CYTOCHROME - 29 



Baker's yeast represents by far the best material for study of 

 oxidation and reduction of cytochrome: 



If a shallow tube (30 mm. high) is half-filled with a suspension of baker's yeast 

 (20 per cent.) in water and the suspension is examined with a Zeiss microspectro- 

 scope, the four absorption bands may be clearly seen; but when air is rapidly 

 bubbled through the suspension the cytochrome becomes oxidised and the bands 

 disappear. If the current of air is stopped the pigment becomes reduced and the four 

 bands rapidly reappear. A similar result can be obtained by shaking a 5 c.c. yeast 

 emulsion in a test-tube and examining it with the microspectroscope. When, instead 

 of air, a current of Ng is passed through the yeast emulsion or when the latter is shaken 

 with Ng, the cytochrome remains in reduced state showing all the time its four 

 characteristic absorption bands. 



Cytochrome cannot be regarded as an alternative to haemoglobin, 

 for in muscles the two pigments are found together. Take for instance 

 the muscle of a perfused guinea-pig; it was here really that Keilin 

 started his researches. If this muscle be treated in the way which 

 has already been described, the muscle being looked at in air through 

 the microspectroscope, the spectrum of Oa-haemoglobin is seen. It 

 is true that the position of the bands is not precisely the same as 

 in the haemoglobin of guinea-pig's blood : the finer points of the placing 

 of haemoglobin bands must be left to the future for discussion. 

 Here it need only be said that the bands seen in guinea-pig's muscle 

 are precisely similar to the bands presented by the haemoglobin of 

 the larva of Gastrophilus — a parasite which is found in the stomach 

 of the horse. But to continue the experiment : shortly after the cover 

 glass is put on, the bands change abruptly. The two bands of O^- 

 haemoglobin give place to the four-banded spectrum of cytochrome. 

 The uninitiated might easily be excused for the supposition that a 

 single pigment was present which gave a two-banded spectrum when 

 oxidised and a four-banded spectrum when reduced. Or in less con- 

 centration, a two-banded spectrum when oxidised and a one-, two- or 

 three-banded spectrum when reduced. It would be futile here to hark 

 back to the polemic (2) as between MacMunn or Hoppe-Seyler, or to 

 investigate the cloud of obscurity which has enveloped the pigments 

 in mammalian muscle since their time. The simple fact, however, 

 is that all the spectral phenomena of guinea-pig's muscle may be 

 imitated, if a suitable mixture be made of sheep's blood haemoglobin 

 and yeast emulsion. When this mixture is shaken with air and rapidly 

 examined with the microspectroscope it shows only the two bands 

 of oxyhaemoglobin, while the cytochrome of the yeast, being oxidised, 

 does not show the absorption bands. Keeping the mixture under 



