438 HERRICK BALTSCHEFFSKY 



The earlier general attempts to explain the requirement for hydrogen 

 donor [17, 19] may be substituted with a more definite hypothesis on the 

 basis of the effect of ascorbate combined with recent results reported by 

 Chance and Nishimura [20]. Ascorbate has been used to reduce directly 

 mitochondrial cytochrome c in experiments designed to determine the P/O 

 ratio in the span cytochrome c to oxygen [21, 22]. From the known redox- 

 potential of cytochrome c^ [23] it may be assumed that ascorbate in a 

 similar manner reduces this electron carrier, which has been reported to 

 participate in the electron transport of light-induced phosphorylation in 

 R. tiibrum [10]. In w^hat appears to be a primary photochemical reaction 

 cytochrome c^ of Chromatium becomes rapidly oxidized in the light, even 



12 16 



mM ascorbate 



Fig. 6. Effect of ascorbate on LIP in R. rubnim. Ascorbate was the only 

 reducing agent added (cf. ref. [3]). The bacteria had been disrupted by grinding 

 with sand. 



at 8o°K. [20]. The logical assumption has been made that chlorophyll is 

 reduced in this reaction [20]. If the electron transport is initiated by light- 

 induced electron transfer from cytochrome r., to chlorophyll it is clear that 

 some cytochrome c.^ must he present in the reduced form in order for the system 

 to operate. Aerobically, this criterion may not be fulfilled, and a hydrogen 

 donor which causes either enzymic or chemical reduction of an appro- 

 priate amount of cytochrome f., has to be present. According to our 

 hypothesis the function of any used hydrogen donor in aerobic light- 

 induced phosphorylation of R. rubrum is to reduce an adequate portion of 

 cytochrome c.y (Fig. 7). 



Table IV shows some difl^erences between menadione and PMS as 

 electron carriers in bacterial light-induced phosphorylation [7]. Quanti- 

 tatively, PMS is a much more potent stimulating agent than menadione, 



