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Title: Effects of light on the membrane-bound enzymes of Sarcina lutea and the role of carotenoids
Author: Anwar, Muhammad
ISNI:       0000 0001 3425 5536
Awarding Body: University of London
Current Institution: Royal Holloway, University of London
Date of Award: 1975
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Photosensitivity of the respiratory enzyme system at high-light intensity (without exogenous photosensitizer) in the isolated membranes of S. lutea has been studied in relation to carotenoid protection. It was observed that malate, NADH and succinate oxidising activities in the membranes of S. lutea were all affected by the blue region of the visible light spectrum and that light of longer wave-lengths was less effective, green and red light being completely ineffective. The effect of light on the above-mentioned activities was greater in non-pigmented than in pigmented membranes when a high light intensity was used for 15 minutes, but no carotenoid protection was observed when the membranes were illuminated using a moderate light intensity for 120 minutes. It was observed that the malate oxidising respiratory chain was photoinactivated at two sites in the normal pigmented membranes and at three sites in the carotenoidless mutants. The two sites observed in the pigmented system were located in the malate dehydrogenase complex and observed in non-pigmented membranes as well. The first site could be assayed independently by PMS reduction whereas the other site assayed by menadione reduction could not be assayed without involving the first one. The second site was more sensitive than the first and it was believed to involve sulphydryl groups because the addition of cysteine could restore the activity. The sensitivity of the first and second sites was not affected by carotenoids. The third photosensitive site, observed in non-pigmented membranes only was shown to be the menaquinone, an essential component of the respiratory chain oxidising malate and NADH but not succinate and functioning between cytochromes and flavoproteindehydrogenase complex. Enzyme activity of illuminated white membrane preparations could partially be restored by the addition of vitamin K1, a homologue of menaquinone (MK) . The percentage loss of MK consequent on irradiation was negligible in yellow membranes but considerable in white membranes. In an in vitro system carotenoid extracted from yellow membranes, when added to vitamin K1, showed a protection of the quinone from visible irradiation. It is concluded that high-intensity light destroys menaquinone - a component of the respiratory electron transport chain, in the carotenoidless mutants but not in the pigmented. Therefore to protect menaquinone from visible light, nature has provided carotenoids in bacteria which are subjected to intense light and air.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Biochemistry