Aiitoxidation of Flavocytochwine bo 



531 



lactate concentration was absent. Figure 10 enables calculation of the lactate 

 concentration giving half-maximal rate (in the presence of excess ferrous 

 ions); it appears to be 0-5 mM. 



500 1000 



l/doctote) 



Fig. 10. The eftect of the concentration of the L(-|-)-lactate in the presence of 



mM FeS04. Abscissa: reciprocal of the molarity of lactate. Ordinate: reciprocal 



of the oxygen consumption (/tl.) in 10 min at pH 6 and 37''C. 



DISCUSSION 



Flavocytochrome b^ preparations catalyse reaction (2) above. 



The reaction proceeds faster with more enzyme and with more oxygen 

 present. The optimal pH is around neutrahty. At pH values lower than 6 

 the reaction slows down. This behaviour is similar to what happens with 

 other oxidants (ferricytochrome c, ferricyanide, phenazine methosulphate) 

 and is similarly explained by the presence of a group on the enzyme dis- 

 sociating with a pA' around 5-65. 



With proper lactate, salt, hydrogen ion and oxygen concentrations the 

 turnover number for oxygen appears to be 27 moles oxygen consumed/mole 

 of enzyme/min as calculated from the values of Fig. 6. Since oxygen accepts 

 four electrons/molecule, the turnover number for oxygen as oxidant is of 

 108 equivalents/mole of enzyme/min, in comparison with 16,000 for ferri- 

 cyanide and ferricytochrome c. Therefore, oxygen under the best conditions 

 is only about 0-7% as efficient as ferricyanide. In terms of —Qo2-> the prep- 

 aration has a value of 260, which proves rather low in comparison with 

 other oxidases. 



Lactate, which is the substrate, inliibits the autoxidation. This means that 

 there is no oxygen consumption without lactate, but the oxygen consumption 



