U.S. lawmakers are concerned a Chinese company's planned $4.7 billion acquisition of pork producer Smithfield Foods could affect the safety and availability of heparin, a blood-thinner widely used in heart surgery and kidney dialysis that is derived from pig intestines.

Members of the House Committee on Energy and Commerce wrote to Smithfield on July 24 asking the company to turn over information on its production of crude heparin, the raw ingredient used to make the drug.

As well as being the world's largest pork producer, with more than 46,000 employees in 25 U.S. states and four countries, Smithfield is also a major supplier of crude heparin.

In a letter to Smithfield's Chief Executive Larry Pope, six Republican committee members said the proposed acquisition of Smithfield by Shuanghui International Holdings "raises questions related to the safety and adequacy of the U.S. heparin supply."

A spokeswoman for Smithfield, Keira Lombardo, confirmed the company had received the committee's letter "and will respond within the timeframe the committee has outlined."

The request comes just weeks after the Senate Agriculture Committee met to examine the potential food safety implications of the deal, which would be the biggest Chinese takeover of a U.S. company.

Smithfield said on Wednesday that the U.S. government has decided to take an additional 45 days to review the planned deal. Smithfield and Shuanghui submitted their proposal in June to the Committee on Foreign Investment in the United States, or CFIUS, an executive branch panel that examines foreign investment for potential threats to national security.

Many deals are approved during an initial 30-day review, but some transactions are given a second 45-day examination.

"The Committee's investigation indicates that the U.S. heparin supply is stressed, and could well be in shortage," the lawmkers' letter said.

"China's heparin market is experiencing its own pressures, and Smithfield Foods under Shuanghui control may be pressured to export its crude heparin product to China instead of supplying U.S. companies."

A spokeswoman for the U.S. Food and Drug Administration, Erica Jefferson, said the agency does not comment on congressional letters. A spokesman for Shuanghui said the company had no comment.

Nearly 150 people in the United States died in 2007 and 2008, and hundreds suffered serious reactions, after they were treated with contaminated heparin, according to the U.S. Pharmacopeial Convention, the body that sets standards for drug quality and purity.

The serious injuries and deaths were associated with the use of heparin that contained active pharmaceutical ingredient (API) from China, the FDA said.

The FDA said it evaluated 94 cases of people who died while receiving heparin during the time the contaminated product was on the market. The agency said many reports did not have sufficient information to determine that the tainted heparin was the cause though in three cases the FDA concluded it was "likely" or "probable."

The House committee is continuing a broader investigation into the FDA's handling of that event.

"Because the contamination case was never adequately addressed by Chinese authorities, at least some of the bad actors responsible for the adulteration presumably are still operating in the Chinese heparin business, and there is little deterrence against, but high economic gain for, new heparin-contamination schemes," the lawmakers' letter said.

The committee members asked Smithfield to provide details of its heparin operations, including the size of its production and a list of current heparin product customers, including any Chinese heparin customers.

They also asked for the names of the key executives at Smithfield Foods involved in making and marketing heparin products, including pig intestines and crude heparin, and details of key managers at any Smithfield facility involved in manufacturing crude heparin.

The Smithfield, Virginia-based company makes ham, sausage, bacon and other prepared meats under labels such as Eckrich, Gwaltney and Armor. It has argued the takeover deal is good for the United States because it will boost pork exports.