Thursday.

Olympus has lost more than $4 billion of its market value in the five trading days since CEO Michael C. Woodford was fired. Following his dismissal, Woodford made public a PricewaterhouseCoopers report that said Olympus may face regulatory and legal scrutiny due to payments made to advisers in the 2008 acquisition of Gyrus Group PLC.