Photooxidase Activity of Heated Chromatophores of Rhodospirillum rubrum

Abstract

\Tan Niel's classical (listinction between bacterial andl green plant photosyntheses is based on the differences in the metabolism of the photooxidant (16). But direct measurement of the oxidizing power in bacterial photosynthesis is, at best, difficult with the -usual spectrophotometric equipment. Light-induced oxidations of endogenous cytochromes have been demonstrated in whole cells and extracts of Rhodospirillini rubruzn (2, 4, 15), but special spectrophotometers were always require(l. The oxidizing power generated by light was easily measured by Vernon and Kamen using as the assay the photooxidation of re(luced indophenol or exogenous mammalian cytochrome c (18,19). However, this photooxidation of exogenous indicators was oxygen dependent and heat resistant causing Smith to doubt its physiological significance (14). The photooxidation of reduced indophenol could merely represent a manifestation of the wvell-known ability of bacteriochlorophyll to mediate a sensitized photooxidation (7). Mild heat has been shown to destroy two absorption bands characteristic of the spectrum of bacteriochlorophyll in the chromatophore of R. rubrumn (3. 8) while the photooxidase assay is relatively heat insensitive (10, 18). By comparing the effect of mild heat treatments on both the spectrum and on the photoxidase activity of the chromatophore, a validation was sought of the photooxidase assay using reduced indophenol. The rate of photooxidation of indophenol was not related to the strength of any absorption peak of bacteriochlorophyll. This participation of other heatlabile factors suggested that this in vitro assay is related to the in vivo metabolism of the oxidant.