Identification of ninhydrin-positive caprolactam metabolites in the rat.

Caprolactam (2-oxohexamethylenimine) is metabolized in the rat partly to 4-hydroxycaprolactam or the corresponding free acid. The 4-hydroxy metabolite rearranges spontaneously in acidic aqueous medium to an equilibrium mixture of which 6-amino-gamma-caprolactone is the major component and 6-amino-4-hydroxyhexanoic acid is a minor component. Approximately 16% of the caprolactam ingested daily by rats fed a diet containing 3% caprolactam was excreted as the 4-hydroxy metabolite and a small amount was excreted as the non-hydroxylated acid, 6-aminohexanoic acid. The metabolites were isolated by ion-exchange chromatography and identified by gas chromatography-mass spectroscopy and infra-red and nuclear magnetic resonance spectroscopy.