Protein of yam tubers (Dioscorea rotundata)
The major proteins of the yam tuber, which were identified as storage proteins by virtue of their abundance (c. 85% of the total protein content) , amino acid composition (high in amide content), and location within the tuber, were isolated by ion exchange chromatography and characterised using in particular, polyacrylamide gel techniques. They consist principally of one size of sub-unit, molecular weight 31,000, N-terminal amino acid glutamine/glutamic acid, of which there are a number of charge isomers; these in general contain one intra-chain disulphide bond. The sub-units associated into polymers depending on the protein concentration, pH and ionic strength of the milieu, and therefore a value for the molecular weight of the native protein(s) is not given. The storage proteins are not glycoproteins, and do not exhibit lectin activity. They are intracellularly located as protein "aggregates" within both cytoplasmic vesicles and cellular protein vacuoles. Their characteristics and other aspects of their biology are compared with those of the storage proteins of the potato tuber. The first nutritionally limiting amino acids of the tuber protein are the sulphur-containing amino acids although tryptophan was not detemined. Comparative amino acid data from several sources, together with the fact that the sub-unit composition of genetically different tubers did not vary greatly, suggests that the potential for breeding for improved protein quality is limited. The protein content of yam tubers can be adequately evaluated by the Kjeldahl technique, since the non-protein nitrogen content of genetically different tubers grown under different environmental conditions was found to be relatively constant.