IDENTIFICATION OF A NOVEL REDUCTASE IN FOLATE METABOLISM

A protein with a molecular weight of 27,500 was co-purified with the e nzyme dihydrofolate reductase (molecular weight 22,000) from the liver of mice less than 8 weeks of age using a methotrexate-Sepharose affin ity matrix. This 27.5 kDa protein crossreacts with dihydrofolate reduc tase against an antiserum raised to the purified 22 kDa enzyme. The pr otein could also reduce dihydrofolate to tetrahydrofolate, thus demons trating the catalytic properties of dihydrofolate reductase. The expre ssion of this 27.5 kDa protein also appears to be age-dependent becaus e it could not be isolated from liver of mice older than four months.