Dihydrofolate Reductase From Diplococcus-Penumoniae - Purification, Amino Acid Composition, And N-Terminal Amino Acid Analysis

Dihydrofolate reductase in preparative amounts was highly purified from an amethopterin-resistant strain of D. pneumoniae. This strain was constructed by genetic recombination and had a high level (120-fold compared to the wild-type) of enzyme. Greater than 20% yield and purity close to 100% were obtained by protamine sulfate precipitation, ammonium sulfate fractionation, molecular sieve chromatography on Sephadex G100, and chromatography on hydroxylapatite and DEAE-cellulose. Purity of the final fraction was determined by amethopterin binding-titration, polyacrylamide disc-gel electrophoresis, and precipitin antibody analysis by immunodiffusion. The amino acid composition of the purified protein is given. Only one N-terminal amino acid, threonine, was identified in an amount suggesting that the protein exists as a single polypeptide chain.