Biochemical studies with 1-(epsilon-n-lysyl)-1-deoxy-D-fructose

Abstract

An investigation has been made of the biochemical properties of 1-(epsilon- N-lysyl)-1-deoxy-D-fructose (fructose-lysine) a derivative which can probably arise from dried milk powder by the action of intestinal enzymes. This compound was synthesised chemically, purified by column chromatography and subjected to a number of analytical procedures including electrometric titration, chromatography and electrophoresis and 13C- and proton-nuclear magnetic resonance, all of which confirmed the authenticity and purity of the preparation. The molecule was unstable in both acid and alkali and at room temperature.

Simple fusions of sugars and lysine (HCl) were studied and methods devised for the preparation of 1-(epsilon-N[U-14C]lysyl)-1-deoxy- D-fructose and 1-(epsilon-N-lysyl)-1-deoxy-[U- 14C] D-fructose. A sample of 1-(epsilon- N-lysyl)-1-deoxy-D-lactulose (lactose-lysine), prepared by heating lactose with lysine HCl, was also obtained by a fusion procedure. This compound is also probably produced from dried milk powder in the alimentary tract.

Growth curves of E. coli B have been studied in the presence and absence of fructose-lysine in the medium. The bacterium was found to be impermeable to fructose-lysine but this compound was able to support colonial growth after the induction of an extracellular cell-bound enzyme(s) capable of the breakdown of fructose-lysine. During growth significant amounts of 14CO₂ were produced from fructose-lysine labelled with 14C in the fructose moiety showing that combined hexose was utilisable as a carbon source. The products of the breakdown of radioactive fructose-lysine were examined by chromatographic and electrophoretic techniques. Lysine was identified in the culture media and this suggested that hydrolysis of the C-N bond linking the amino acid to the sugar occurred. A stable compound derived from labelled fructose-lysine and produced apparently without cleavage of the C-N bond was also detected but not fully characterised. The possible existence of other intermediates has been postulated.

Rat liver slices, homogenates and mitochondrial preparations were shown to be capable of slowly metabolising fructose-lysine as judged by 14CO₂ formation from fructose-lysines labelled in either the fructose or the lysine moieties with 14C and as shown by the appearance of degradation products in the reaction mixtures. The sugar-amino acid C-N bond was not thought to be cleaved by liver enzymes.

The possible role of fructose-lysine as an enzyme inhibitor was examined using enzyme systems in vitro and the molecule was observed to serve as a weak inhibitor of the amino acid-metabolising enzymes, lysine decarboxylase and L-amino acid oxidase, and of some glycosidases. Some preliminary kinetic studies suggest that inhibition was of the competitive type. Fructose-lysine served as a substrate for oxidation by L-amino acid oxidase Lactose-lysine was hydrolysed by beta-galactosidase to fructose-lysine and galactose and also acted as an inhibitor of lactose hydrolysis by this enzyme.

The fate of the sugar amino derivatives from milk powder is discussed in relation to their possible effects on digestion and general metabolism of infants.