In vitro peptidase activity of rat mucosa cell fractions against glutamine-containing dipeptides

Instability and limited solubility hampers the inclusion of free glutamine in aqueous preparations intended for clinical nutrition. This drawback can be overcome by using synthetic stable and highly soluble glutamine-containing dipeptides. Rapid utilization in the target tissue is a prerequisite for their rational use. Hydrolytic capacity of rata small intestine against L-alanyl-L-glutamine and glycyl-L-glutamine was assessed by incubating mucosa cell fractions (brush border and basolateral membrane vesicles, soluble fraction) under controlled and standardized conditions. Subsequently dipeptide disappearance and the simultaneous liberation of the constituent free amino acids were monitored (liquid chromatography) enabling estimation of the Michealis constants and the maximum velocity of the reaction. The estimated Michealis constants (Eadie-Hofstee plot) for membrane-bound peptidases appeared to be lower for alanyl-glutamine that those for glycyl-glutamine (brush border membrane: 0.99 +/- 0.23 vs. 4.59 +/- 2.01 mmol/L; basolateral membrane: 1.02 +/- 0.38 vs. 4.80 +/- 0.89 mmol/L). The estimated capacity for hydrolysis, expressed per mg mixed protein, was 3- to 5-fold higher for alanyl-glutamine that for glycyl-glutamine in all preparations. The results indicate that mammalian intestinal mucosa is well equipped to enable an efficient hydrolysis of glutamine-containing dipeptides either delivered luminally or intravenously.