110 THE FUNCTIONS OF 



the following arguments are advanced. First, Hopkins and 

 others have shown that such compounds appear to be present in 

 all living cells and may control oxidation and reduction processes. 

 Secondly, the oxidation of cysteine to cystine is catalysed by 

 metals. 1 Thirdly, Giroud and Bulliard have shown that zinc 

 has a specific effect in stabilizing the nitroprusside colour 

 reaction of the sulphydryl (SH) group. 2 Reed and Dufrenoy 

 also refer to Maze's finding that roots of maize grown in a culture 

 solution deficient in, though not completely free from, zinc, 

 contain sulphides in the ash, indicating that the sulphur meta- 

 bolism of the plant was adversely affected. 



Chandler (1937) has pointed out that zinc deficiency has its 

 most serious effects in plants where carbohydrates have accumu- 

 lated, and he suggests that zinc deficiency brings about in- 

 hibition of some process of carbohydrate transformation. This, 

 as Reed (1938) suggests, may mean that zinc catalyses oxidation 

 processes which in its absence may run the other way. 



Following on their earlier work, Reed and Dufrenoy (1942) 

 have made a cytological study of catechol aggregates which 

 arise in the vacuoles as a result of zinc deficiency. They consider 

 that they form by a process which they call coacervation, in 

 which disperse phase particles of the colloidal system con- 

 stituting the vacuole become aggregated into spherical masses. 

 This process is regarded as something more than a separation 

 of phases, as the aggregates become surrounded by a precipita- 

 tion membrane composed of orientated molecules of a phospho- 

 lipoid. Tests for oxidase show (Dufrenoy and Reed, 1942) that 

 these aggregates are not only centres of catechol derivatives, 

 but also for catechol oxidase activity. 



1 It is stated, however (cf. Meldrum, 1934), that the most active 

 metals are iron, copper and manganese. 



2 This test, due to Morner, consists in adding a 5 or 10 per cent solu- 

 tion of sodium nitroprusside rendered alkaline with ammonia to the 

 liquid to be tested, and shaking. A violet colour, which soon fades, is 

 produced if a cysteine peptide such as glutathione is present. Some other 

 substances, including creatinine and acetone, give somewhat similar 

 colours. According to Giroud and Bulliard the addition of salts of zinc 

 gives a red colour much more stable than the violet colour of the reaction 

 as usually produced. It appears to be quite specific for the sulphydryl 

 group and is not given by either creatinine or acetone. 



