296 MISCELLANEOUS NITROGEN AND SULFUR COIVlPOUNDS 



Aporphine alkaloids are presumed to be formed from two molecules of tyrosine, and 

 aristolochic acid might be formed by an oxidation of such an alkaloid (66). Peroxidase 

 catalyzes the oxidation of amines to nitroso compounds (67) which might be oxidized fur- 

 ther to nitro derivatives. 



Since these compounds are so specialized, the original papers should be consulted 

 for details of isolation and identification. 



HYDROXYLAMINE DERIVATIVES 



Hydroxylamine itself has not been isolated from higher plants although it is a postu- 

 lated intermediate in nitrate reduction (Chapter 10) and possibly in formation of the mus- 

 tard oil glycosides (q. v. ). A small group of compounds so far restricted to such grasses 

 as maize (Zea mays), rye {Lolium spp. ), and wheat (Triticum spp. ) can be regarded as 

 complex hydroxamates which occur as glucosides (18). Upon hydrolysis the aglycones 

 tend to rearrange forming oxazoles (68, 69): 



N=C=0 



UREA AND RELATED COMPOUNDS 



In the older literature urea was often reported to be present in higher plants, but 

 the analytical methods used were not beyond reproach and many workers questioned such 

 reports. More recently it has been found that the ornithine cycle functions in plants 

 (Chapter 10), and the occurrence of free urea in some plant tissues has been well-estab- 

 lished. Urea may also arise from purine breakdown via allantoin and allantoic acid (Chap- 

 ter 11). Other compounds which can be regarded as related to urea have also been iso- 

 lated from plants. Some structures are given below with their occurrence. The whole 

 area of urea, ureide and guanidine metabolism in plants has been reviewed (70). 



H 



/\_ 



CH2 C— hydantoin 



I ' I 



0=C NH 



H5NCNHCHoCH==C. 

 2 II ^ \ 



(Beta vulgaris) 



NH CH 



galegine 



3 (Galego officinalis) 



H2NCNH(CH2)4NH2 



M|j agmatine 



(Ricinus communis) 



