Legumes amino acid content

Protein contents of selected oilseeds and legume seeds, and food protein ingredients prepared by various procedures, are shown in Table I. Amino acid contents and protein efficiency ratios (PER s)... 

Table 2.13 Amino acid content of legumes, oilseeds and nuts (in g per 16 g of nitrogen).

The grain or pulse forms of legumes have a high total protein content (20-26%) and can therefore be used as a natural supplement to cereals. Pulses are normally deficient in the essential amino acids methionine and cystine but contain enough lysine, whereas cereals are deficient in lysine but contain enough methionine and cystine. 

Extraction is an essential step when analyzing solid samples. In some cases homogenization with a solvent suffices, but in others the sample must first be coimninuted. Water, solutions of acetic acid or sodium chloride, or more complex saline solutions are used as solvents. Mixtures of water and methanol or water and ethanol are also employed. The choice of solvent depends on the degree of selectivity desired in the extraction and whether the extraction yield is intended for quantitative analysis. Optimization of the extraction procedure is required in all cases, to fit the nature of the sample to be analyzed and the range of molecular weights of the peptides to be separated. For example, water has been used as the extraction solvent for cheese (33) and legumes (34). Saline solutions have been utilized to extract peptides from meat (35-38) and flour (39,40). Benedito de Barber et al. (41) examined differences in the extractability of amino acids and short peptides in various solvents (1M acetic acid, 70% ethanol, and distilled water) they concluded that extraction with 1M acetic acid yielded the maximum amino acid and peptide contents. 

Methionine and cysteine, the sulfur-containing amino acids, are related meta-bolically. Methionine can be converted, in the body, to cysteine. In other words, methionine is the source of the sulfur atom in the synthesis of cysteine in the body Methionine nutrition is of occasional concern, because legume proteins have a relatively low methionine content. Therefore, legume-based diets, including those based on beans and peas, may not result in maximum growth rates for infants or animals. 

The seed canavanine is rapidly metabolized on germination to canaline and urea [Eq. (1), Fig. 7] by the enzyme arginase which is able to catalyze the hydrolysis of both arginine and canavanine. Urea is further hydrolyzed to ammonia and CO2, the N is presumably utilized for protein amino acid biosynthesis (Rosenthal, 1970 Whiteside and Thurman, 1971). An analysis of a large number of canavanine-containing legumes showed that there was a correlation between canavanine content and urease activity (Rosenthal, 1974). There is limited evidence that canaline is further broken down to homoserine and ammonia. 

Methionine Animal proteins contain 2-4% of methionine, whereas plant proteins contain only 1-2% (average content is 1.7%). Methionine is the limiting amino acid in legumes. Methionine (and cysteine) is present only in small amounts in histones and is completely lacking in protamines. 

The pyridine (dihydropyridine) derived amino acid P-(3-hydroxy-4-pyridon-l-yl)-L-alanine, L-mimosine (2-62), is thyreotoxic (causes over-activity of the thyroid gland) to nonruminant animals. It occurs in the subfamily Mimosoideae of the legume family Fabaceae, in plants of the genera Mimosa and Leucaena. Leucaena leucocephala (syn. Mimosa glauca) is native to the tropical and subtropical Americas, known as White Leadtree, and is used as livestock fodder. The mimosine content in seeds can reach up to 5% of dry matter (see Section 10.3.2.7.1). The amino acid pyridosine arising from lysine in the Maillard reaction can also be considered a dihydropyridine derivative (see Section 4.7.5.12.3). 

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