Amino acid composition of seaweeds

All amino acids are presented in seaweeds (Matanjun et al., 2009). With respect to total EAAs in the FAO/WHO (1991) pattern, seaweeds (especially red and green) seem to be able to contribute to adequate levels of total EAA (Wong and Cheung, 2000). On the contrary, Matanjun et al. (2009) reported higher amounts of amino acid (AA) in green seaweeds than in red and brown. Despite this, many papers indicated that EAAs in red algae represented almost half of total AAs and it meant the ratio of EAA to AA about 0.4-0.5. The ratio of EAA to nonessential amino acids (NEAAs) was about 0.7-0.9 (Gressler et al., 2010; Norziah and Ching, 2000). These data agreed with the results of Galland-Irmouli et al. (1999) who reported that proteins of Palmaria palmata contained 26-50% of EAAs of AA, and its protein profile was close to the profile of egg protein. Wong and Cheung (2000) found EAA on the level of 42-48% in red and green seaweeds. Methionine and cysteine were detected in a high amount in red seaweeds than in green and brown (Qasim, 1991), but the value showed low amounts in red algae, less than 0.3% and 0.1%, respectively (Gressler et al., 2011). The highest EAA was phenylalanine in species belonging to three groups: red, green, and brown algae (Matanjun et al., 2009). Glycine, alanine, arginine, proline, glutamic, and aspartic acids composed together a large part of the AAs fraction, whereas AAs tyrosine, methionine, and cysteine occurred in a lower amount (Gressler et al., 2010; Norziah and Ching, 2000) on contrary to the results of Qasim (1991). However, Ortiz et al. (2006) concluded that brown alga Durvillaea antartica stood out of a high level of histidine and valine. Seaweeds have the amounts of glutamic and aspartic acids in a range from 15% to 44%.

In the case of NEAAs, glutamic and aspartic acid constituted a predominant quantity of AA. In brown seaweeds, it represented 20-44% (Fleurence, 1999a; Munda, 1977; Wong and Cheung, 2000), in green seaweeds 26-32% (Fleurence et al., 1995), and in red seaweeds 14-19% (Fujiwara-Arasaki et al., 1984), respectively. Glutamic acid (or a salt form) is intricately involved in sustaining proper function of the brain and its mental activity. Aspartic acid, in a form of energy, helps initialize two of the body's most important pathways (Krebs and urea cycles) (Braverman et al., 2003). But it is needed to consider that no author analyzed all AAs, namely, tryptophan and cysteine, consequential in differences in a sum of AAs (Gressler et al., 2011). In general, all three groups, green, brown, and red seaweeds, contain the similar amount of NEAA (Matanjun et al., 2009). Seaweeds contain also nonprotein nitrogenous materials, such as free AAs, chlorophyll, nitrate and nitrite nitrogen, ammonium ions, and nucleic acids. Only free AAs are probably in a connection with typical flavors and taste, especially glutamic and aspartic acid (Yaich et al., 2011), and also glycine and alanine (Norziah and Ching, 2000). Therefore, it could be assumed that this amount is not significant because of a very comparable value of the crude protein and AA content (Qasim, 1991).

Many methods with different analysis condition are used for the determination of AAs. These methods conclude two steps: hydrolysis of substrate, and chromatographic separation and detection of residues. Hydrolysis is the most critical part which is affected by several factors such as temperature, time, hydrolysis agent, and additives (Fountoulakis and Lahm, 1998). Weiss et al. (1998) focused on the effects of the hydrolysis method on the AA content and composition of protein with a conclusion that the conventional hydrolysis delivered more accurate data in comparison with the microwave radiation-induced hydrolysis.

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