MISCELLANEOUS COMPOUNDS 285 



The sugar moiety of the isothiocyanate is glucose. The isothio- 

 cyanate (R — NCS) is formed enzymatically by intra-molecular rear- 

 rangement accompanied by hberation of glucose and sulfate. The 

 enzyme, myrosinase, is relatively nonspecific for naturally occurring 

 isothiocyanates but is highly specific in that other types of glucosidic 

 linkages are not attacked. In some seeds the enzyme is located in 

 special cells and the glucoside in other cells so that the isothiocyanate 

 is produced only after the tissue is damaged. According to Kjaer 

 (1960), three different pathways for enzymatic attack upon the in 

 vivo glucosides may exist: (1) intramolecular rearrangement to isothio- 

 cyanate, (2) rearrangement to form thiocyanate, and (3) formation of 

 nitriles and elementary sulfur with no change in the carbon skeleton. 

 Lepidium sativum produces substances of Types 1 and 2, apparently 

 enzymatically, while a related species, L. ruderale forms only the 

 thiocyanate derivative. 



Representative isothiocyanates are illustrated in Table 14-3. 

 It is not unusual to find several members of a particular series 

 occurring within a related group of plants, and representatives of more 

 than one series may also occur together in a single species. Arrange- 

 ment of the isothiocyanates into homologous series is possible, and the 

 series are somewhat similar to those in which the cyanogenetic glyco- 

 sides are arranged (Chapter 10). 



Correspondence between isothiocyanates such as glucoputran- 

 jivin and cyanogenetic glycosides such as hnamarin may be dupHcated 

 by other examples: 



CH3 SCeHnOs ^^ OCeHnOs 



H C^ « >' '^-^ 



"3^ NOSO3- H3C 



glucoputranjivin linamarin 



Also, both isothiocyanates and cyanogens are modified enzymatically 

 upon damage to the tissue and this fact suggests that further relation- 

 ships exist between the two groups. Nonetheless, little or no taxo- 

 nomic overlap occurs in the distribution of cyanogenetic glycosides 

 and isothiocyanates. Functional equivalence is suggested in the two 

 groups, but little is known concerning either the function or mode of 

 biosynthesis of mustard oils. 



Mustard oils have been found to be common in only a few 

 families: Cruciferae, Capparidaceae, Moringaceae, Resedaceae, and 

 Tropaeolaceae. The compounds also occur infrequently in Caricaceae, 

 Euphorbiaceae, Limnanthaceae, Salvadoraceae, Phytolaccaceae, and 

 Plantaginaceae (Kjaer, 1960). 



It is notable that the first four families, together with the 



