Indicators indispensable amino acids

The chemical score is determined as follows. The contents of the indispensable amino adds of egg albumin (ovalbumin), which is used as a reference protein, and the food protein in question arc determined. Egg albumin is considered to be ideal and nutritionally complete The test requires several steps. The protein must be purified, hydrolyzed to the constituent amino acids, and then subjected to analysis using an amino acid analyzer. The values for each of the indispensable amino acids for ovalbumin and the test protein are then listed as shown in Table 8.14. The contents of the various amino acids in the two proteins are then compared. The amino acid in the test protein that is present in the low cst level, on a percentage basis, can be found by examining the table. This amino acid is called the limiting amino acid of the protein. The value of the percentage is the chemical score. Table S.14 indicates that the quantity of lysine in oat protein is 51% that in egg protein. Hence, the chemical score of oat protein is 51,... 

Another method of determining protein requirements is the indicator amino acid oxidation technique. This method utilizes a carbon-labeled isotope (L-[1- C]) tracer that is ingested orally, and oxidation of this labeled carbon is measured in expired breath as COa. This method is based on the assumption that if one indispensable amino acid is dehcient, all other amino acids will be oxidized until that particular indispensable amino acid is available in adequate amounts, at which point oxidation of the amino acid pool, including the tracer, will be the lowest [32]. Using the indicator amino acid technique, researchers reported that protein recommendations are as much as 30 % higher than the WHO recommendations that are based on nitrogen balance studies [33]. The indicator amino acid oxidation method may help researchers reevaluate current protein requirements [34, 35]. 

Fig. 22.2 Main limiting indispensable amino acids of cereal grains (g/16 g N). Straight lines indicate requirements for chicks dotted lines indicate requirements for growing pigs.

Site-directed mutagenesis is an indispensable technique for determining the effect of substituting a specific amino acid writh another. For example, enzyme reaction rates can be measured for both wdld type and mutant enzymes, and changes in enzyme kinetics can be monitored to assess the possible catalytic role of a given residue. The complete absence of activity in a mutant enzyme indicates that the mutated residue is essential for catalysis. 

---