Spektrale Änderungen von Kaninchenlebermikrosomen durch Cumarin

Abstract

The addition of coumarin to rabbit liver microsomes causes a spectral change with the appearance of a maximum absorption at 390 and a minimum at 452 nm. The extent of the spectral change depends on the concentrat-tions of protein and substrate. The maximum spectral change is given by 5.10-2 M coumarin. The extinction of liver microsomes from rabbits treated with phenobarbital and from untreated controls was measured at 425 nm after the addition of the same amount of coumarin to each. The ratio of the extinctions (control: experimental) varied from 1: 5 to 1:7; the corresponding ratio for the cytochrome P-450 [liver micro-somal cytochrome binding carbon monoxide] content of the microsomes was only about 1:2. The measured cytochrome P-450 cannot therefore be limiting for the binding of coumarin to the endoplasmic reticulum. The spectral dissociation constant (Ks) for the binding of coumarin to rabbit liver microsomes is 9.10-4 M for both control and phenobarbital-treated animals. This is about 100 times higher than the previously determined Michaelis constant for the hydroxylation of coumarin (6. 10-6 M).