David Y. Cooper

PHOTOCHEMICAL ACTION SPECTRUM OF THE TERMINAL OXIDASE OF MIXED FUNCTION OXIDASE SYSTEMS.

The reversal of the carbon monoxide inhibition by bands of monochromatic light was determined for the oxidative demethylation of codeine and monomethyl-4-aminopyrine and the hydroxylation of acetanilide by rat liver microsomes and for the hydroxylation of 17-hydroxyprogesterone at carbon-21 by bovine adrenocortical microsomes. Maximum reversal occurred at 450 millimicrons, the light absorption maximum of the CO compound of the CO-binding pigment of microsomes. The agreement between photochemical action spectrum and spectrophotometric difference spectrum supports the conclusion that the CO-binding pigment is the terminal oxidase of mixed function oxidase systems of mammals.

[96] Methods of determining the photochemical action spectrum

Publisher Summary If the degree of promotion or inhibition of a biological reaction by bands of monochromatic light of equal quantum intensity is plotted as a function of wavelength of irradiating light, a photochemical action spectrum results, which depicts the light absorption spectrum of the biocatalyst responsible for the light sensitivity of the reaction. The essential technical requirements for this method are a light source from which, monochromatic bands of sufficient intensity can be isolated, and a radiometer for the accurate measurement of the quantum energy of the bands. Carbon arcs, high pressure xenon lamps, and projector type metal filament lamps furnishes continuous spectra convenient for scanning a broad spectral range with the same light source and for resolving details of the action spectrum. Xenon lamps and tungsten filament lamps are presently the preferred sources of continuous spectra. The metal vapor lamps are mainly used to provide reliable reference points for the photochemical action spectrum, and they do not permit the resolution of every detail of the spectrum. The energy of the irradiating light is usually measured with thermopiles or bolometers. The photochemical action spectrum is usually determined at infinitely small light absorption of the reaction system in order to ensure that every enzyme molecule along the light path is exposed to the same intensity of radiation. The determination of the photochemical action spectrum of CO derivatives of the respiration enzyme is based on the assumption that the distribution of the enzyme between O 2 and CO. The chapter also discusses the determination of photochemical activation and inactivation spectra of various biocatalysts.