Haem a Compounds as Models for Cytochromes of Group a. II. Absorption Spectra of Haem a Compounds with Detergents

The absorption spectra of compounds of haem a, CO-haem a and haematin a with nitrogen-free detergents have been studied. In contrast to the compounds with nitrogenous ligands, those with detergents show a CO-shift towards shorter wavelengths; this shift is small with cationic or anionic detergents, but is more than half that assumed for cytochrome a3 with non-ionic detergents of the Tween class, particularly with the polyoxyethylene sorbitan monooleate Emasol 4130. Emasol in the compound is not displaced by serum album in and in the same w/v concentration haem a combines more strongly with Emasol. Based on those observations and that of Japanese authors that non-ionic detergents reactivate a cytochrome oxidase previously inactivated by cholate, it is postulated that the linkage between haem a and protein in cytochrome a3 is through a lipid, not through a bond between haem-iron and protein-nitrogen. The importance of this hypothesis for understanding the problem of the relationship between cytochromes a and a3 is discussed.