Natural Inhibitors of the Hill Reaction 



K. A. CLENDENNING,* T. E. BROWN, and E. E. WALLDOV, 



Charles F. Kettering Foundation, Yellow Springs, Ohio 



Although healthy green leaves invariably have high photosynthetic 

 capacities, chloroplasts freshly isolated from them often have low or 

 negligible capacities for the Hill reaction (1, 2). Hill (3) and McClen- 

 don (4) attributed this phenomenon to physical factors (plastid rup- 

 ture, unfavorable osmotic conditions), but these do not account for 

 the following observations: Chloroplasts from many species are con- 

 sistently inactive when isolated intact in ice-cold sugar solution 

 (1) ; chloroplasts having high Hill reaction capacities in the unbroken 

 state retain this ability when finely disintegrated (5-7) ; the osmotic 

 en\'ironment of leaf chloroplasts in vivo is far from constant (8, 9), and 

 their Hill reaction activity in vitro is insensitive to osmotic pressure 

 changes (10). 



The first evidence of natural inhibitors was obtained by Kumm and 

 French (1), who noted that inactive chloroplasts often were associated 

 with tannins or related compounds which darkened Hill's solution. 

 Clendenning and Gorham (2) demonstrated the presence of heat- 

 stable, water-soluble inhibitors in inactive chloroplast sources, whose 

 action was irreversible. Apart from this incidental information, little 

 was known concerning natural inhibitors of the Hill reaction. 



The grana of blue-green algae and the plastids of led algae retain 

 Hill reaction activity only when isolated and tested in concentrated 

 solutions of polymers such as dextrin (11) or polyethylene glycols 

 (4). Phycocyanin and phycoerythrin otherwise are dissolved, with an 

 accompanying loss of photochemical activity (11). McClendon (4) 

 noted instances in which the capacity for water photolysis in green 

 plant chloroplasts was improved by using 30% w/v Carbowax 4000 

 instead of 0.5M sucrose in their isolation, but this result was not 

 obtained consistentl3^ No stabihzing action of polyethylene glycols 



* Present address: Division of Marine Biology, Scripps Institution of Ocean- 

 ography, La Jolla, California. 



274 



