Porsche moved Wednesday to put a brake on volatility in Volkswagen shares by selling some options and Germany's financial regulator announced a formal investigation into Volkswagen's recent stratospheric stock gains.

Porsche said it would be selling up to 5 percent of its options for common shares in the automaker — causing Volkswagen's stock to slump drastically Wednesday after several days of eye-popping gains.

The shares plunged 43.2 percent to 522.50 euros ($652.64) in Frankfurt trading.

The options move comes after a weekend announcement that Porsche had bought options to increase its ownership in Volkswagen to 75 percent, building upon its previous ownership of 42.6 percent in common stock.

"We're only willing to reduce our options position as little as possible," and Porsche won't sell "one single share" of the 42.6 percent it holds in common shares, Porsche spokesman Frank Gaube said.