SAN FRANCISCO (REUTERS) - Apple Inc O Chief Executive Tim Cook's one-time stock award worth nearly US$413 million (S$525.4 million) is now partly subject to the performance of the shares in a move requested by the company's largest shareholders.

Cook, who has presided over a 42 per cent drop in Apple's share price since it touched a high of US$705 in September, approached the board to impose a performance criteria on his yet-to-be-vested stocks, according to a filing with the Securities and Exchange Commission.

Under the new system, part of Mr Cook's grant is subject to Total Shareholder Return, a measure of Apple's stock performance and dividends.

The company's board had in January of 2012 granted Mr Cook one million restricted stock units to signal its confidence in Mr Cook after Steve Jobs turned over the helm of the iPhone and iPad maker to his long-time lieutenant in August.