Comparison of the stability of d -phenylglycine- l- dopa and l -Gly- l -Phe in rat intestinal mucosa suspension. Each point represents mean ± SE. of 3 experiments. A methanolic solution (100 μl) of the test compound (1 mg/ml) was diluted with an isotonic mannitol buffer solution (pH 6.5, 2.4 ml) as the stock solution. This stock solution (1 ml) was mixed with the freshly prepared mucosal suspension (1 ml). The mixture was incubated in a water bath at 37°C and subjected to sampling at intervals between zero to 90 min of incubation. Each sampled solution (200 μl) was denatured with 0.8 ml of MeOH and centrifuged at 6,600 g for 5 min. Each of the supernatant (20 - 100 μl) was subjected to HPLC assay.