The ε-(γ-Glutamyl)lysine Moiety in Crosslinked Casein Is an Available Source of Lysine for Rats

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Abstract
To determine bioavailability, expressed as the protein efficiency ratio (PER) and biological value (BV) in rats, of the ε-(γ-glutamyl)lysine [ε-(γ-Glu)Lys] moiety in crosslinked proteins, we prepared heavily crosslinked [21.5 µmol ε-(γ-Glu)Lys/g casein] and intermediately crosslinked [13.6 µmol ε-(γ-Glu)Lys/g casein] casein, using microbial transglutaminase. In Experiment 1, rats were assigned to one of four diets (heavily or intermediately crosslinked caseins, intact casein or non-protein diet) for 4 wk to evaluate the bioavailability of the ε-(γ-Glu)Lys moiety in crosslinked casein as the sole source of dietary protein. Rats that were fed intact casein and the two crosslinked caseins had similar growth rates, PER, and BV, indicating that crosslinked caseins supported the growth of rats similarly to the intact casein. In Experiment 2, heavily crosslinked casein was added to wheat gluten-based diets in concentrations of 20 and 40 g/kg diet to evaluate the bioavailability of lysine in the ε-(γ-Glu)Lys moiety of the casein as a lysine supplement for lysine-poor gluten. One of six diets (heavily crosslinked or intact casein diets in the two concentrations, gluten diet, or non-protein diet) was fed to rats for 4 wk. No significant differences in food intake, body weight gain, PER or BV were observed among rats fed the intact or crosslinked casein diets at either 2 or 4 g/100 g casein. These results suggest that the ε-(γ-Glu)Lys moiety in crosslinked caseins are absorbed and therefore supplement the gluten. HPLC analysis of urine and feces of rats fed the crosslinked caseins actually confirmed that ∼99% of the ε-(γ-Glu)Lys moiety was absorbed in the body.