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The Respiratory Chain of Chlorella protothecoides

I. Inhibitor Responses and Cytochrome Components of Whole Cells ¹

^a Department of Botany, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514

The respiration and cytochrome properties of "glucose-bleached" Chlorella protothecoides Krüger, Indiana strain 25, were studied. This organism, when grown heterotrophically with high glucose and a low organic nitrogen source, has no chlorophyll, little carotenoid, and diminished chloroplast structure—factors which make it suitable for respiration studies.

Whole cell endogenous oxygen uptake rates are either stimulated or only slightly inhibited by cyanide, azide, CO, and antimycin. When these inhibitors are used with m-chlorobenz-hydroxamic acid (mCLAM), an inhibitor of higher plant mitochondrial alternate oxidase, O₂ uptake is inhibited. There is little effect of mCLAM by itself on the rate of oxygen uptake. The inhibition by CO of O₂ uptake in the presence of mCLAM is reversed by light.

The cytochrome chain of C. protothecoides consists of cytochromes aa₃, b, and c, as revealed by room temperature difference spectra. In common with mitochondria of higher plants, there is a further reduction of cytochrome b with dithionite. In the presence of antimycin, the cytochromes aa₃ and c are oxidized and cytochrome b is reduced. Cyanide causes a partial reduction of cytochromes aa₃ and c while cytochrome b remains oxidized. This general response is characteristic of higher plant mitochrondria having large amounts of cyanide-resistant respiration. Carbon monoxide spectra reveal one CO-combining pigment. The cytochrome b region differs from that of higher plants in that the typical complex spectrum does not appear at low temperature (–190 C).

The concentration of cytochrome aa₃ per cell volume was observed during the greening of "glucose-bleached" cells. The concentration of these cytochromes nearly tripled during the 24 hours of the initial stages of greening.

¹ This work was supported by an American Cancer Society Institutional Subgrant. This paper is a portion of a dissertation submitted to the University of North Carolina by N. G. Grant.