Plant-based aquaculture feeds and amino acid supplementation
Fishmeal has been the preferred protein source in aquatic feed formulations, but it is becoming less available and more expensive. Cheaper plant-based protein sources have been popularised within the aquaculture feed industry, particularly soybean- and corn-based meals. Although the crude protein content of plant-based ingredients might be comparable to that of fishmeal, such ingredients are generally less digestible and deficient in one or more essential amino acids (EAA). In particular, soybean meal-based aquafeeds are deficient in total sulfur amino acids (TSAA) and cornmeal-based aquafeeds are deficient in lysine (Lys).
Moreover, deficiency in TSAA increases with increasing levels of plant ingredients in aquafeed formulations (Goff and Gatlin III, 2004). Thus, crystalline amino acids, mainly Lys and methionine (Met), are increasingly being used to fill EAA deficiencies in plant-based aquafeeds in order to sustain protein synthesis and optimal performance.
Because Met is a precursor of cysteine (Cys), the levels of dietary Cys will affect the requirement of dietary Met. Thus, these two EAA show overlapping functions, and many studies determine the TSAA requirement (i.e., Met + Cys) instead of the Met requirement alone [National Research Council (NRC), 2011].
Although both the Met requirement alone and the TSAA requirement should be considered in feed formulation, as they vary widely among species (see Table 1; NRC, 2011), EAA requirements have generally been determined under optimal experimental conditions (i.e. temperature, oxygen supply, available food, and population density) but no information is available for EAA requirements under natural production conditions.
Methionine supplementation
Methionine can be supplemented in three different synthetic forms available on the market: L-Met (99 percent active substance), the biologically-active form of Met in vegetable or animal proteins; DL-Met (99 percent active substance), a racemic mixture of 50 percent D-Met and 50 percent L-Met, produced by chemical synthesis and used as a feed additive for decades, and hydroxymethionine (OH-Met), a hydroxyl acid with a hydroxyl group (OH) instead of an amine group (NH2) at the asymmetric carbon (See Figure 1). OH-Met is available either as a liquid product containing 88 percent of active substance and 12 percent of water or as a calcium salt.
Differences in the chemical structure lead to differences in the absorption, conversion, and utilisation of these molecules. Similar to other EAA, DL-Met is absorbed via active transport. In contrast, because OH-Met is an organic acid, it is absorbed by both passive diffusion and active transport (Martín-Venegas et al., 2007), which is Na+/H+ dependent and operated by monocarboxylate transporter 1.
Moreover, while both D- and L-Met are absorbed along the gastrointestinal tract, OH-Met absorption is completed at the end of the duodenum (Richards et al., 2005; Jendza et al., 2011). The conversion of L- and D-OH-Met to L-Met is a two-step process, each compound being converted first to ketomethylthio-butanoic acid (KMB) and then transaminated to L-Met (Vázquez-Añon et al. 2017) (See Figure 2). This conversion is driven by different enzymes present in the gastrointestinal tract, liver and kidney (Fang et al. 2010).
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The Aquaculturists
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