Jon Corzine’s return to Wall Street may turn out to be shorter than expected.. Shares of his firm, the commodities and derivatives brokerage house MF Global, have plummeted 54% so far this week amid concerns about the firm’s exposure to European sovereign debt. Troubles at MF Global could mean big blows to Corzine's position as head of the firm. Worries about Europe have buffeted other American financial firms in recent months, but MF Global is showing signs of becoming the first to face a full-blown panic. Investors began running to the exits after Moody’s Investors Service on Monday cut its ratings on the company to one notch above junk status and Standard & Poor’s on Wednesday threatened a downgrade. A ratings reduction below investment grade would hurt the company’s futures clearing business, analysts said. Both ratings agencies said they were concerned about MF Global’s capital position given its exposure to $6.3bn in debt from Italy, Spain, Belgium, Ireland and Portugal — among the most troubled economies that use the common currency of the euro. S&P noted that the debt is 5.2 times the company's total equity. Earlier this month, MF Global disclosed that FINRA had ordered it in August to set aside additional capital. Moody’s said the debt exposure and the need to inject capital highlighted “the firm’s increased risk appetite and raises questions about the firm’s risk governance.” Corzine’s arrival last year heralded a potential renaissance for the firm. As Goldman’s chief executive from 1994 to 1999, he led the banking titan’s responses to several challenges, including the Asian financial crisis and the collapse of the Long-Term Capital Management hedge fund. His crowning achievement was Goldman’s initial public offering in 1999, though the controversial move led to a power struggle that ultimately cost him his job. While investors have been fleeing, it is not clear whether its clients are staying put. One thing is clear, however: continued woes at MF Global could mean the exit of Corzine. [Dealbook, 10/25/11]