O-Methylisourea Can React with the α-Amino Group of Lysine: Implications for the Analysis of Reactive Lysine

The specificity of O-methylisourea (OMIU) to bind to the ε-amino group of Lys, an important supposition for the OMIU-reactive Lys analysis of foods, feeds, ingredients, and digesta, was investigated. Crystalline l-Lys incubated under standard conditions with OMIU resulted in low homoarginine recoveries. The reaction of OMIU with the α-amino group of Lys was confirmed by MS analysis, with double derivatized Lys being identified. None of the changes in reaction conditions (OMIU pH, OMIU to Lys ratio, and reaction time) with crystalline l-Lys resulted in 100% recovery of homoarginine. The average free Lys content in ileal digesta of growing pigs and broilers was found to be 13% of total Lys, which could result in a significant underestimation of the reactive Lys content. The reaction of OMIU with α-amino groups may necessitate analysis of free Lys to accurately quantify reactive lysine in samples containing a large proportion of Lys with a free α-amino group.