The remarkable P450s: a historical overview of these versatile hemeprotein catalysts

The P450s are remarkable because there are so many of them and they do so many things. They are the products of a gene superfamily that includes representation from all phyla and many diverse species. At last count the cDNAs for more than 300 P450s had been cloned and se-quenced (1), and many have been expressed as functional recombinant enzymes in transformed yeast, bacteria, or mammalian cells. The P450s are claimed to catalyze more than 60 different types of chemical reactions , including aromatic and aliphatic hydroxylations, N-and S-oxidations, epoxidations, and dealkylations, to name but a few examples. The P450s are characterized spectrophotometrically by the presence of an absorbance band at 450 nm when a sample of the reduced hemepro-tein is gassed with carbon monoxide (2), hence the name P450 to designate a pigment absorbing light at 450 nm. This characteristic absorbance fingerprint identifying a P450 is due to the presence of a thiolate ligand to the heme iron of the hemeprotein (3), a fact that also contributes to its unique properties as an " oxygen-activating " enzyme. The P4505 enjoy a rich history based in the classic studies of the i940s and 1950s by 1) B. B. Brodie and colleagues at the National Institutes of Health, who pioneered the field of drug metabolism (4), 2) R. biochemical toxicology by his detailed studies defining the metabolism of a wide variety of chemical xenobiotics (5), 3) Elizabeth and James Miller of the McArdle Labo-who established the principles of chemical carcinogenesis by demonstrating the role of metabolic activation of chemical procarcinogens (6), 4) the large number of steroid biochemists who deciphered the pathways of cholesterol biotransformation to androgens, estrogens, mineralo-, and glucocorticoids (7), and 5) studies of alternate pathways of lipid metabolism, such as the omega-hydroxylation of fatty acids (8). This foundation in chemistry, pharmacol-ogy, biochemistry, endocrinology, and cancer biology was set more than 50 years ago (Fig. 1). Critical to the discovery and characterization of the P450s were two other advances that occurred in the 1950s. Osamu Hayaishi (9) and Howard Mason (10) simultaneously and independently discovered the oxy-genases-enzymes that catalyze the incorporation of molecular oxygen into organic molecules. Second was the work of Britton Chance (11), who designed and used rapid and sensitive spectrophotometric techniques for the study of turbid suspensions of cells and cell membranes. In 1955, Martin Klingenberg, a young postdoctoral fellow from Germany, was working in the …