The metabolism of dopamine, NN-dialkylated dopamines and derivatives of the dopamine agonist 2-amino-dihydroxy-1,2,3,4-tetrahydronaphthalene (ADTN) by catechol-O-methyltransferase

Catechol-O-methyltransferase: substrate-specificity and stereoselectivity for beta-adrenoceptor agents

Isoprenaline, isoetharine, rimiterol, dobutamine and nadolol were investigated as substrates for purified pig-liver catechol-O-methyltransferase using a sensitive spectrophotometric assay. Kinetic parameters, Km and Vmax, were defined and the apparent first-order rate constant (Vmax/Km) was derived. On the basis of the apparent first-order rate constant, rimiterol was found to be a 1.5-fold and dobutamine a 5-fold better substrate for catechol-O-methyltransferase than isoprenaline; isoetharine shows no improvement over isoprenaline. Nadolol is not a substrate for catechol-O-methyltransferase. O-Methylation of isoprenaline- and noradrenaline-enantiomers was found to be stereoselective: catechol-O-methyltransferase shows selectivity towards the laevo (-) isomer with respect to the (+) form or racemic mixture. The investigation indicated stereochemical and steric determinants important in the interaction of catechol-O-methyltransferase with physiologically and clinically important beta-adrenoceptor agents.

In-vitro and in-vivo metabolism of the presynaptic dopamine agonist 3-PPP to a catecholic analogue in rats

The dopamine agonist 3-PPP and its enantiomers are hydroxylated in-vitro by rat liver microsomes to the catecholamine 3-(3,4-dihydroxyphenyl)-N-n-propylpiperidine (4-OH-3-PPP) with Km and Vmax values of about 1 microM and 2 nmol (mg protein)-1 min-1 respectively. As the catecholamine formed appears to be a good substrate for catechol-O-methyltransferase, in-vivo catecholamine formation in rats from 3-PPP was only detectable after inhibition of COMT by tropolone. The resulting brain levels of 4-OH-3-PPP, as measured by HPLC with electrochemical detection 45 min after administration, were about 350 pmol g-1 after i.p., and about 100 pmol g-1 after s.c. injection of 45 mumol kg-1 3-PPP, with no significant difference between racemic, ( + ) or (-) 3-PPP. It was estimated that these catecholamine levels represent about 1-5% of the 3-PPP levels after i.p., and about 0.2-0.5% after s.c. administration of 3-PPP. The relevance of this metabolic conversion of 3-PPP for its pharmacological profile is discussed.

Brain and serum concentrations of dopamine analogues after peripheral administration to rats

Concentrations of dopamine (DA) analogues were determined in rat brain regions and serum after peripheral administration of the drugs. The time course of the concentrations of four N,N-dialkylated DA derivatives is reported in relation to the simultaneously measured effects on DA metabolism. Maximum brain concentrations were reached at about 10 min after injection, followed by a rapid elimination of the parent compounds. O-Methylation was found to be of major importance in this early disappearance. The 3-O-methyl metabolite of N,N-dipropyl-DA (DiPr-DA) was very rapidly formed and was eliminated much more slowly than the parent compound. Inhibition of O-methylation as well as subcutaneous, instead of intraperitoneal (ip), administration resulted in higher brain and serum levels of DiPr-DA. Brain concentrations of 11 DA analogues were determined 10 min after ip injection and were compared with their octanol/water (pH 7.4) partition coefficients. Within one group of compounds with a similar metabolic profile the brain concentrations and partition coefficients showed a good correlation.