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Anomalous Results Obtained in the Determination of Available Lysine in Casein Using 2,4,6-Trinitrobenzenesulfonic Acid

Dairy Products Laboratory, Eastern Utilization Research and Development Division, ARS, USDA, Washington, D.C. 20250

ABSTRACT

When acid precipitated caseins were evaluated for their available lysine using 2,4,6,-trinitrobenzenesulfonic acid, amounts greater than the total lysine content were found. Detailed investigation of several casein fractions showed that the ^s1 variants and ß casein yielded available lysine amounts consistent with total lysine. However, available lysine of the -fraction was higher than its total lysine content. This is ascribed to hexosamines which are part of the structure of this molecular species. This finding indicates that caution should be observed in using 2,4,6-trinitrobenzenesulfonic acid for the determination of available lysine in foods containing significant amounts of glycoproteins.