GUANIDINATION OF LYSINE IN SELECTED DIETARY PROTEINS

Lysine in proteins can be converted to homoarginine by the guanidination reaction with O-methylisourea (MIU). Food proteins in which all the lysine has been converted to homoarginine can be used in nutrition studies to directly determine endogenous lysine excretion from the gut. The pH of the reaction solution is known to affect the completeness of conversion of lysine to homoarginine. Optimum pHs of 11.0, 10.7, and 10.8 were found for sodium caseinate, gelatin, and soy isolate protein, respectively, when 2% (w/v) of each protein in 0.6 M MIU was incubated at 20 °C for 144 h. As the protein concentration in 0.6 M MIU was increased above 2% (w/v), the extent of conversion declined markedly for sodium caseinate and the soy isolate but there was only a very small decline for gelatin. With gelatin, a maximum conversion of lysine to homoarginine of 95% was achieved, indicating that guanidinated gelatin is a suitable protein source for directly determining the endogenous excretion of lysine in the mammalian gut. © 1990, American Chemical Society. All rights reserved.