52 OXIDATION-REDUCTION POTENTIALS 



in diameter. Both ends of this short glass rod are ground and polished, one end 

 is held in close proximity to the objective lens and the other dips into the cell 

 suspension. The bung is jS.tted into a piece of glass tubing 15 mm. in bore. When 

 the glass rod is lowered into the cell suspension this outer tube dips into the annulus 

 of mercury and forms an air-tight seal. If a slide 1| in. wide, instead of the usual 

 1 in., be used, more ample clearance between the overlapping glass tubes can be 

 allowed, and by means of side-tubes any desired gas may be circulated through the 

 inner chamber. 



The inexpensive apparatus described has the following advantages : — 



(1) The glass tube dipping into the suspension avoids the serious loss of 

 illumination inevitable at an air-suspension interface, and brings the objective 

 optically close to the suspension whilst actually separating it. 



(2) The ordinary focussing arrangement of the microscope, by raising or 

 lowering the glass rod attached to the objective, provides an effective method of 

 varying the depth of suspension through which the light passes. 



(3) The mercury seal prevents drying up of the suspension during observations 

 and effectively prevents accidental scattering of organisms, a desirable precaution 

 in the case of pathogenic bacteria. 



(4) The objective of the microscope is protected and all parts of the apparatus 

 coming in contact with bacteria may be sterilised in boiling water, the simple Canada 

 balsam seals, being readily renewed after use. 



Cytochrome occurs in aerobic bacteria but not in strict anserobes, and to its 

 presence has been attributed the heat-stable peroxidase of bacteria, but successful 

 isolation of cytochromes makes possible the direct testing of this point. (Yaoi and 

 Tamiya, 1928). Facultative angerobes may be deficient in one or more of the 

 cytochrome constituents. It is not, however, possible to obtain a close relation 

 between the type of metabolism of a bacterium and its cytochrome composition 

 since a number of alternative respiratory enzymes may be present. 



As with some other biologically interesting systems the determination of the 

 oxidation-reduction potentials of the cytochromes has presented some difficulties. 

 Wurmser and Filitti-Wurmser (1938) found that the potential of cytochrome -C 

 reached an equilibrium only after several hours but the value obtained (E^„+0'253 v.) 

 was confirmed by studying the equilibrium between cytochrome and reductinic 

 acid. The oxidation-reduction potential was constant between pH 5 and 8. The 

 usual potential equation was found to be obeyed : — 



T? —TT 1 ^"^ 1 [K'educed cytochrome] 

 ° F [Oxidised cytochrome] 



By studying spectrophotometrically the equilibria between cytochrome and 

 oxidation-reduction dyes Stotz, Sidwell and Hogness (1938), obtained the value 

 Eq^ + 0*262 V. Using quinhydrone and cytochrome -C in washed muscle Laki 

 (1938) found Eo^ + 0-28 v. at pH 7-4, and Ball (1938) gives the value + 0-27 at pH 

 7-4. Green (1934) previously reported Eq^ + 0-127 v. with yeast cytochrome -C. 

 Working with heart muscle Ball (1938) has given figures for the E^^ values of three 

 cytochrome components : — 



Cytochrome-a, -f 0-29 v. ; b, — 0-04 v., ; c, -{- 0-27 v. 



The oxidation-reduction potential behaviour of cytochrome-C has been studied 

 by Paul (1947) and Rodkey and Ball (1947) who find an E„ value of about +0-26 v. 



