THE PHOTOINACTIVATION OF THE RESPIRATORY CHAIN IN <i>SARCINA LUTEA (MICROCOCCUS LUTEUS)</i> AND PROTECTION BY ENDOGENOUS CAROTENOID

Abstract

Abstract. Irradiation of isolated membranes of Sarcina lutea (Micrococcus luteus) with blue light rapidly inactivated the respiratory malate oxidase system under aerobic but not anaerobic conditions. This inactivation was much faster than that seen in whole cells suggesting that the intact organism possesses protective mechanisms capable of preventing or repairing light damage. Three photosensitive sites have been detected by comparing the effect of blue light on membranes from the carotenoid‐containing wild‐type and a carotenoidless mutant. The sites have been identified as the initial malate dehydrogenase enzyme, a flavoprotein assayed by phenazine methosulphate reduction, a sulphydryl group associated with the dehydrogenase complex but not involved in phenazine methosulphate reduction and the respiratory quinone, menaquinone. Menaquinone was found to be sensitive only in carotenoidless membranes and not in membranes from the pigmented wild‐type. Studies of the variation of photosensitivity with wavelength suggest that the three sites are sensitized by different chromophores and that the quinone acts as its own photosensitizer.