The American Stock Exchange's new Amex Commodity Exchange won federal approval yesterday to begin selling interest rate futures in Government National Mortgage Association certificates.

The 5-to-9 vote by the Commodity Futures Trading Commission cleared the way for ACE, as the new exchange is known, to begin trading on a special section of the exchange floor on Sept. 12. ACE spokesman said.

The commission also approved the rules under which the new exhange will operate.

The action marks the approval of New York's first new commodity exchange since 1933. ACE also will be the first exchange outside of Chicago to trade futures on the government certificates, which also are called Ginnie Maes.

Futures contracts in Ginnie Maes had been available only through the nation's largest futures market, the Chicago Board of Trade. More than 400,000 Ginnie Mae contracts with an estimated value near $40 billion exchanged hands via the CBT from January through July, more than double the volume of the previous year.

Nathan Most, former director of Amex's commodity options development who now serves as president of ACE, expressed "deep satisfaction" over the government's decision.

A statement indicated that ACE will make 154 seats available to interested traders in addition to those reserved for regular Amex members.

Ginnie Mae certificates are issued on pools of federal guaranteed mortgage loans. Those holding futures contracts promise to buy or sell a certain amount of the certificates at a set price during a specific period. Such transactions can be used as investors as a hedge against inflation.