摘要：

Soya beans, like other legumes, contain low concentrations of the nutritionally essential sulphur amino acid, methionine. Cysteine, although not an essential amino acid because it can be synthesized from methionine, also influences the nutritional quality of soya bean products when it is only present in low levels. A low cysteine content will also aggravate a methionine deficiency. Soya bean lines deficient in 7S protein subunits have been identified. The 7S proteins contain substantially less methionine and cysteine than the 11S proteins. With the myriad of genetic null alleles for these subunits it may be possible to tailor the 7S/11S storage protein ratio and their total composition in seeds to include only those subunits with the richest sulphur amino acid composition. Cotyledon feeding experiments, using isolated soya bean cotyledons, demonstrated that addition of methionine to the culture media caused increased synthesis of both proteins and free amino acids but the mechanism by which this takes place is not clear. Biotechnological approaches to improve nutritional value of soya beans include elevated expression of genes that originate from other species which encode high-sulphur proteins. High level expression of a 2S Brazil nut albumin gene in soya bean resulted in raised methionine concentration although the Brazil nut gene is highly antigenic and therefore will not be useful in production agriculture. Modification of glycinin to increase sulphur amino acid content is possible, and these gene products are capable of normal assembly into trimers invitro although are rapidly degraded invivo by the asparaginyl endopeptidase responsible for post-translational modification of proglycinin. Solutions to the methionine deficiency may be anticipated from a combination of approaches followed in laboratories worldwide. Many of these approaches are not without difficulty but, despite this, the likelihood is that soya beans with improved nutritional quality (which may not be confined to sulphur-containing amino acids as other nutritionally essential amino acids are also valuable) will be available in the near future. It will be essential to confirm that the increased total methionine (or other amino acid) is digestible to the animal to at least the same degree as conventional cultivars.

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