Sensitization of Chloroplast Reactions. I. Sensitization of Reduction & Oxidation of Cytochrome C by Chloroplasts

The abundant evidence for two photoreactions in photosynthesis, sensitized by different pigment groups, has accrued from three different types of observation (8). First, the chromatic transients found by Blinks (1) showed that lights of different wavelengths, though provoking equal steady state rates, may require different transitory adjustments to attain these rates. Second, it was found by Emerson et al. (4) that two beams of different color together could yield a rate of oxygen evolution higher than the sum of the individual rates. This enhancement effect implied that two photoprocesses might assist or even require each other in photosynthesis. Third, Kok and Gott (10) observed that the momentary oxido-reduction level of the photocatalyst P700 was affected in opposite fashion by lights of different wavelengths. This was interpreted as a push and pull effect by two photoreactions on this intermediate of the oxidoreduction chain. One approach towards an understanding of the specific roles of the two photoreactions is a search for chloroplast converisions which might require only one of them or require the two in a different ratio than complete photosynthesis. The first example to become known of an abnormal sensitization is that of photophosphorylation mediated by phenazine methosulfate (11) which was found to attain its highest rate in (strong) light of long wavelength (X > 690 mu). This is in contrast to the generally observed decline of the quantum yield in this spectral area which was discovered by Emerson and Lewis (5). A convenient substrate for chloroplast conversions is cytochrome c. The strong and narrow absorption band at 550 mfi of its reduced form allows sensitive spectrophotometric observation, and in the red spectral area it does not interfere with chloroplast absorption. Holt (8) observed that isolated chloroplasts reduce ferricytochrome c in the light. This process was later found to be stimulated by the enzyme photosynthetic pyridine nucleotide reductase (PPNR) (2, 18). Nieman et al. (16, 17) discovered that upon treatment with digitonin, chloroplasts lose this photoreductive capacity and, instead, will photooxidize ferrocytochrome c. They also observed that this photooxidation was catalyzed by a soluble chloroplast enzyme which was termed a photooxidase. It therefore is possible to compare the wavelength dependency of the quantum yield in photoreduction and photooxidation of cytochrome c by rather similar photosynthetic particles. In this paper we report an anomalous wavelength dependency of the quantum yield of photooxidation of ferrocytochrome c.