Identification of a novel reductase in folate metabolism.

A protein with a molecular weight of 27,500 was co-purified with the enzyme dihydrofolate reductase (molecular weight 22,000) from the liver of mice less than 8 weeks of age using a methotrexate-Sepharose affinity matrix. This 27.5 kDa protein crossreacts with dihydrofolate reductase against an antiserum raised to the purified 22 kDa enzyme. The protein could also reduce dihydrofolate to tetrahydrofolate, thus demonstrating the catalytic properties of dihydrofolate reductase. The expression of this 27.5 kDa protein also appears to be age-dependent because it could not be isolated from liver of mice older than four months.