THE EPSILON-(GAMMA-GLUTAMYL)LYSINE MOIETY IN CROSS-LINKED CASEIN IS AN AVAILABLE SOURCE OF LYSINE FOR RATS

To determine bioavailability, expressed as the protein efficiency rati o (PER) and biological value (BV) in rats, of the epsilon-(gamma-gluta myl)lysine [epsilon-(gamma-Glu)Lys] moiety in crosslinked proteins, we prepared heavily crosslinked [21.5 mu mol epsilon-(gamma-Glu)-Lys/g c asein] and intermediately crosslinked [13.6 mu mol epsilon-(gamma-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 ev aluate the bioavailability of the epsilon-(gamma-Glu)Lys moiety in cro sslinked casein as the sole source of dietary protein. Rats that were fed intact casein and the two crosslinked caseins had similar growth r ates, PER, and BV, indicating that crosslinked caseins supported the g rowth of rats similarly to the intact casein. In Experiment 2, heavily crosslinked casein was added to wheat gluten-based diets in concentra tions of 20 and 40 g/kg diet to evaluate the bioavailability of lysine in the epsilon-(gamma-Glu)Lys moiety of the casein as a lysine supple ment for lysine-poor gluten. One of six diets (heavily crosslinked or intact casein diets in the two concentrations, gluten diet, or non-pro tein diet) was fed to rats for 4 wk. No significant differences in foo d 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. T hese results suggest that the epsilon-(gamma-Glu)Lys moiety in crossli nked caseins are absorbed and therefore supplement the gluten. HPLC an alysis of urine and feces of rats fed the crosslinked caseins actually confirmed that similar to 99% of the epsilon-(gamma-Glu)Lys moiety wa s absorbed in the body.