Expression of functionally active human cytochrome P-450c21 (CYPXXIA2) in Escherichia coli and one-step purification of the recombinant protein by metal chelate affinity chromatography

Functionally active human cytochrome P-450c21 has been expressed in Escherichia coli and purified to apparent homogeneity. To increase the expression level, the N-terminal sequence of the cDNA was modified. The C-terminal sequence of the cDNA was modified by inserting an additional four histidine residues to allow the one-step enzyme purification procedure using metal chelate affinity chromatography. The recombinant cytochrome P-450c21 is expressed in E. coli in the amount 40-50 nmoles per liter of the growth medium and is inserted into the bacterial membranes. Modification of the N- and C-terminal sequences of cytochrome P-450c21 does not change the K_m and V_max for progesterone and 17α-hydroxyprogesterone hydroxylation. The recombinant cytochrome P-450c21 expressed in E. coli was purified from solubilized bacterial membranes using metal chelate affinity chromatography. The highly purified hemoprotein migrates in SDS-PAGE as a single band corresponding to molecular weight 54 kD and shows type I binding spectra with progesterone and 17α-hydroxyprogesterone. Hydroxylation activity of the purified cytochrome P-450c21 was reconstituted in the presence of purified NADPH-cytochrome P-450 reductase and an NADPH-regenerating system. The K_m values for the highly purified recombinant cytochrome P-450c21 in the reconstituted system were 12.2 μM and 3.21 μM for 17α-hydroxyprogesterone and progesterone, and the V_max values were 192.9 nmoles/min and 198 nmoles/min per nmole P-450c21, respectively. The dissociation constant determined from the difference binding spectra was 31.1 μM for 17α-hydroxyprogesterone and 14.7 μM for progesterone.