Metabolism of normetanephrine-H3 in rat brain--identification of conjugated 3-methoxy-4-hydrophenylglycol as the major metabolite

Abstract

Normetanephrine-H³ injected into the cisterna magna of rats is rapidly metabolized and disappears from brain with an initial half-life of about 12 min. Monoamine oxidase inhibition prevents almost completely the conversion of normetanephrine-H³ to other metabolites and markedly diminishes the rate of disappearance of radioactivity from brain ( ${\text{T}}_{\frac{1}{2}}=2·4\text{hr}$). These data show that normetanephrine is normally metabolized primarily by monoamine oxidase and that unaltered normetanephrine does not readily pass out of the brain. Free 3-methoxy-4-hydroxyphenylglycol (MHPG) and 3-methoxy-4-hydroxymandelic acid (VMA) formed from intracistemally injected normetanephrine-H³ represent only a small fraction of the radioactivity in brain. The major metabolite was identified as the sulfate conjugate of MHPG. After intracisternal administration of norepinephrine-H³, 3-methoxy-4-hydroxyphenylgIycol sulfate (MHPG-SO₄) was also found to be the major metabolite present in brain. These findings suggest that deamination, reduction, and subsequent corrugation with sulfate is the primary route of metabolism of normetanephrine in rat brain and that norepinephrine is also metabolized to this sulfate conjugate.

Fig. 1

Parglyine (75 mg/kg, i.p.) or saline (control) was injected 45 min prior to the intracisternal injection or normetanephrine-H³. Total radioactivity and normetanephrine-H³ were assayed at various times after the intracisternal injection. Each point represents the mean ± S.E.M. of 8 determinations.

Fig. 2

Normetanephrine-H³ was injected intracisternally. Animals were killed 1 hr after the injection and brains were homogenized in 0·4 N perchloric acid. Aliquots of supernatant fluid were chromatographed as described in Methods.

Fig. 3

The radioactive conjugate formed in rat brain after the intracisternal injection of normetanephrine-H³, was isolated (see text). This conjugate was incubated with “glusulase” for 1 min (un- hydrolyzed) or for 20 hr (hydrolyzed). The incubates were chromatographed and assayed for radioactivity; histograms were constructed for hydrolyzed and unhydrolyzed samples. The two resulting histograms are superimposed in this figure.

Fig. 4

Normetanephrine-H³ was injected intracisternally. Normetanephrine-H³ and metabolites were analyzed in brain at various times after injection. Each point represents the mean ± S.E.M. of 8 determinations.