564 Discussion 



preparations of Morton and colleagues are inhibited slightly by L-malate and strongly 

 by D-malate (Morton et al., this volume, p. 501). 



It should be noted that both our preparation, and that of the Australian workers, 

 fail to dehydrogenate reduced triphosphopyridine nucleotide (TPNH), whereas this is 

 dehydrogenated by the preparation of Okunuki's school (Horio, Yamashita, Yaman- 

 aka, Nozaki and Okunuki, this volume, p. 552). 



With reference to observations of Nygaard, in my experience the cytochrome 6, acts 

 specifically on L(+)-lactate. Another enzyme of the yeast, present in anaerobic yeast, 

 was shown by Slonimsky and his collaborators to act on d(— )-lactic acid. It is not a 

 flavo-cytochrome, but a flavoprotein. Work is going on in our laboratory on this 

 enzyme which we prefer to define, with a terminology different from that of Slonimsky, 

 as a D-hydroxyacid dehydrogenase. 

 Armstrong: The substrate specificity of crystalline cytochrome Zjj was shown by Appleby 

 and Morton (Biochem. J. 73, 539, 1959) to include glycolate and a-hydroxybutyrate. 

 The further studies of Armstrong and Morton (see Table 5, this volume, p. 512) show 

 that a number of a-hydroxymonocarboxylic acids are substrates. Additional 

 hydroxycarboxylic and certain other groups probably interfere with attachment if 

 sufficiently close to the a-hydroxyl group. In D-malate, the hydroxyl group has the 

 same configuration as in L-lactate if viewed from the /9-carboxyl end. Thus it is not 

 surprising that D-malate is a much more powerful inhibitor of the reaction with 

 L-lactate than is L-malate. One might expect that certain esters of malate might be 

 substrates of cytochrome b.,. It would appear from the pH optima reported by Horio, 

 Yamanaka, Yamashita, Nozaki and Okunuki (this volume, p. 552) that malate and 

 TPNH are dehydrogenated by enzymes other than cytochrome b., in the preparation 

 used by these workers. 



On the Cytochrome b Components in the Respiratory Chain in Yeast 



Chance: The following relates to the possible function of cytochrome 62 in the respiratory 

 chain in yeast. In 1954 we were actively considering the possibility that the b com- 

 ponent of yeast cells could be identified with components ranging from cytochrome b.^ 



Fig. 1. Spectrum of reduced cytochrome ^2 compared with 

 that of cytochrome c at liquid air temperatures (RE-129a). 



540 560 580 



to peroxidase (Chance, 1954: In Mechanisms of Enzyme Action: Johns Hopkins Press, 

 Baltimore, p. 399). We have, however, many new data to offer on this point, particu- 

 larly low temperature data on cytochrome b., (Chance, Klingenberg and Boeri, Fed. 

 Proc. 15, 231, 1956). It has been found that the low temperature spectrum of reduced 



