Ch

NO H

OH OH

CH2OH

HOOC-C-NH2

(CH2)4 lysine NH2

H3C.

furosine

COOH

COOH

pyridosine

Fig. 11.9 Compounds derived from fructosyllysine decomposition.

Fructosyllysine is unstable in the acid conditions of protein hydrolysis, producing about 30% furosine, pyridosine (a minor cyclisation product) and about 50% lysine (Fig. 11.9). Furosine was first detected in foods by Erbersdobler and Zucker (1966) and can be easily analysed by HPLC: thus furosine quantification is considered a good estimate of nutritionally unavailable lysine. Milk proteins, owing to their nutritional relevance, have been considered with particular attention. Owing to the presence of lactose, the Amadori compound in this case is lactulo-syllysine and furosine is again a useful marker of lysine unavailability. For this reason several authors have used the furosine method for determining the progress of the Maillard reaction in different foods (Chiang, 1983; Hartkopf and Erbersdobler, 1993 and 1994, Henle et al, 1995; Resmini et al, 1990). However, today very powerful analytical techniques are disclosing new possibilities, permitting, for example, the direct determination of fructosyllysine (Vinale et al, 1999) by the use of a stable isotope dilution assay performed in liquid chromatography - mass spectrometry (LC-MS). This method overcomes the problems of hydrolytic instability of the analyte and the incompleteness of the enzymatic digestion technique.

Other possible markers of lysine transformation are N-e-carboxymethyllysine (CML) and 5-hydroxymethylfurfural (HMF) (Fig. 11.10). CML was detected for the first time in milk by Buser and Erbersdobler (1986) and an oxidative mechanism was proposed for its formation (Ahmed et al, 1986). The formation of HMF in foods has been explained in two ways: via the Amadori products through eno-lisation (in the presence of amino groups) and via lactose isomerisation and degradation, known as the Lobry de Bruyn-Alberda van Ekenstein transformation (Ames, 1992). Because of this, it has recently been proposed to measure separately the HMF formed only by the acidic degradation of Amadori products and

HOH2

pyrraline

CO2H

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