Overexpression and large-scale purification of recombinant hamster polymorphic arylamine N-acetyltransferase as a dihydrofolate reductase fusion protein.

N-Acetyltransferases (NATs) are enzymes that catalyze the detoxification and/or bioactivation of a variety of xenobiotics. Rapid kinetic, biophysical, structural, and bioactivation studies on NATs require quantities of purified enzyme capable of being obtained only through recombinant DNA technology. This laboratory has previously developed a protein expression and purification system in which NATs are expressed as proteins fused to a FLAG octapeptide followed by a thrombin-cleavage site to allow liberation of the rNAT. Typically, however, only 0.5-1.5 mg of the recombinant NAT's could be readily purified in a single isolation sequence by immunoaffinity chromatography. Therefore, the expression system was modified by inserting the L54F dihydrofolate reductase (DHFR) mutant gene sequence between the FLAG octapeptide and the thrombin-cleavage site. Expression was carried out with TOPP3 Escherichia coli cells. The new purification methodology utilizes the unique pH dependence of binding to a methotrexate (MTX)-affinity column by the L54F DHFR mutant. Unfortunately, the affinity chromatography strategy did not work satisfactorily. Although the specific activity of the purified rNAT2 was comparable to that of NAT2 obtained from hamster tissue, only 3% of the activity was recovered. The apparent cause of the low recovery is the unanticipated irreversible binding of rNAT2 to MTX. Ion-exchange chromatography was investigated as an alternative purification method. An initial DEAE anion-exchange column resulted in partial purification of the fusion protein. The fusion protein was cleaved with thrombin and reapplied to a DEAE anion-exchange column. The second DEAE column resulted in not only the separation of rNAT2-70D from FLAG-L54F DHFR, but also the purification of rNAT2-70D to near homogeneity. Application of the nearly homogeneous rNAT2-70D to a gel-filtration column resulted in recovery of homogeneous protein. The ion-exchange method of purifying rNAT2-70D is inexpensive and simple and yields more than 8 mg of pure enzyme from 1 liter of cell culture.