498 Discussion 



DISCUSSION 

 Cytochromes c^ and bg of Particulate Components of Plants 



By R. K. Morton (Adelaide) 



Morton: In 1953-1955 in collaboration with E. M. Martin, I investigated some of 

 the haemoproteins of plants. From silver-beet petioles and from wheat roots, 

 mitochondria and microsomal fractions were prepared, the origin and nature of 

 which were investigated by electron microscopy (Hodge, Martin and Morton, /. 

 Biochem. Cytol. 3, 61, 1957). We showed that the microsomal fraction largely 

 originated from two types of lipoprotein membranes which we designated as 'endo- 

 plasmic reticulum' and 'golgi zones' respectively. Incidentally, this was the first 

 identification of such structures in higher plants. 



When plant mitochondria were heated (to destroy other haemoprotein components), 

 and reduced with ascorbate, cytochrome c (a-band, approximately 550 m/< at room 

 temperature) was detected. However, when reduced with DPNH, and to some extent, 

 with succinate, and especially with dithionite, a band appeared close to 554 mfi (at 

 room temperature), which could be readily resolved by lovz-dispersion microspectro- 

 scopy from the cytochrome c band at 550 m// and from the cytochrome b band at 

 560-564 m/t. The band at 554 m// at first suggested to us contamination with cyto- 

 chrome /(a-band, 555 m/{ at room temperature). However, when it was detected even 

 more strongly in mitochondria from plant roots, it was recognized as a component 

 not previously described in plants and called cytochrome Cj of plant mitochondria 

 (Martin and Morton, Biochem. J. 65, 404, 1957; see also Morton, Rev. pure appl. 

 Chem. 8, 161, 1958). This nomenclature was, at that time (1958), consistent with the 

 report of cytochrome c^ as a haemoprotein of a-band at approximately 554 mn (at 

 room temperature) in liver and muscle mitochondria and in other animal tissues. At 

 that time, no function had been allocated to cytochrome c^; and, indeed, we would be 

 on very dangerous ground today if we attempted to classify cytochromes on a functional 

 basis. Moreover, it appeared undesirable to introduce a new alphabetical designation 

 for every component found in each new tissue examined. Lundegardh (Biochim. 

 biophys. Acta 11, 355, 1958) confirmed the presence of this component from spectro- 

 photometric studies of plant roots, and showed that its oxidation-reduction state 

 changed during aerobiosis and anaerobiosis. Moreover, in our studies of the eff"ect of 

 inhibitors such as antimycin A (Martin and Morton, Biochem. J. 64, 221, 1956; 

 Biochem. J. 65, 404, 1957) and later of HOQNO and amytal (Wiskich, Morton and 

 Robertson, Aiist. J. Sci. 13, 109, 1960) on plant respiration, we observed that cyto- 

 chrome Ci was partly reduced in the presence of these respiratory inhibitors. 



We therefore tentatively included this cytochrome as a component of the respiratory 

 chain of plant mitochondria (see also Wiskich, Morton and Robertson, loc. cit.). 

 This, of course, needs verification. Chance's trapped steady-state procedure (this 

 volume, p. 457) will undoubtedly be valuable for clarifying this question. 



In the microsomal fraction, at room temperature only a single oxidizable-reducible 

 haemoprotein was detected, with an a-band, when reduced, at 559 m// (at room 

 temperature). This new component we first called 'cytochrome b^' (Martin and 

 Morton, Nature, Lond. 176, 113, 1955) (not wishing to add another cytochrome name 

 to the literature) and later 'cytochrome b^ (M & M)' (Martin and Morton, 1957, loc. 

 cit.) to distinguish this pigment from the component described by Hill and Scarisbrick 

 (New Phytol. 50,98, 1951). 



Cytochrome b^ of wheat-root microsomes was reduced very rapidly by DPNH and 

 more slowly by TPNH ; no other substrate was found. The ferrocytochrome is very 

 rapidly oxidized in air -so rapidly that, by analogy with rates experienced with 

 cytochrome b-, in microsomes, it appeared possible that a specific cytochrome 63 

 oxidase was present (Martin and Morton, 1955, loc. cit.). In this connexion, we isolated 

 from silver-beet petiole a very small amount of an oxygen-combining haemoprotein 

 which appeared to have some analogy with the haemoglobins of yeast and fungi 

 described by Keilin and Tissieres (see Martin and Morton, 1957, loc. cit.) The 



