GTCR, a Chicago-based private equity firm, plans to buy the American Stock Exchange, the third largest US stock market, for $110m (€94m), in a rare move for the private equity industry.

The National Association of Securities Dealers (NASD) put the exchange up for sale two years ago to avoid conflict with its role as regulator. A year ago, it was in preliminary merger talks with the New York Stock Exchange, but no deal transpired.