Racemization kinetics of free and protein-bound lysinoalanine (LAL) in strong acid media. Isomeric composition of bound LAL in processed proteins

Abstract

The racemization kinetics of free and protein-bound lysinoalanine (LAL) in deuterated hydrochloric acid was determined by measuring the deuterium incorporation in LL- and LD-LAL diastereomers by GC/MS, and a method was developed to determine LAL isomeric composition in processed proteins. The method includes a limited acid hydrolysis of the samples in 6 N DCl, thus avoiding the complete racemization of LAL residues, and the measurement of unlabeled and labeled LAL isomer distributions. A range of heated or alkali-treated proteins of plant or animal origin were analyzed. All samples contained LL- and LD-LAL in similar amounts with, however, a significant stereoselectivity in favor of LD-LAL in samples subjected to moderate processing conditions (low temperature, short time, or low alkaline pH). A minimum relative concentration of ca. 40% was found for LL-LAL in the least-treated proteins. Computer modeling indicated a possible difference in the heats of formation between LL- and LD-LAL isomers which could explain the observed asymmetric distributions. The presence of both LAL isomers in similar proportions in all proteins indicates that the postulated two-step mechanism for LAL formation applies for both cyst(e)ine and serine-phosphate as precursors.