Abstract
Abstract 1. A cytochrome f preparation was obtained from spinach chloroplasts with an absorbance ratio of 3.0 at 421:278 nm. The minimum molecular weight calculated from sodium dodecyl sulfate gel electrophoresis was 34,000 and from amino acid analysis 36,000. It contained 2 half-cystines and 3 histidines per heme. The purified preparation was soluble in water and appeared to be in an octameric form. 2. Photosystem I particles were depleted of cytochromes by treatment with digitonin and differential centrifugation. These particles photooxidized purified reduced cytochrome f, which in the presence of ascorbate was again reduced in the dark. The oxidation was markedly accelerated by addition of plastocyanin at low concentrations (1 to 10 nm) approximating the P700 concentration. The photooxidation was inhibited by salt and unaffected by nonionic detergents. From the stoichiometry and the high rate of photooxidation, it was concluded that cytochrome f, plastocyanin and Photosystem I form a catalytic complex.
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