Origin of the Urease Inhibition of Metschnikowia pulcherrima Extracts: Comparative Assays with Synthetic Pulcherriminic Acid and Cyclo-dileucine

The objective of this work was to determine whether pulcherriminic acid was responsible for the urease inhibition activity of the extracts of the yeast Metschnikowia pulcherrima. Pulcherriminic acid was synthesized through a seven-step pathway from l-leucine, starting with the thermal cyclodimerization of l-leucine to the corresponding 2,5-diketopiperazine, followed by oxidation to the 2,5-dichloropyrazine through three consecutive steps without purification of the intermediates, oxidation to the corresponding di-N-oxide, dechlorination by nucleophilic aromatic substitution with benzyloxide, and deprotection with trifluoroacetic acid without isolation of an intermediate. The urease inhibition assay showed 57 ± 2.3% inhibition of the urease activity at 500 ppm of pulcherriminic acid, much lower than the percent inhibition obtain with the extract, in which pulcherriminic acid was not detected. The cyclic dimer of l-leucine was present in the extract, and its inhibitory capacity was also tested, showing a percent inhibition of 56.1 ± 6.11% of the urease activity at 400 ppm, again much lower than the percent inhibition of the extract. This work demonstrates that the inhibitory capacity of the extracts of the yeast M. pulcherrima is not due to either only pulcherriminic acid or only its cyclic dipeptide precursor.