Dopamine monooxygenase

Analogous side-chain oxidations occur in various biosynthetic pathways. The neurotransmitter norepinephrine, for instance, is biosynthesized from dopamine by a benzylic hydroxylation reaction. The process is catalyzed by the copper-containing enzyme dopamine /3-monooxygenase and occurs by a radical mechanism. A copper-oxygen species in the enzyme first abstracts the pro-R benzylic hydrogen to give a radical, and a hydroxyl is then transferred from copper to carbon. 

It has been suggested that the activation of dopamine 3-monooxygenase by ascorbate was due to the formation of OJ by reduction of O, as the enzyme was activated by O J and inhibited by BESOD This inhibition of the enzyme could not be confirmed Dopamine P-monooxygenase could, moreover, be activated... 

K. Lerch, Copper Monooxygenases Tyrosinase and Dopamine )3-Monooxygenase (R), Metal Ions Biol. Syst. 13 (1981), 143-186. 

In the experiments with galactose oxidase and dopamine -monooxygenase very low enzyme concentrations have often been used. The addition of an inert protein might help to approximate the natural conditions by providing a protection against surface denaturation and against radicals, and by regulating the trace metal concentrations. 

Fig. 4.18 Hydroxylation of dopamine by the non-coupled binuclear copper center in dopamine-/ -monooxygenase.

The first successful observation and characterization of the ascorbate free radical was carried out with ESR (14,15). A 1.7-G ESR doublet was reported and it was correctly concluded that the observed spectrum represented the anionic form (A ) of the radical. These measurements (14,15) showed that the enzyme-generated radical (horseradish peroxidase-hydrogen peroxide-ascorbate) was present as a free radical and decayed by second-order kinetics (see Figure 2). Recent experiments (16,17) have shown that ascorbate oxidase and dopamine-monooxygenase also generate unbound ascorbate radicals. 

Central Nervous System. Dopamine monooxygenase (DMO) is an enzyme that requires copper, as a cofactor and uses ascorbate as an electron donor. This enzyme catalyzes the conversion of dopamine to norepinephrine, the important neurotransmitter. There are soluble and membrane-bound forms of the enzyme, the latter being found in the chromaffin granules of the adrenal cortex. Monoamine oxidase, one of the numerous amine oxidases, is a copper-containing enzyme that catalyzes the degradation of serotonin in the brain and is also involved in the metabolism of the catecholamines. 

Type 2 Copper Proteins Overview Cu, Zn Superoxide Dismutase Dopamine-/ -Monooxygenase (D/3M) and Peptidyiglycine a-Amidating Monooxygenase (PAM)... 

Second, dopamine-/ -monooxygenase (D/ M) and peptidylglycine a-hydroxylating monooxygen-ase (PHM), respectively, catalyze the reactions in Equations (3) and... 

Dopamine / -monooxygenase (D/3M, EC 1.14.17.1) is an enzyme found in mammalian brains that catalyzes the aerobic hydroxylation of dopamine to norepinephrine (Equation (3))." Ascorbate is the physiological electron donor to the enzyme, so that two equivalents of ascorbate are... 

Lerch, K. Copper monooxygenases, tyrosinases and dopamine-monooxygenase, in Metal Ions in Biological Systems. Copper Proteins, Siegel, H., Ed., Dekker, New York, 1981, p. 143. 

The transformations described above constitute models for several oxidations (see Figure 10) catalyzed by copper-dependent mono- and dioxygenases [2,3]. Reactions (18) and (19), for example, are models for trypophan dioxygenase (EC 1.13.11.11) and quercetin dioxygenase (EC 1.13.11.24), respectively. Other examples include dopamine monooxygenase (EC 1.14.17.1) and tyrosinase (EC 1.14.18.1). 

Because LCEC had its initial impact in neurochemical analysis, it is not, surprising that many of the early enzyme-linked electrochemical methods are of neurologically important enzymes. Many of the enzymes involved in catecholamine metabolism have been determined by electrochemical means. Phenylalanine hydroxylase activity has been determined by el trochemicaUy monitoring the conversion of tetrahydro-biopterin to dihydrobiopterin Another monooxygenase, tyrosine hydroxylase, has been determined by detecting the DOPA produced by the enzymatic reaction Formation of DOPA has also been monitored electrochemically to determine the activity of L-aromatic amino acid decarboxylase Other enzymes involved in catecholamine metabolism which have been determined electrochemically include dopamine-p-hydroxylase phenylethanolamine-N-methyltransferase and catechol-O-methyltransferase . Electrochemical detection of DOPA has also been used to determine the activity of y-glutamyltranspeptidase The cytochrome P-450 enzyme system has been studied by observing the conversion of benzene to phenol and subsequently to hydroquinone and catechol... 

Cysteine string protein (CSP) Cytochrome b561 Peripheral membrane protein that is paimitoylated on >10 cysteines. May have a role in Ca2+ sensitivity of exocytosis. Electron-transport protein required for intravesicular monooxygenases in subsets of secretory vesicles. Required for dopamine- -hydroxylase and peptide amidase activity. 

DBM dopamine P-monooxygenase GDPH glycerol phosphate dehydrogenase... 

The first step is catalysed by the tetrahydrobiopterin-dependent enzyme tyrosine hydroxylase (tyrosine 3-monooxygenase), which is regulated by end-product feedback is the rate controlling step in this pathway. A second hydroxylation reaction, that of dopamine to noradrenaline (norepinephrine) (dopamine [3 oxygenase) requires ascorbate (vitamin C). The final reaction is the conversion of noradrenaline (norepinephrine) to adrenaline (epinephrine). This is a methylation step catalysed by phenylethanolamine-jV-methyl transferase (PNMT) in which S-adenosylmethionine (SAM) acts as the methyl group donor. Contrast this with catechol-O-methyl transferase (COMT) which takes part in catecholamine degradation (Section 4.6). 

Miller, S.M. and Klinman, J.P. (1982). Deduction of kinetic mechanisms from primary hydrogen isotope effects dopamine beta-monooxygenase - a case history. Methods Enzymol. 87, 711-732... 

Miller, S.M. and Khnman, J.P. (1985). Secondary isotope effects and structure-reactivity correlations in the dopamine heta-monooxygenase reaction evidence for a chemical mechanism. Biochemistry 24, 2114-2127... 

Figure 11.17 Supplementation of diet with y-linolenic acid to overcome a deficiency of A desaturase Supplementation of a diet with DOPA to overcome a deficiency of monooxygenase in Parkinson s disease. A desaturase is a rate-limiting enzyme in the synthesis of arachidonic acid. Supplementation of diet with y-linolenic acid bypasses this enzyme. Damage to neurones in the brain that use dopamine as a neurotransmitter causes a deficiency of rate-limiting a supplement - enzyme, tyrosine monooxygenase, which is bypassed by a supplement, DOPA (dihydroxyphenylalanine). DOPA (usually, described as L-DOPA) is considered by the medical profession as a drug but, in reality, it is a dietary supplement.

Tyrosinase is a monooxygenase which catalyzes the incorporation of one oxygen atom from dioxygen into phenols and further oxidizes the catechols formed to o-quinones (oxidase action). A comparison of spectral (EPR, electronic absorption, CD, and resonance Raman) properties of oxy-tyrosinase and its derivatives with those of oxy-Hc establishes a close similarity of the active site structures in these proteins (26-29). Thus, it seems likely that there is a close relationship between the binding of dioxygen and the ability to "activate" it for reaction and incoiporation into organic substrates. Other important copper monooxygenases which are however of lesser relevance to the model studies discussed below include dopamine p-hydroxylase (16,30) and a recently described copper-dependent phenylalanine hydroxylase (31). 

---