892 7. MERCURIALS 



Photophosphorylation to form ATP is not necessarily coupled with NADP 

 reduction (J. S. C. Wessels, 1959), but nevertheless one might predict that 

 it would be reduced by mercurials. It has been found that the incorporation 

 of Pj into ATP in illuminated chlorophasts is inhibited around 50% by 

 p-MB at 0.05-0.1 mM (Arnon et al, 1956; J. S. C. Wessels, 1958, 1959; 

 Jagendorf and Avron, 1959), and similar effects were reported for Rhodo- 

 spirillum rubrum (Smith and Baltscheffsky, 1959). Photophosphorylation 

 is not inhibited as potently as photoreduction. 



The photochemical fixation of C^^Og by chloroplasts is inhibited 14% and 

 88% by 0.01 and 0.05 mM p-MB, respectively (Gibbs and Calo, 1959 b), 

 but a reconstructed system (extract + chloroplast fragments) is more sen- 

 sitive, 61% and 94% inhibition being exerted by these concentrations of 

 p-MB (Gibbs and Calo, 1960 b). It is not known if this implies some barrier 

 to penetration in the intact chloroplast. Both the initial and steady-state 

 rates of fixation of CO2 in illuminated dahlia leaves are only slightly re- 

 duced (15-25%) by 0.5 mM p-MB, even though plenty of time is provided 

 for penetration (Massini, 1957), and in Scenedesmus obliquus photosynthesis 

 is inhibited only 50% by 1 mM p-MB after 260 min exposure (Horwitz, 

 1957). The failure to inhibit more potently in these cases can at present 

 be explained only on the basis of inadequate penetration into the cells or a 

 certain structural integrity of the photosynthetic apparatus which makes 

 it difficult for a mercurial to exert such inhibition as is observed with isolat- 

 ed chloroplasts. No detailed study of the effects of mercurials on the rapidly 

 labeled C compounds has been made, but Miyachi (1960) has found that 

 p-MB decreases the level of what he calls the primary photogenic agent 

 (measured by 3-sec C^^Og fixation) in ChloreUa, although it does not inter- 

 fere with the participation of this substance in the subsequent photosyn- 

 thetic pathway. Nonphotosynthetic C^^Og fixation is usually inhibited 

 strongly by mercurials, e.g., the autotrophic fixation by Hydrogenomonas 

 facilis (McFadden and Atkinson, 1957) or the fixation associated with sul- 

 fide oxidation in Thiobacillus thiooxidans (Iwatsuka et al., 1962), both being 

 inhibited around 50% by 0.01 mM p-MB — which is not surprising con- 

 sidering the sensitivity of the various enzymes usually involved in COg 

 fixation. This dark fixation is possibly related in some manner to photo- 

 synthesis, and it has occasionally been pointed out that the same inhibitors 

 are effective in both. 



THE CELL MEMBRANE AS A SITE FOR MERCURIAL ACTION 



In the following section we shall discuss the effects of the mercurials on 

 permeability and membrane transport systems, as a background for under- 

 standing the responses of tissues to these inhibitors, but it may serve to 

 clarify the problem if we take up the theory of the role of the cell membrane 



