Glutamine and Asparagine in Nutritional Products

Glutamine and asparagine in enteral therapeutic nutritional products, and in the milk, casein, soy, and whey protein ingredients with which they were formulated, have been estimated by an indirect approach: from published amino acid sequence and individual protein distribution data. A second set of estimates, based on experimental GLX and ASX determinations, differed by ≤ 10% from the sequence-based estimates. Further corroboration was obtained by determinations of amide nitrogen, which also differed by ≤ 10% from the sum of the estimated glutamine + asparagine. Determinations of GLX, ASX, and amide nitrogen were also used to estimate glutamine and asparagine in wheat protein ingredients and in wheat protein–fortified nutritional products. The resistance of protein-bound glutamine and asparagine to deamidation during nutritional product processing was verified. Free ammonia in protein hydrolysates was correlated with degree of hydrolysis, suggesting that glutamine content decreases as the degree of hydrolysis increases. The study indicates that, pending comparison and verification by direct enzymatic hydrolysis methods of glutamine and asparagine determination, the concentrations in common protein ingredients and in many commercial nutritional products may be reliably estimated from published data.