Plastein, amino acid composition

Figure 2. A process of enzymatic protein degradation and resynthesis for producing a plastein with improved acceptability and an improved amino acid composition (11)...

Solubilization of Protein. Fish protein concentrate has high nutritional quality as determined both from its essential amino acid composition and from animal feeding experiments. Unfortunately, the concentrate is quite insoluble in water because of its denaturation by the solvent extraction method used in processing thus it contributes no functional properties to a food and must be used in bakery products primarily. A potentially useful method of solubilizing the protein is by proteolysis (9-12). As is the case with protein hydrolysates of casein and soybean protein, bitter peptides are formed during the hydrolysis. Papain and ficin produce more of these bitter peptides than does Pronase, for example (12). Pronase was found to produce a more brothy taste (13). A possible method of removing the bitter peptides is to convert the concentrated protein hydrolysate to plastein by further proteolytic enzyme action (14) to remove the bitter peptides. 

Model Experiment. Any L-amino acid ester (D-amino acid esters do not work Table III), when added to a reaction medium containing a protein hydrolysate, is generally incorporated during the resynthesis reaction under the appropriate reaction conditions. In this way it is feasible to prepare a plastein whose amino acid composition has been altered. Therefore, the plastein reaction, accompanied by new amino acid incorporation, would be expected to be much more valuable than when plastein formation is carried out in the absence of new amino acids. 

Arai et al. (69) have prepared a similar plastein from soybean protein on an enlarged scale and purified it by precipitation with 70% ethanol. Table IV shows the amino acid composition of this purified plastein in comparison with that of the soybean protein used in preparation of the protein hydrolysate. 

The purpose of the above study of incorporating lysine, methionine, and tryptophan was to improve the essential amino acid patterns of the algal, bacterial, and leaf proteins. Figure 5 compares the relative essential amino acid compositions of the three proteins and those of the plasteins in each case the FAO/WHO-suggested relative composition for the... 

Table VI. Amino Acid Compositions of Low-Phenylalanine High-Tyrosine Plasteins from Soybean and Fish Proteins (wt %)...

The plastein reaction was also investigated for the possible use of proteins in novel food applications. This was done via establishing a relationship between the microenvironmental conditions of the plastein reaction and the amino acid composition of the products. At low substrate concentration, produced by manipulation with additives, the plastein reaction was enhanced via the condensation pathway 174,75]. The plastein activity in the presence of a-chymotrypsin as catalyst increased with substrate concentration in the range of 10-30% (w/v). The substrate of this plastein reaction was a peptic hydrolysate of albumin obtained at 40°C, pH 7.0. The content of hydrophobic amino acids, in this case lie, Leu, Val, and Tyr, increased in the plastein products, while the content of Asp, Glu, Ser, and... 

Thr decreased. Interestingly, the increase in substrate concentration resulted in basically different amino acid compositions of the plastein products. 

The plastein reaction can help to improve the biological value of a protein. Figure 1.52 shows the plastein enrichment of zein with tryptophan, threonine and lysine. The amino acid composition of such a zein-plastein product is given in Table 1.41. 

Table 1.44. Amino acid composition (weight-%) of plasteins with high tyrosine and low phenylalanine contents from fish protein concentrate (FPC) and soya protein isolate (SPI)...

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