A study on the chiral inversion of mandelic acid in humans

Mandelic acid is a chiral metabolite of the industrial pollutant styrene and is used in chemical skin peels, as a urinary antiseptic and as a component of other medicines. In humans, S-mandelic acid undergoes rapid chiral inversion to R-mandelic acid by an undefined pathway but it has been proposed to proceed via the acyl-CoA esters, S- and R-2-hydroxy-2-phenylacetyl-CoA, in an analogous pathway to that for Ibuprofen. This study investigates chiral inversion of mandelic acid using purified human recombinant enzymes known to be involved in the Ibuprofen chiral inversion pathway. Both S- and R-2-hydroxy-2-phenylacetyl-CoA were hydrolysed to mandelic acid by human acyl-CoA thioesterase-1 and -2 (ACOT1 and ACOT2), consistent with a possible role in the chiral inversion pathway. However, human α-methylacyl-CoA racemase (AMACR; P504S) was not able to catalyse exchange of the α-proton of S- and R-2-hydroxy-2-phenylacetyl-CoA, a requirement for chiral inversion. Both S- and R-2-phenylpropanoyl-CoA were epimerised by AMACR, showing that it is the presence of the hydroxy group that prevents epimerisation of R- and S-2-hydroxy-2-phenylacetyl-CoAs. The results show that it is unlikely that 2-hydroxy-2-phenylacetyl-CoA is an intermediate in the chiral inversion of mandelic acid, and that the chiral inversion of mandelic acid is via a different pathway to that of Ibuprofen and related drugs.