The metabolism and biosynthesis of (±)-o-octopamine and (±)-o-synephrine in the rat

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Abstract
The metabolism of (±)-o-octopamine and (±)-o-synephrine by rats was studied quantitatively by a gas chromatography-mass spectrometry-selected ion monitoring (g.c.-m.s.-s.i.m.) method using deuterated internal standards. When o-octopamine was injected intraperitoneally into rats four metabolites were excreted in the urine: (i) unconjugated o-hydroxymandelic acid (OHMA) (16%), (ii) unconjugated o-hydroxyphenylglycol (OHPG) (4·5%), (iii) an acid-hydrolysable conjugate of OHPG (28%) and (iv) unconjugated o-octopamine (10%). When o-synephrine benzoate was similarly administered six metabolites were excreted in urine: (i) unconjugated OHMA (13·5%), (ii) unconjugated OHPG (3·3%), (iii) an acid-hydrolysable conjugate of OHPG (15·6%), (iv) unconjugated o-synephrine (10%), (v) an acid-hydrolysable conjugate of o-synephrine (8·5%) and (vi) unconjugated o-octopamine (0·3%). Adult rats normally excreted OHMA (1·0 μg day−1) but OHPG, o-octopamine and o-synephrine could not be detected in urine. After the administration of a monoamine oxidase inhibitor, unconjugated o-octopamine (0·3 μg day−1) was excreted in urine but OHPG and o-synephrine could not be detected. o-Tyramine given to rats afforded urinary o-octopamine (75 ng day−1) and this was increased 10-rold upon co-administration of a monoamine oxidase inhibitor and o-tyramine.