468 C. F, Strittmatter 



incomplete reduction of the substance with DPNH and by apparent shght 

 spectral asymmetry is not resolved as yet. 



Observations in Invertebrates 



While there have been many observations in invertebrate tissues of haemo- 

 chromes absorbing maximally in the range 555-560 m/^, the most 

 exphcit demonstration of a microsomal cytochrome-hke component in 

 invertebrates has come from studies of cytochrome components in the 

 Cecropia silkworm. Many tissues of the larval form, most notably the midgut 

 wall, showed on reduction a broad indivisible absorption band over the 

 range 551-562 m//, with maximum at about the centre, which was ascribed 

 to a cytochrome x by Sanborn and Williams (1950). In further studies with 

 larval midgut homogenates, Pappenheimer and Williams (1954) concluded 

 from the absorption spectra and such properties as cyanide-insensitive autox- 

 idizability that this Cecropia pigment was a Z?-type cytochrome and called 

 the material 'cytochrome b^. They considered it to be closely related to 

 microsomal cytochrome, but also tentatively identified the pigment with 

 cytochrome e (now termed q). The insect cytochrome b^ has not been 

 purified and its precise relationship with the microsomal cytochrome is not 

 established, but the analogy between the two pigments has recently been 

 extended by observations on intracellular distribution: in both the larval 

 midgut mucosa and the wing epithelium of the developing adult Cecropia, the 

 cytochrome b^ was localized in the microsome fraction while cytochromes b, 

 c, a and a^ were concentrated in the mitochondria (Shappirio and Williams, 

 1957). 



Plants 



An apparently analogous haemoprotein in certain plant tissues is cyto- 

 chrome ^3, first described by Hill and Scarisbrick (1951). In studies of a 

 variety of non-photosynthetic plant tissues, Martin and Morton (1955, 1957) 

 subsequently localized cytochrome b^ in microsomal fractions. Similar to 

 the mammalian microsomal cytochrome, cytochrome />;} shows absorption 

 bands in the reduced state at about 558-560 m/n, 525-529 m/n and 425 m/u, 

 is apparently autoxidizable and does not combine with carbon monoxide. 



Micro-organ isms 



Other than having the general characteristics of a Z?-type cytochrome, 

 cytochrome b^ of various bacteria shows no particular relationship to the 

 microsomal cytochrome, but preparations of cytochrome bj from yeast 

 resemble microsomal cytochrome closely in certain haemoprotein properties, 

 including the position of the haemochrome absorption bands, the reduction 

 of the haem moiety by specific enzyme-linked flavin, and its rapid reoxidation 

 by cytochrome c or ferricyanide. However, the nature and definition of the 



