Cytochrome b^ 563 



Historically, the name 'cytochrome 6,' was given by Dixon and co-workers (Bach, 

 Dixon and Keilin, Nature, Land. 149, 21, 1942; Bach, Dixon and Zerfas, Biochem. J. 

 40, 229, 1946) to a haemoprotcin seen in yeast extracts, which was reduced in the 

 presence of lactate and oxidized by cytochrome c and by oxygen. If we retain the 

 prefix 'cytochrome' — given by Keilin to naturally-occurring intracellular haemoprotcin 

 pigments capable of reversible oxidation and reduction — then the name 'cytochrome 

 bo should be used for the intact, homogeneous protein containing haem and flavin 

 groups which was first isolated and described by Appleby and Morton {Nature, Load. 

 173, 749, 1954). This crystalline protein has L(+)-lactate dehydrogenase activity. 

 The evidence in favour of this view has been set out by Appleby and Morton (1954, 

 loc. cit.; Biochem. J. 71, 492, 1959; and Biochem. J. 73, 539, 1959) and by Morton 

 (Society of Biological Chemists, India, Silver Jubilee Souvenir, p. 177, 1955; Rev. pure 

 appl. Chem. 8, 161, 1958). 



Although we hold the view that the polydeoxyribonucleotide (Morton, 1958, loc. 

 cit.; Appleby and Morton, Biochem. J. 73, 539, 1959; Biochem. J. 75, 258, 1960), 

 is an integral component of the naturally-occurring material, as yet there is no evidence 

 that this polynucleotide has any direct function in relation to the enzymic activity. 

 Hence the minimum requirement for lactate dehydrogenase activity is the flavohaemo- 

 protein. 



Boeri refers to this flavohaemoprotein having enzymic activity as 'flavocytochrome 

 b^, but there seems no good reason for the addition of 'flavo' other than to distinguish 

 this material from derived proteins devoid of the flavin group. 



Okunuki and co-workers refer to 'cytochrome b^ as a haemoprotcin, devoid of 

 L(+)-lactate dehydrogenase activity, which they isolated from yeast cells (Yamashita, 

 Higashi, Yamanaka, Nozaki, Mizushima, Matsubara, Horio and Okunuki, Nature, 

 Lond. 179, 959, 1957). The behaviour of this material as reported by Okunuki and 

 co-workers (see Horio, this volume, p. 552 and p. 560), and our own observations 

 (Morton, 1958: loc. cit., Morton, Armstrong and Appleby, this volume, p. 501; 

 Armstrong, Coates and Morton, this volume, p. 571) indicate that this haemoprotcin 

 is modified from intact cytochrome b^. The relationship of this material to intact 

 cytochrome b^ is somewhat analogous to the relationship of the peptic (or tryptic) 

 'core' of heart-muscle cytochrome c to cytochrome c. I think that no one would refer 

 to the 'core' as 'cytochrome c\ I therefore consider that the enzymically-inactive 

 haemoprotcin derived from cytochrome 60 should not be called 'cytochrome b^ but 

 some other name, possibly 'haemoprotcin 557 from cytochrome b.^, which would have 

 the value of indicating the position of the a-absorption band and the relationship to 

 cytochrome b.^, 



As suggested by the observations of Slonimski and colleagues (Slonimski and 

 Tysarowski, C. R. Acad. Sci., Paris 246, 1111, 1958; Labeyri, Slonimski and Naslin, 

 Biochim. biophys. Acta 34, 262, 1959) and of Nygaard (Biochim. biopliys. Acta 35, 212, 

 1959 and this volume, p. 544), it is likely that there is more than one 'lactate dehydro- 

 genase of bakers' yeast', particularly if the yeast is grown under partially-anaerobic 

 conditions. The term 'lactate dehydrogenase' refers to the protein which specifically 

 activates lactate for dehydrogenation. Cytochrome ^2 's one such dehydrogenase 

 (certainly the principal L(+)-lactate dehydrogenase of aerobic bakers' yeast) and the 

 term 'cytochrome b^^ is therefore preferable to the less-specific 'lactate dehydrogenase 

 of bakers' yeast' when the flavohaemoprotein is implied. 



The Substrate Specificity of Cytochrome h.^ 



BoERi: With reference to our non-crystalline preparations of cytochrome b^, and the 

 crystalline preparation obtained by Appleby and Morton, the following points should 

 be noted. 



(a) As found by Dickens and collaborators, our preparations dehydrogenate 

 glycerate whereas the preparations of Morton and co-workers (Morton, Armstrong 

 and Appleby, this volume, p. 501) do not. 



(b) Our preparations are inert to both isomers of malate, whereas the crystalline 



