Exchange operator Nasdaq OMX Group will rebalance its benchmark Nasdaq 100 index, cutting the weighting of widely held stocks, such as Apple Inc, and causing volatility as investors adjust their portfolios.

Nasdaq OMX is rebalancing the index by cutting the weighting of 82 securities to bring the weighting more in line with market capitalizations, the exchange said on Tuesday.

The rebalancing hit shares of Apple and boosted shares of Microsoft Corp and was one reason for the Nasdaq futures falling before the opening bell.

The rebalancing will affect the relative weights of all the securities in the index and cause popular index-tracking funds such as the PowerShares QQQ to buy and sell shares to match the new composition of the index.

Apple fell 1.7 percent to $335.50 while Microsoft rose 1.4 percent to $25.91.

Once the rebalancing takes effect May 2, the projected weight of Apple will be 12.33 percent of the index compared with a current weight of 20.49 percent, Nasdaq said on its website.

Microsoft's weighting will rise 4.9 percent. Other big changes include Oracle, whose weighting will rise 3.4 percent, Intel up 2.5 percent, Cisco Systems, up 2.1 percent and Google up 1.6 percent.

Apple will continue to remain the largest component of the Nasdaq-100 index.

Nasdaq decided to enact a special rebalance in order to bring the weights of the index securities closer in line with their actual market capitalizations, the exchange said.

The special rebalancing of the Nasdaq 100 Index will be enacted based on index securities and shares outstanding as of March 31, the company said.

(Editing by Kenneth Barry)