Recently a number of bacterial cytochromes have been obtained in soluble form, purified to varying degrees, and some of the chemical and physical properties studied. I t is not surprising t h a t most of t h e bacterial cytochromes which have been obtained in soluble form are those of t y p e c, since these are the most stable. Sometimes methods used for the isolation of cytochrome c from mammalian tissues were unsuccessful in work with t h e bacterial pigments; consequently several other types of extraction pro
cedure have been d e v e l o p e d .2 2 , 3 5'5 , 4·1 0 9·1 1 0
T h e first cytochrome to be isolated from bacteria was one from a
photo-7. CYTOCHROMES IN AEROBIC ELECTRON TRANSPORT 387 synthetic bacterium, Rhodospirillum rubrum.111 This cytochrome appears t o be involved in light-induced reactions in these bacteria.1 1 2 Cytochrome pig
ments have also been isolated from other photosynthetic bacteria and from anaerobic sulfate and from nitrate-reducing organisms. These bacteria all h a v e an oxidative metabolism, although the oxidant is not oxygen. T h e cytochromes of anaerobic metabolism are described in Chapter 8.
Cytochromes of type c have been isolated from Azotobacter vinelandii,22' 55 Micrococcus denitrificans,35' 54 Pseudomonas aeruginosa,63'109 Pseudomonas denitrificans,35'54 an unidentified pseudomonad,1 1 3 and from Pseudomonas fluorescens.m As discussed in Section IV, there is considerable specificity in the interaction between the c-type cytochromes and t h e cytochrome oxidases. Of the c-type cytochromes isolated, only t h a t from M. denitri
fieans was observed t o be oxidized b y mammalian cytochrome c oxidase, and this reaction appeared t o have a rather low r a t e .5 4 I n all cases where observations were made, the bacterial c-cytochromes were rapidly oxidized and reduced b y oxidases and reductases on particles prepared from t h e corresponding b a c t e r i a .2 2 , 3 5-5 4 ·6 3 T h e purified cytochromes from Azotobac
ter vinelandii were not oxidized b y particulate preparations from E. coli or Acetobacter peroxydans22 T h e preparation of cytochrome c from Pseudo
monas fluorescens contains a peroxidase which will oxidize the reduced cyto
chrome c on addition of H202.1 1 4 T h e relationship between t h e cytochrome c peroxidase and the cytochrome c oxidase of these bacteria is not clear.
Two c-type cytochromes were isolated from Azotobacter vinelandii?2 and Pseudomonas aeruginosa,109'115 and there is evidence for two in an unidenti
fied pseudomonad.1 1 3 These are reminiscent of the mixture of cytochromes c and Ci in mammalian and yeast cells. As with cytochromes c and C i , t h e wavelengths of the α-absorption peaks of the two c-type cytochromes are so close t h a t only one fused peak is observed in the mixture a t room temperature. T h u s they are recognized as separate entities only b y physical separation or b y observations of the absorption spectra a t the temperature of liquid air.2 2 I n Pseudomonas aeruginosa it is claimed t h a t electron trans
port has been observed between t h e two c-type cytochromes.1 1 5
Some of the properties of the isolated c-type bacterial cytochromes are summed u p in Table V (the cytochromes of t h e photosynthetic bacteria are excluded). I n all cases these cytochromes were shown to have absorp
tion spectra, chemical reactions, and stability similar t o mammalian cyto
chrome c and to form t h e same pyridine and cyanide hemochromogens.
T h e y t h u s have t h e same prosthetic group as mammalian cytochrome c.
T h e isoelectric points are usually quite different from t h a t of the m a m malian pigment, indicating a different protein moiety. K a m e n and T a k e d a1 1 0 showed t h e amino acid composition of t h e cytochrome c from Pseudomonas aeruginosa to be quite different from t h a t of mammalian cytochrome c.
TABLE V PROPERTIES OF SOLUBLE BACTERIAL CYTOCHROMES Cytochrome* Bacteria from which cyto chrome isolated Activity c-Type cytochrome (63, 110, 115) Pseudomonas aeruginosa Intermediate electron carrier Ci-Type cytochrome (63, 110, 115) Pseudomonas aeruginosa Intermediate electron carrier c-Type cytochrome (35, 54) Micrococcus denitrificans Intermediate electron carrier can be oxidized by mammalian cyto chrome c oxidase c-Type cytochrome (35, 54) c-Type cytochrome (114) c-Type cytochrome (113)
Pseudomonas denitrificans Pseudomonas fluorescens Unidentified pseudomonad Intermediate electron carrier Preparation of the cytochrome contains a peroxidase which oxidizes the cytochrome in the presence of H202 Intermediate electron carrier Ci-Type cytochrome (113) Unidentified pseudomonad Intermediate electron carrier Chemical and physical properties E0' = 0.25 volts at pH 7; not au- toxidizable; hemin like cyto chrome c E0' = 0.225 volts at pH 7; not au- toxidizable; hemin like cyto chrome c E0' = 0.25 volts at pH 7; isoelec tric point at acid pH; not au- toxidizable; hemin like cyto chrome c EQ = 0.32 volts at pH 7; hemin like cytochrome c EQ « 0.26 volts at pH 7; isoelec tric point at acid pH; hemin like cytochrome c EQ = about 0.20 volts; isoelec tric point at alkaline pH; slowly^ oxidized in air
Peaks in absorp tion spectrum of reduced pigment 551, 521, 416 ταμ 554, 525, 416 ταμ 550, 522, 416 τημ 551-52, 523, 417 ΤΆμ 550, 520, 415 τημ 550, 520, 416 ταμ 553 , 523, 419 ταμ
Cytochrome c4 (22, (55) Cytochrome c6 (22, 55) Cytochrome b4 (117) bi-Type cytochrome (35) bi-Type cytochrome (35)
Azotobacter vinelandii Azotobacter vinelandii Halotolerant bacteria Micrococcus denitrificans Pseudomonas denitrificans
Intermediate electron carrier; not oxidized by oxidases of mammalian tissues or E. coli or Acetobacter peroxydans Intermediate electron carrier; not oxidized by oxidases of mammalian tissues or E. coli or Acetobacter peroxydans Oxidized by nitrate; reduced by by cellular reductases in the presence of succinate or DPNH autoxidizable; does not com bine with cyanide Oxidized by nitrate; reduced by cellular reductases in the pres ence of succinate or DPNH; autoxidizable β In this column, numbers in parentheses refer to references.
Eo' = 0.32 volts at pH 7; not au toxidizable; does not combine with cyanide or carbon mon oxide; relatively stable to heat or alkali, denatured in acid; hemin like cytochrome c; iso electric point at acid pH; con tains 0.46% iron; mol. wt. around 12,000 Eo' = 0.30 at pH 7; not autoxidiza-| ble; does not combine with cy anide or carbon monoxide; rela tively stable to heat or alkali, denatured in acid; hemin like cytochrome c; isoelectric point at acid pH Preparation is probably a mix ture, the predominant pigment being a c-type cytochrome Protohemin is the prosthetic group; not reduced by ascorbate or f errocyanide; isoelectric point at alkaline pH Protohemin is the prosthetic group; not reduced by ascorbate or ferrocyanide; isoelectric point at alkaline pH
551, 522, 416 τημ (mM extinction coefficients are 23.8, 17.6, 157.2) 555, 526, 420 τημ 554, 521, 418 τημ 559, 528, 426 πΐμ 559, 528, 426 ιημ
7. CYTOCHROMES IN AEROBIC ELECTRON TRANSPORT 389
390 LUCILE SMITH
T h e redox potentials of the bacterial c-cytochromes vary considerably, some being even more oxidizing t h a n t h a t of mammalian cytochrome c.
T h e cytochrome from M. denitrificans has a potential very similar to t h e mammalian pigment, and is slowly oxidized by the mammalian oxidase.5 4
An Azotobacter cytochrome has been obtained in crystalline form.1 1 6 T h e two c-type cytochromes isolated from Azotobacter vinelandii have been given the special names cytochrome c4 and c5; they do not interact with oxidases of mammalian tissues or other species of bacteria. So far t h e c-type cytochromes isolated from other bacteria have not been given special names. If further work shows similar species specificity, a new system of nomenclature will certainly be necessary. T h u s far it seems likely t h a t t h e c-type cytochromes described here all perform similar functions.
Cytochromes of t y p e b have been isolated from Μ. denitrificans and Pseudomonas denitrificans*6 These were found to be autoxidizable and did not combine with cyanide. These cytochromes were reduced by particulate preparations from the corresponding bacteria in the presence of succinate or D P N H .
T h e cytochrome isolated from halotolerant bacteria1 1 7 and named cyto
chrome b4 appears to be a mixture of more t h a n one pigment,1 1 8 the pre
dominant one being a c-type rather t h a n a b-type cytochrome.
T h e pigment isolated from Pseudomonas aeruginosa and called cyto
chrome G B (for g r e e n - b r o w n )1 0 9 , 1 1 5 has such an unusual absorption spec
t r u m t h a t it seems to be a fraction containing more t h a n one pigment.
This fraction shows some oxidase activity for one of the c-type cytochromes from these bacteria.