Evidence for the in vitro metabolism of allylisopropylacetamide to reactive intermediates. Mechanistic studies with oxygen-18.

Abstract

Metabolism of allylisopropylacetamide (AIA) (1) in microsomal preparations from phenobarbital-pretreated rats is shown to proceed by way of three cytochrome P-450-dependent pathways: (i) aliphatic (C-3') hydroxylation, (ii) allylic (C-3) hydroxylation and (iii) olefin oxidation. The latter represents the major route of biotransformation and leads ultimately to the formation of the gamma-butyrolactone 2. In order to elucidate the mechanism by which AIA is converted to this gamma-lactone, and to gain information on the nature of chemically reactive intermediates in the process, the metabolism of AIA to 2 was investigated in 18O2 or H218O and the pattern of label incorporated into the product was determined by gas chromatography/mass spectrometry (GC/MS). The results support the formation of AIA epoxide as an initial product of olefin oxidation and indicate that this species undergoes rapid intramolecular rearrangement to a protonated iminolactone which, in turn, is hydrolysed to the stable gamma-lactone. On the other hand, the 'dihydrodiol' metabolite of AIA, which would be expected to result from direct hydrolysis of AIA epoxide, was not detected in incubation products and, furthermore, the 18O labeling data specifically exclude the possibility that it served as a precursor of 2. It may be concluded, therefore, that AIA epoxide and the protonated iminolactone to which it gives rise represent reactive intermediates in the oxidation of AIA which may play a key role in the alkylation of certain cellular constituents which accompanies metabolism of AIA by liver enzymes.