976 7, MERCURIALS 



requires a 1 : 1 ratio of mercurial to SH groups, as assumed by these work- 

 ers. Nevertheless, in the face of no negative evidence, it is felt that, the 

 mercurials do inhibit growth by reacting with some SH groups — the prob- 

 lem is with what SH groups, since there are many different SH-containing 

 substances in the cell. Do the mercurials inactivate some SH enzyme, or 

 enzymes, involved in an important metabolic pathway, or react with SH 

 groups in the membrane to block active transport of necessary substances 

 into the cell, or alter permeability so that intracellular components are lost, 

 or directly stabilize the membrane to prevent division, or interfere with the 

 utilization of ATP, or disturb metabolism by reacting with some thiol co- 

 enzyme ? Experiments showing that certain substances protect against mer- 

 curials are not easy to interpret. Thus Pershin and Shcherbakova (1958) 

 found that histidine, glutamate, methionine, and particularly thiamine pro- 

 tect E. coli against Hg++, and interpreted this as indicating that the metab- 

 olism of these compounds is interfered with by Hg++, but it is also possible 

 that the protection is simply due to complexes formed with the Hg++. 

 Theories involving various physicochemical properties of Hg++ and other 

 heavy metal ions — such as solution pressure, solubility products, electro- 

 negativity, and ionization potential (e.g., Shaw, 1954; Somers, 1959) — 

 do not warrant serious consideration since they simplify the biological 

 system beyond recognition and, even if true, would not help us appreciably 

 to understand how the mercurials act. 



Viruses 



Most viruses and phages can be inactivated by the mercurials but it 

 requires fairly high concentrations relative to those inhibiting bacterial 

 growth (Table 7-27). In most work a virus suspension is incubated with the 

 mercurial for a certain period and the infectivity is then tested. As with 

 the effects on microorganisms in general, the degree of inactivation by the 

 mercurials depends on many factors, particularly the medium in which the 

 virus is suspended, the temperature, and the exposure time. The rates of 

 inactivation are quite different for various viruses: ECHO 7 virus is 50% 

 inactivated by 0.05 roM p-MB in 1 min and 99% inactivated in 6 min 

 (Choppin and Philipson, 1961), whereas tobacco mosaic virus is not com- 

 pletely inactivated after 24 hr exposure to 18.5 mM Hg++ (Kassanis and 

 Kleczkowski, 1944). This would be expected since the virus SH groups 

 must vary widely in reactivity as do the SH groups of proteins in general. 

 The inactivation is first order with respect to virus. Staphylococcus phage 

 infectivity declines exponentially when exposed to Hg++ according to the 

 equation: 



dPIdt - kiRgCU) (Po - Pi) 



where Pq is the phage initially present and P, the inactivated phage (Krueger 

 and Baldwin, 1933). This equation holds fairly well over most of the range. 



