264 C. p. WHITTINGHAM 



sibility that carbon dioxide is necessary in catalytic amounts, and if 

 so this must then be in a form not freely exchangeable with external 

 CO2; there is no evidence to support this. 



Until recently the maximal rates of oxygen production obtainable 

 with chloroplast preparations at high light intensities (Qo^^^^) were 

 considerably lower than the corresponding rates of photosynthesis. 

 This may be due to the use of unsuitable reagents, e.g., quinone. A 

 very active isolated chloroplast system seems to be that obtained 

 by Hill and Davenport using chard chloroplasts, the heme factor 

 from pea, and cytochrome c. Qo^^^ of the order of 4000 have been 

 obtained at 20° C, well within the range of values for photosynthesis. 



KINETICS OF THE HILL REACTION; THE RELATIONSHIP 

 BETWEEN RATE AND LIGHT INTENSITY USING THE "HEME" 



FACTOR 



The chloroplast reaction is inhibited by many substances which in- 

 hibit photosynthesis, e.g., hydroxylamine, o-phenanthroline, and 

 phenylurethane. The inhibition is similar to that of photosynthesis, 

 and these results indicate that there are reaction steps common to the 

 two systems. Other substances which markedly inhibit photosyn- 

 thesis inhibit the chloroplast reaction only after prolonged exposure, 

 e.g., cyanide, or not at all, e.g., SH inhibitors such as iodoacetamide 

 and p-chloromercuribenzoate. Hence the primary photochemical 

 reaction of photolysis need not require free SH groups for activity. 

 On the other hand, the reduction of cytochrome c (or methemoglobin) 

 in the presence of the Hill and Davenport factor is inhibited by p- 

 chloromercuribenzoate and the inhibition is reversible by addition of 

 cysteine (2). This presumably then is at least a two-step reaction 

 in which the second link depends on SH groups; possibly the factor 

 is itself unable to act catalytically without free SH groups. The 

 participation of SH groups in chloroplast photochemistry cannot then 

 be excluded except in the simpler photolytic reactions, e.g., quinone 

 reduction. 



The relationship between rate of oxygen production and light in- 

 tensity has been compared for the Hill reaction and Chlorella photo- 

 synthesis. Clendenning and Ehrmantraut (6) compared oxygen 

 production from Chlorella in presence and absence of quinone as a 

 function of light intensity. They found the same initial slope (i.e., 

 quantum efficiency) and maximal rate but a difference in rate for me- 



