In Vivo Studies on the Metabolism of the Monoterpene Pulegone in Humans Using the Metabolism of Ingestion-Correlated Amounts (MICA) Approach: Explanation for the Toxicity Differences between (S)-(−)- and (R)-(+)-Pulegone

The major in vivo metabolites of ( S )-(−)-pulegone in humans using a metabolism of ingestion-correlated amounts (MICA) experiment were newly identified as 2-(2-hydroxy-1-methylethyl)-5-methylcyclohexanone (8-hydroxymenthone, M1 ), 3-hydroxy-3-methyl-6-(1-methylethyl)cyclohexanone (1-hydroxymenthone, M2 ), 3-methyl-6-(1-methylethyl)cyclohexanol (menthol), and E -2-(2-hydroxy-1-methylethylidene)-5-methylcyclohexanone (10-hydroxypulegone, M4 ) on the basis of mass spectrometric analysis in combination with syntheses and NMR experiments. Minor metabolites were be identified as 3-methyl-6-(1-methylethyl)-2-cyclohexenone (piperitone, M5 ) and α,α,4-trimethyl-1-cyclohexene-1-methanol (3- p -menthen-8-ol, M6 ). Menthofuran was not a major metabolite of pulegone and is most probably an artifact formed during workup from known ( M4 ) and/or unknown precursors. The differences in toxicity between ( S )-(−)- and ( R )-(+)-pulegone can be explained by the strongly diminished ability for enzymatic reduction of the double bond in ( R )-(+)-pulegone. This might lead to further oxidative metabolism of 10-hydroxypulegone ( M4 ) and the formation of further currently undetected metabolites that might account for the observed hepatotoxic and pneumotoxic activity in humans. Keywords: Metabolism of ingestion correlated amounts (MICA); urinary metabolites; pulegone; menthol; piperitone; 2-(2-hydroxy-1-methylethyl)-5-methylcyclohexanone; 3-hydroxy-3-methyl-6-(1-methylethyl)cyclohexanone; 3-methyl-6-(1-methylethyl)cyclohexanol; E-2-(2-hydroxy-1-methylethylidene)-5-methylcyclohexanone; 3-methyl-6-(1-methylethyl)-2-cyclohexenone; α,α,4-trimethyl-1-cyclohexene-1-methanol