The Mt. Gox bitcoin exchange in Tokyo filed for bankruptcy protection Friday and its chief executive said 850,000 bitcoins, worth several hundred million dollars, are unaccounted for. The exchange’s CEO Mark Karpeles appeared before Japanese TV news cameras, bowing deeply. He said a weakness in the exchange’s systems was behind a massive loss of the virtual currency involving 750,000 bitcoins from users and 100,000 of the company’s own bitcoins. That would amount to about $425 million.