HEMOGLOBIN AS A CYTOCHROME OXIDASE MODEL 393 



ity in the mode of hein/globin formation by autoxidation of hemo- 

 globin to that by oxygen in the presence of reducing substances. 



The kinetics of hemoglobin autoxidation were closely studied by 

 Brooks {341,342), who found the following law to hold: 



^ = t|„(a-.)| ^^ (1) 



at 1 + opo, 



where k and b are constants, a is the initial hemoglobin concentration, 

 a is the quotient [hemoglobin]/ [total hemoglobin], and x is the hemr- 

 globin concentration. 



Brooks claimed therefore that only hemoglobin reacts with oxygen 

 (hence the proportionality to a) and that oxygen caused an additional 

 inhibition expressed in the term 6/(1 + bpQ,). He rejected the idea 

 that intermediates between Hb4 and Hb4(02)4 played a role, since 

 his equation did not fit the assumption of a reaction of any one of the 

 intermediates, e.g., Hb402 or Hb4(02)2, with oxygen. The inhibiting 

 action of oxygen expressed in the term 6/(1 + 6^02) remained 

 unexplained. As Brooks pointed out, an alternative explanation, in 

 harmony with equation 1, would be that the rate of autoxidation is 

 proportional to the concentration of unoxygenated heme and to a 

 Fe* O2 compound acting as oxidative catalyst. The latter explana- 

 tion is preferable to the first. 



In a theoretical study Legge {1666) reinterpreted Brooks' results 

 as indicating the spontaneous breakdown of the intermediate com- 

 pound Hb4(02)2 to form hem/globin.* This fails to explain, however, 

 why the autoxidation of myohemoglobin, which has only one heme 

 group per molecule, has also a maximal rate at low oxygen pressure 

 (Brooks, 3W)- 



It remains to explain how the Fe^ O2 group can become an 

 oxidative catalyst and why in the hemoglobin molecule with four 

 heme groups just the intermediate Hb4(02)2 gives rise to hem/globin. 

 From the fact that the reduction of oxygen involves four equivalents, 

 one would have rather expected the compound Hb4(02) to be the one 

 to undergo a spontaneous transformation to hem/globin. (This has 

 erroneously been given as the result of Legge's study in Holden's 

 review {1317).) With Hb4(02)2 such a reaction is stoichiometrically 

 possible only if hydrogen donors participate in it: 



Hb4(()2)2 + 2 HoX -* Flb(0n)4 + 2 X (2) 



* III ;i private communication Dr. Brooks has informed us that tlie rate of hemf- 

 glol)in formation at liigh oxygen pressures is not in agreement with I^egge's theory. 



