Purification and properties of a shortened form of cytochrome P-450 2E1: deletion of the NH2-terminal membrane-insertion signal peptide does not alter the catalytic activities.

As reported previously, alcohol-inducible cytochrome P-450 2E1 lacking the hydrophobic NH2-terminal segment is located primarily in the inner cell membrane when expressed in Escherichia coli and is active with a typical substrate. To study the catalytic properties in detail, we have purified the truncated P-450 lacking residues 3-29 to electrophoretic homogeneity from the solubilized bacterial membrane fraction in the presence of 4-methylpyrazole as a stabilizing agent. The resulting heme protein with a specific content of 15.8 nmol of P-450 per mg of protein has a reduced CO difference spectrum identical to that of the full-length enzyme, with a Soret maximum at 452 nm. The rates of catalysis of four reactions in the reconstituted enzyme system, including the oxygenation of ethanol to give acetaldehyde, the oxidative dealkylation of N-nitrosodiethylamine to give ethylene and acetaldehyde, and the ring hydroxylation of aniline and p-nitrophenol, are the same with the shortened and full-length enzymes. The apparent Km of p-nitrophenol is also the same, as is that for NADPH-cytochrome P-450 reductase and for cytochrome b5, which stimulates p-nitrocatechol formation about 3-fold. Moreover, the requirement for phosphatidylcholine for full catalytic activity is unchanged despite the absence of the NH2-terminal segment. Although this highly hydrophobic segment is believed to play a role in the intact cell as a membrane-insertion signal sequence, we conclude that it has no function in the catalytic activity of the cytochrome as an oxygenase, including interactions with the other components of the enzyme system.