Cytochrome h-^, 



569 



performed in both directions, by addition of ferricyanide to reduce flavocytochrome 

 62 or by addition of L(+)-lactate to oxidized flavocytochrome b.^,. The titrations were 

 performed in anaerobic conditions, by following the extinction changes at 557 m/< 

 in a Thunberg-shaped cuvette. In both instances, plots of the equivalents used versus 

 the moles of enzyme reduced showed strict adherence to a ratio of 2 equivalents/mole 

 for the first part of the curve, up to about 50 % reduction and oxidation. The second 



20 40 60 80 100 



m// moles L(+) lactate added 



Abscissa: millimicromoles of L(+)-lactate added from the burette. 

 Ordinate: reduction as judged from the increase at 557 m//. Line a: 

 theoretical for a stoichiometry lactate: enzyme = 1:2; line b for a stoi- 

 chiometry 1 : 1, c for a stoichiometry 2:3; line d is the best fitting line. 

 Conditions of the experiment: pH 70, phosphate buffer 0-01 M, 20°C, 



part of the curve showed that to obtain complete reduction or oxidation more equiva- 

 lents of the titrating agent were necessary. Our experiments were performed with about 

 50 millimicromoles of flavocytochromes b.^. In view of the 2: 1 ratio obtained in these 

 experiments, we favour a reaction of the type: 



Lactate + Prot - FMNH - cyt+++-^ Pyruvate + Prot - FMNHg - cyt++ 



in which the enzyme exchanges one electron on the haematin iron and one only on 

 the flavin moiety. This hypothesis can account for the fact that the spectrum of 

 oxidized flavocytochrome Z^g does not show the bands of oxidized flavin. 



Experiments were begun, in co-operation with A. Ehrenberg of the Medical Nobel 

 Institute of Stockholm, to detect the eventual presence of a free radical by electron 

 spin resonance measurements. 



The Function and Bonding of the Flavin Group of Cytochrome bg 



The Bonding between the Flavin Group and Apoprotein of Cytochrome ^2 



By J. McD. Armstrong, J. H. Coaxes and R. K. Morton (Adelaide) 

 Morton: The studies reported here suggest that the riboflavin phosphate prosthetic 

 group of cytochrome b^ is bonded to the protein by thiol groups of cysteine residues. 

 The linkage is extremely labile, and irreversible dissociation of the flavin may occur 

 P 



