O-methylation and decarboxylation of alpha-methyldopa in brain and spinal cord: depletion of S-adenosylmethionine and accumulation of metabolites in catecholaminergic neurones

Inhibition of the activity of sympathetic preganglionic neurones and neurones activated by visceral afferents, by alpha-methylnoradrenaline and endogenous catecholamines

The responses of electrophysiologically identified sympathetic preganglionic neurones (SPGN) and neurones activated by visceral afferents (VA) to iontophoretic application of: (1) the intraneuronal metabolites of alpha-methyl-DOPA (alpha-MD): alpha-methylnoradrenaline (alpha-MNA) and alpha-methyldopamine (alpha-MDA); (2) noradrenaline (NA) and adrenaline (Ad); and (3) N-methyl-beta-hydroxy-phenylethylamine (NMPEA), were tested in the upper and lower segments of the thoracic spinal cord of the cat. alpha-Methylnoradrenaline, NA and Ad had an inhibitory action on the majority of spontaneously firing neurones. Inhibition of the activity of preganglionic neurones and neurones activated by visceral afferents induced by alpha-MNA was usually equal to the effect of NA, but in some neurones alpha-MNA was more potent. The transport numbers of both amines are similar. The alpha adrenoceptor antagonists, thymoxamine, piperoxan and yohimbine, antagonized the inhibitory effects of alpha-MNA in spontaneously-active preganglionic neurones and neurones activated by visceral afferents. Piperoxan antagonized also the inhibitory effects of NA. The inhibitory effect of alpha-MDA was weaker and that of NMPEA was much weaker than that of alpha-MNA and NA. The inhibitory effects of alpha-MNA and NA in the cerveau isolé preparation resembled those observed in anaesthetized animals. In reserpinized cats, with catecholamine levels in brain stem and spinal cord reduced by 98-99%, the inhibitory effects of alpha-MNA were preserved. It is postulated that alpha-MNA modulates the activity of preganglionic neurones and that this action, leading to a decrease in sympathetic output and mediated by alpha 1 and alpha 2 adrenoceptors, could be the most important factor in the antihypertensive effect of alpha-MD. The inhibition of the activity of neurones activated by visceral afferents by alpha-MNA indicates that alpha-MD may also attenuate the response of the central nervous system to the input from the heart and other organs.

Substrate stereospecificity and selectivity of catechol-O-methyltransferase for DOPA, DOPA derivatives and alpha-substituted catecholamines

The substrate specificity of highly purified pig liver catechol-O-methyltransferase has been investigated kinetically. This enzyme shows stereospecificity towards the naturally occurring L-isomer of 3,4-dihydroxyphenylalanine (DOPA) which has a higher affinity and maximal velocity as a substrate than the D-form. We have confirmed the implication of the in vivo study of Ito et al. [1], that methylation of 5-S-L-cysteinyl-L-DOPA is catalysed extremely slowly by catechol-O-methyltransferase, despite the comparatively high affinity of the enzyme for the substrate. Salbutamol is not a substrate for the enzyme and DL-threo-3,4-dihydroxyphenylserine (DOPS) is such a poor substrate that accurate kinetic analysis proved impossible. Alpha-substitution of DOPA, noradrenaline and isoprenaline causes a decrease in the affinity of catechol-O-methyltransferase for these compounds. However, the "suicide' inhibitors of aromatic-L-amino acid decarboxylase (DOPA decarboxylase), fluoro- and difluoro-alpha-methyl DOPA are more superior catechol-O-methyltransferase substrates than alpha-methyl DOPA, presumably because the electron-withdrawing effect of the presence of fluorine in their structure overcomes the steric influence of the alpha-methyl group. A DOPA decarboxylase inhibitor in clinical use, benserazide, is, however, a much superior catechol-O-methyltransferase substrate and may have the therapeutic advantage of decreasing methylation of L-DOPA [2]. Alpha-Methyl dopamine has a lower Km and higher Vmax than the parent compound.

Mechanism of action of methyldopa in the rat. Role of 3-O-methylated metabolites

We studied the effect of 3-O-methyl-methyldopa (OMMD), the 3-O-methylated metabolite of the antihypertensive drug methyldopa (alpha-methyldopa, AMD), on blood pressure in the spontaneously hypertensive rat. OMMD lowered blood pressure in a dose-related manner when given orally or intraperitoneally. Its action lasted longer than that of AMD, and daily oral administration produced a cumulative fall in blood pressure. Oral and intraperitoneal OMMD produced similar reductions of blood pressure and similar tissue OMMD levels. After intraperitoneal injection of different doses, levels of OMMD measured in brain, spinal cord, and plasma correlated with the magnitude of the antihypertensive effect. No AMD was detected in tissues after either route of administration, which suggests that the antihypertensive effect was not based on demethylation of OMMD to AMD. Peripheral inhibition of the enzyme, aromatic amino acid decarboxylase (AAAD), failed to suppress OMMD's effect on blood pressure; in contrast, central inhibition of AAAD did decrease OMMD's antihypertensive effect. These observations suggest that 3-O-methylated metabolites may participate in the antihypertensive effect of AMD.

Diet and uptake of aldomet by the brain: competition with natural large neutral amino acids

The rise in levels of aldomet in the brains of rats after an injection of the alpha-methylated amino acid was depressed when large neutral amino acids, but not acidic amino acids, were coadministered with the drug. This result suggests that aldomet is transported into brain by the carrier for natural large neutral amino acids. The prior ingestion of a carbohydrate meal, which lowers levels of neural amino acids in the serum, enhanced the uptake of aldomet into brain; the consumption of a protein-containing meal inhibited the subsequent uptake of aldomet into the brain. Antecedent diet can thus affect the availability of aldomet to the central nervous system; the mechanism of this effect probably involves the blood-brain barrier uptake system for large neutral amino acids.