Discovery of P-450
A major class of oxidative transformations was initially characterized in 1955
by O. Hayaishi in Japan and H.S. Mason in the United States. Since one class
of oxygenases had requirements for both an oxidant (molecular oxygen) and a
reductant (reduced NAPD), it was given the trivial name “mixed-function oxidases.”
Ryo Sato |
An understanding of the biochemical nature of these reactions grew from
early studies by Garfinkel and Klingenberg, who observed a carbon monoxide-binding
pigment with unique absorption at 450 mµ in liver microsomes. Based on
the unusual spectrum, he concluded that it was probably not an iron hemoprotein.
Klingenberg found the CO-binding pigment in rat liver microsomes, again
reporting the unique aspects of difference spectrum. The pigment was ultimately
characterized as a cytochrome in 1962 by Omura and Sato, using rabbit
liver microsomes. Through the use of detergent solubilization, they showed
that the resultant “P-420” was a cytochrome with typical a,
b, and Soret absorption bands.
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Tsuneo Omura |
The role of this unique cytochrome in drug metabolism was then uncovered in
studies using adrenal microsomes for the catalysis of the hydroxylation of 17-hydroxy
progesterone to corexolone. Having prepared detergent solubilized adrenal microsomes,
Cooper and co-workers found that they contained a CO-binding pigment similar
to that of pig liver, as reported by Klingenberg. Relying on the studies of
Ryan and Engel that had shown that the adrenal C-21 hydroxylation reaction was
inhibited by CO, and the inhibition could be reversed by white light, Estabrook,
Cooper, and Rosenthal performed classic photoactivation experiments and proved
that this cytochrome was the oxygen-combining component in the C-21 hydroxylation
of steroids.
In the 35 years since its identification as the terminal component of oxygenation
reactions, cytochrome P-450 has grown beyond drug metabolism to influence many
other sciences. The first crystallization of a mammalian P-450 was reported
in 1999 by Eric Johnson and co-workers.
