384 U. LUMRY AND J. D. Sl'IKES 



of the iH^lation l>etweeii pH and tlio Hill reaction rate of isolated 

 chloroplasts will nltimately provide the kind of fundamental in- 

 formation now hosinning to ajipear in the field of enzyme mechanism. 

 The most detailed study of the pH dependence of the Hill reaction is 

 the recent work of Rieske (4), who determined the pH dependence of 

 Ax and />■/, as shown in Fifz;. 4. The olTect of />>, closely approximates the 

 relation between pH and the quantum yield of isolated chloroplasts, 

 as reported by Wayrynen et al. (9) in that both show a maximum at 

 approximately pH 6.3. This result was to be expected since the quan- 

 tum yield is proportional to Ax. 



A similar maximum is shown by ko but at much higher pH. Above 

 7.5 the Hill reaction becomes very sensitive to small changes in sus- 

 pending media and preparative conditions and in particular to traces 

 of inorganic ions in a manner not yet understood. For this reason the 

 true optimum of A:^ may lie at a higher pH than thus far found. There is 

 a further complication resulting from the occasional appearance of an 

 inhibition by light. Just how much parallelism will ultimately appear 

 between these pH studies and similar studies on algae and intact 

 cells is, of course, not yet known. However, if much similarity does 

 exist those growth conditions and reaction components upon which 

 the internal cell pH depends will ha\^e to be much more carefully con- 

 trolled than in present practice. For example, a shift in internal pH 

 of 0.5 unit could result in a two fold change in quantum yield (see 

 Fig. 4). 



EFFECTS OF DEUTERIUM OXIDE ON THE HILL REACTION 



Most studies under steady-state illumination and in flashing light 

 of the effects of deuterium oxide on photosynthesis have yielded 

 relatively simple results. The only effect has appeared to be a decrease 

 in the rate of the limiting dark reaction (10). Although these studies 

 have not received much theoretical consideration they might be 

 interpreted in terms of water entering the overall photosynthetic 

 process at the Blackman step, since the changes in apparent free 

 energy of activation observed are comparable in magnitude to those 

 found in a simpler chemical system where activated-complex forma- 

 tion inA'oh-es the breaking of bonds containing hydrogen. Horwitz 

 (11) and Krasnovskii and Brin (12) studied the effects of deuterium 

 oxide on the Hill reaction of whole Chlorella cells and on leaf-chloro- 

 plast preparations, respectively. 



