NATURAL INHIBITORS OF THE HILL REACTION 281 



high Hill reaction rates {Qo^^ = 1000 at 10° C). Chloroplasts of this 

 species are characterized by high polyphenol oxidase activity, and 

 the leaf macerates are extremely \ascous. The polysaccharide causing 

 the viscosity, however, is not in itself responsible for the initially low 

 activity: the polysaccharide was isolated from "blanched" leaves as 

 well as from the inner bark according to Gill, Hirst, and Jones (30) ; 

 upon providing similar viscosities with the polysaccharide from these 

 two sources, the Hill reaction rates of "active" chloroplasts were not 

 inhibited by more than 30%. The remarkable improvement of red 

 elm chloroplasts by exhaustive washing cannot be explained in terms 

 of saponins, acids, or tannins, and so should be ascribed to a fourth 

 type of interference. 



CHLOROPLAST STABILIZATION BY CARBOWAX* 



When leaves are ground in 30% Carbowax 4000, the cytoplasm is 

 precipitated on the chloroplasts. (The supernate from the first cen- 

 trifugation is devoid of heat-coagulable protein.) This "salting out" 

 phenomenon is regularly associated with an increased stability of 

 the chloroplasts' capacity for water photolysis. When the chloroplasts 

 are freed of cytoplasm by isolation in 0.5 M sucrose, they do not ex- 

 hibit enhanced stability when subsequently suspended in 30% Carbo- 

 wax 4000. The preservative effect of Carbowax which is exerted via 

 the cytoplasm is magnified by prolonged storage and by the use of 

 relatively high temperatures in isolating and testing the chloroplasts. 

 It is also exhibited, however, in freshly isolated chloroplasts (isolated 

 at 0°C., tested at 10° C), pro\aded limiting numbers of chloroplasts 

 are used in the acti\'ity measurements. Examples were encountered 

 of chloroplasts which showed no higher initial activity when isolated 

 in 30% Carbowax 4000 vs. 0.5 M sucrose, just as McClendon ob- 

 served (4). However, with the use of successively smaller aliquots of 

 such chloroplast suspensions (e.g., Ailanthus aUissima) the observed 

 capacity for the Hill reaction regularly became higher in the Carbo- 

 Avax than in the sucrose preparations. The different results obtained 

 with Carbowax vs. sucrose by McClendon at two stations (4) may 

 have been caused by the provision of chloroplast-limiting conditions 

 at one station and not the other. 



* Further information on this subject is reported by the authors in Physiologia 

 Plantarum, 9: 519-532, 1956. 



