*3.4. WY Peptide Inhibited the Activity of MAO*

We evaluated the effect of WY dipeptide and tryptophan on MAO-B activity. Tyrosine and YW dipeptide were not tested in this assay because the compounds could not be dissolved in the assay buffer. Treatment with 1 mM WY dipeptide decreased MAO-B activity by 48 ± 1.95% compared to that of the control treatment. By contrast, treatment with 1 mM tryptophan did not inhibit MAO-B activity (Figure 4A). These results suggested that the WY dipeptide increased the dopamine content in the hippocampus by inhibiting MAO-B activity and that its chemical structure (Figure 4B) was important to inhibit the MAO-B activity.

**Figure 4.** The inhibition of monoamine oxidase by the WY dipeptide and tryptophan and the chemical structure of the WY dipeptide. (**A**) The activity of monoamine oxidase B (MAO-B) was tested in the presence of 1 mM WY dipeptide or tryptophan (W). Data represent the mean ± SEM of triplicate per sample. The *p* values shown were calculated using the Dunnett's test. \*\* *p* < 0.01. (**B**) The chemical structure of the WY dipeptide.
