Availability of the true Schiff's bases of lysine. Chemical evaluation of the Schiff's base between lysine and lactose in milk.

Abstract

During the heat-treatment of milk, the Maillard reaction which takes place between the epsilon-amino group of lysine and lactose leads to the formation of two well-defined chemical types : the Schiff’s base in equilibrium with its aldosylamine form and the deoxyketose (Amadori product). Rat growth assays showed that the synthetic ε-N-deoxyketosyl-L-lysine was not utilized as a source of lysine and that the true Schiff’s bases resulting from the reaction of lysine with aromatic aldehydes were 100% utilized indicating that the Schiff’s base ⇄ aldosylamine are also 100% utilized. Taking the “in vitro” enzymatic method as references for the determination of available lysine, the classic acid hydrolysis and the reduction method by NaBH4 were used to evaluate lysine bound to lactose as Schiff’s base ⇄ aldosylamine or deoxyketose forms. It was demonstrated that a) the deoxyketose is the only form of unavailable lysine in milk, b) the furosine content in the acid hydrolysates multiplied by the conversion factor 3.1 gives the quantity of the deoxyketose form present, c) the reduction by NaBH4 permits one to evaluate totally the two types of binding; the difference between the values obtained by both methods corresponds to the Schiff’s base ⇄ aldosylamine form.