The proteins of grasses

A FEW years ago Robertson [1928] suggested that since the keratin of wool contained such a high proportion of cystine (about 13 %) the carrying capacity of any country for sheep which are grown for wool production might well be determined by the capacity of its pasture plants to produce cystine. Attention thus became focused on the cystine content of grasses and other pasture plants. Now the major part of this cystine will presumably be present as a constituent of the leaf proteins, and to determine this protein-cystine it is necessary to hydrolyse the protein with strong acid or (less satisfactorily) to digest it with appropriate proteolytic enzymes. As no method for preparing these proteins has hitherto been available earlier workers have found it necessary to submit the whole grass or pasturage itself to acid hydrolysis or to enzymic digestion. This means, in effect, that the proteins in the leaf material have been treated in the presence of from three to six times their own weight of cellulose, sugars, organic acids etc., and it is not at all surprising that under these very unsatis- factory conditions the highly specific colour reaction of Sullivan for cystine was found difficult to carry out and indicated the presence of only a trace of this amino-acid Evans [1931] also, assuming the alkali-labile sulphur to represent two-thirds of the cystine-sulphur, found that the various samples of pasturage examined by him might contain from 0 07 to 0x18 % of cystine on the basis of dry matter. He pointed out however that not all the labile sulphur of proteins can correctly be ascribed to cystine and suggested that the true cystine content of grass must be less than 0.1 % of the dry matter.