D£-Dopa

An interesting example of the above difference is l-DOPA 4, which is used in the treatment of Parkinson s disease. The active drug is the achiral compound dopamine formed from 4 via in vivo decarboxylation. As dopamine cannot cross the blood-brain barrier to reach the required site of action, the prodrug 4 is administered. Enzyme-catalyzed in vivo decarboxylation releases the drug in its active form (dopamine). The enzyme l-DOPA decarboxylase, however, discriminates the stereoisomers of DOPA specifically and only decarboxylates the L-enantiomer of 4. It is therefore essential to administer DOPA in its pure L-form. Otherwise, the accumulation of d-DOPA, which cannot be metabolized by enzymes in the human body, may be dangerous. Currently l-DOPA is prepared on an industrial scale via asymmetric catalytic hydrogenation. 

Tyrosinase, a copper-containing oxidoreductase, catalyzes the orthohydroxy-lation of monophenols and the aerobic oxidation of catechols. The enzyme activity will be assayed by monitoring the oxidation of 3,4-dihydroxyphenyl-alanine (dopa) to the red-colored dopachrome. The kinetic parameters Ku and Vmax will be evaluated using Lineweaver-Burk or direct linear plots. Inhibition of tyrosinase by thiourea and cinnamate will also be studied. Two stereoisomers, L-dopa and D-dopa, will be tested and compared as substrates. 

Part D Choose one inhibitor and do four assays with L- or D-dopa. 

The treatment of results will be described for L-dopa. The procedure for D-dopa is identical. Prepare a table of L-dopa concentration per assay (mmo-lar) vs. A/i/min. Convert all AA/mm units to /xmoles/min as described in part B. Prepare a Michaehs-Menten curve (/xmoles/min vs. [S]) as in Figure E5.1 and a Lineweaver-Burk plot (l//xmole/min vs. 1/[S]) as in Figure E5.2. Alternatively, you may wish to use the direct linear plot. Estimate Ku and Vmax from each graph. The intercept on the rate axis of the Lineweaver-Burk plot is equal to 1/V-. For example, if the line intersects the axis at 0.02,... 

Some enzymes are able to differentiate between D and L isomers of substrates. Dopa, with a chiral center, exists in enantiomeric pairs, D and L. The naturally occurring dopa molecule has the L configuration, so it might be expected that the enzyme-catalyzed oxidation of L-dopa is more facile than that of D-dopa. The relative reactivity can be determined in a quantitative way by comparing Ku or Vmix values. Does tyrosinase exhibit stereoselectivity ... 

A substance to be carried forms a complex with a component of the membrane on one side the complex is then carried through the membrane, the drug or substance is released, and the carrier returns to the original surface and state to repeat the process. The carrier shows specificity for instance, L-dopa but not D-dopa is transported. 

Poly(L-lysine)-Cu(II) complex catalyzes the oxidation of D-3,4-dioxyphenyl-alanine (d-DOPA) much more than L-DOPA, with the condition that poly(L-lysine) gives a-helical configuration (135,136). The a-helical structure serves to hold the... 

Figure 9.4 Elution pattern of AADC incubation mixtures with the homogenate of rat cerebral cortex as enzyme from HPLC. The standard incubation mixture contained 0.5 mg of rat cerebral cortex. (A) Experimental incubation with L-dopa. (fl) Blank incubation with D-dopa. Dihydroxybenzylamine (100 pmol) was added to each sample after incubation. DHBA, dihydroxybenzylamine DA, dopamine. (From Nagatsu et al., 1979.)...

Azad, B. B., Chirakal, R. and Schrobilgen, G. J. (2007) Trifluoromethanesulfonic acid, an alternative solvent medium for the direct electrophilic fluorination of DOPA new syntheses of 6-[18F]fluoro-L-DOPA and 6-[18F]fluoro-D-DOPA. J Labeled Comp. Radiopharm., 50, 1454. 

Both reduction and oxidation processes have been found to be biphasic. Thus, in kinetic studies of the reduction of synthetic oxidation with Fe , respectively, a fast first electron-exchange step was followed by a slow second step (210). Whereas the quinone-quinol relationship involves an exchange of two electrons, only 0.5 electrons were accounted for the fast reaction step between d,/-dopa melanin and Tp similarly, only 0.02 electrons per indole unit was exchanged with Fe (210). From the 25 I ratio for the fast reduction versus oxidation steps, it was concluded that melanin in an air atmosphere exists predominantly in the quinonoid form. This finding was further supported by an experiment in which reduced d,/-dopa melanin was partially reoxidized by air. 

Related to the tunichromes are halocyamines A (41) and B (42) from Halocynthia roretzi. [39]. These are tetrapeptides which contain a Dopa residue (note the authors have depicted a D-Dopa residue in their structure). Both halocyamines displayed antimicrobial activity against B. subtilis (MIC 50pg /mL), B. megateruine (MIC 50 pg/mL), B. cereus (MIC 100 pg /mL) and the yeast Cryptococcus neoformans (MIC 100 pg /mL). Halocyamine A was also found to be cytotoxic to neuroblastoma N-18 cells at 160 pM for 24 hours. It caused degeneration of neurite and soma in cultured rat fetal brain cells at 100 pM and death of Hep-G2 cells at 200 pM for 24 hours. 

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