Mitsubishi said Monday it was reiterating its commitment to Morgan Stanley “in response to recent market volatility.” The New York-based firm dropped to its lowest share price since December 2008 as concern intensified that Europe’s debt crisis will infect U.S. bank balance sheets.

“The special relationship we have formed remains core to our global business strategy,” the Tokyo-based bank said in a statement.

Mitsubishi invested $9 billion in Morgan Stanley in October 2008 as the firm’s stock price collapsed after the bankruptcy of Lehman Brothers Holdings Inc. The banks agreed in April to convert $7.8 billion of preferred shares into common stock and completed the conversion in June, making the Japanese bank Morgan Stanley’s largest common shareholder.

Morgan Stanley fell $1.04, or 7.7 percent, to $12.47 at 4 p.m. in New York Stock Exchange composite trading, the lowest closing price since Dec. 2, 2008. The shares have fallen 54 percent this year.

Mitsubishi UFJ still holds about $500 million of non- convertible preferred stock with a 10 percent dividend. Morgan Stanley can buy back that stake for about $572 million beginning on Oct. 15, 2011, according to the firm’s annual 10-K filing.