Electron Transfer from Cytochrome b5 to Cytochrome P450

There are four electron carriers in the endoplasmic reticulum of liver cells, two flavoproteins: NADH cytochrome b5 reductase (FP1) and NADPH cytochrome P450 reductase (FP2) and two hemoproteins: cytochrome b5 and cytochrome P450. The two reductases were first evidenced in 1950 by Hogeboom and Schneider (1) by their strong cytochrome c reductase activity. They were isolated respectively by Spatz and Strittmatter (2) and Van der Hoeven and Coon (3). The cytochrome b5 was described by Strittmatter and Ball (4). It was isolated after tryptic digestion of microsomes (5) and then purified with full length protein chain by Spatz and Strittmatter (6). The last electron carrier discovered in microsomes was cytochrome P450, the CO-binding pigment evidenced by Klingenberg (7) and Garfinkel (8) in 1958.