Four foundations run by billionaire industrialists Charles and David Koch hold a combined $310 million in assets according to tax filings obtained by the Center for Public Integrity.

The documents also show that the brothers, principal owners of the second-largest privately held company in the United States, combined in 2011 to donate $24 million through those foundations with much of the money going to support free-market and libertarian think tanks and academic centers.

A $4.5 million grant to the George Mason University Foundation makes up nearly 15 percent of the university foundation’s revenue for 2011. The school is the largest recipient of Koch foundation money since 1985, and it houses several free-market and libertarian research centers including the Institute for Humane Studies, which received $3.7 million from the Koch foundations.

The D.C.-based American Legislative Exchange Council received $150,000 to help finance its activities, including meetings where corporate representatives draft model legislation with state legislators. The Koch brothers have decades-long connections with ALEC, which gave the brothers the Adam Smith Free Enterprise Award in 1994.

Two of the Koch-run foundations are among dozens of conservative endowments that give money to Donors Trust, a charitable vehicle that has passed $400 million in anonymous grants to “liberty-minded” think tanks in the last decade.

Other think tanks that received Koch foundation grants in 2011:

• The Bill of Rights Institute: $350,000
• The Federalist Society: $260,000
• The Jack Miller Center: $250,000
• American Enterprise Institute: $200,000
• Manhattan Institute: $200,000
• Pacific Research Institute: $100,000
• Ayn Rand Institute: $50,000
• Heartland Institute: $25,000

The above grants came from foundations run by Charles Koch. His brother David’s foundation focused all of its $10 million grant giving in 2011 to the renovation of a theater in New York City.

The Koch brothers' complete Internal Revenue Service Form 990 tax filings for 2011, which were not publicly available before now, may be viewed here:

• Claude R. Lambe Charitable Foundation
• Charles Koch Foundation
• Knowledge and Progress Fund
• David H. Koch Charitable Foundation