Halogenated Biogenic Amines In Biochemistry And Pharmaco

Diabetes, Digestive, Kidney Diseases

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Abstract

Biogenic amines play key roles in neurotransmission, metabolism, and in control of various physiological processes. Using a variety of synthetic methodologies, including novel procedures developed by us, we initially prepared in racemic form a series of biogenic amines with fluorine substituted at various ring-positions. These ring-fluorinated biogenic amines continue to find applications in a multitude of studies, including research on the mechanisms of transport, storage, release, metabolism, and modes of action of these amines. Of particular significance was the discovery that 6-fluoronorepinephrine (6-FNE) is a selective alpha-adrenergic agonist and 2-fluoronorepinephrine (2-FNE) is a selective beta-adrenergic agonist. This behavior also was seen with fluorinated epinephrines (FEPI) and other phenethanolamine agonists. Fluorinated DOPS and DOPAs. In a recent extension of this work we prepared the R- and S-enantiomers of 2- and 6-FNE and the R- and S-enantiomers of 2- and 6-FEPI. In order to have available alternate biological precursors for R-2-FNE and R-6-FNE, we also prepared the 2S,3R-isomers of 2- and 6-fluoro-dihydroxyphenylserine (2F- and 6F-L-threo-DOPS), first using Evans enantioselective aldol strategy, and more recently using an efficient Sharpless asymmetric aminohydroxylation procedures. We examined these analogues as substrates for dopa decarboxylase. We also completed enantioselective syntheses of 2-F-, 5-F-, 6-F-and 2,6-diF-L-DOPA in order to study the effects of ring-fluorination on the rate of enzymatic decarboxylation of these analogues. The predominant influence of fluorine substitution on decarboxylation is related to an increase in Km for all fluorinated analogues (5-F > 2-F > 6-F > Dopa). This implies that the presence of fluorine substituents at ring position 2, 5 or 6 affects binding to the active site. In contrast, the effects of fluorine substitution on the kcat for decarboxylation are minimal. Thus, the fluorinated analogues are decarboxylated less efficiently by DDC as a result of lower affinity for the enzyme. Of the analogues, 6-F-Dopa is the best substrate, with a kcat/Km some 10% that of L-Dopa. [18F]6F-L-DOPA has been used extensively in PET studies to quantify dopaminergic function, and rates of decarboxylation are thus important considerations. Aromatic amino acid decarboxylase converts 6-F-L-threo-DOPS to R-6-FNE, but a competing Pictet Spengler cyclization with the pyridoxal phosphate cofactor diminishes the efficiency of the process. With 2F-L-threo-DOPS, the Pictet Spengler reaction predominates and decarboxylation is essentially blocked. We plan to use this observation that fluorine in the 2-position facilitates Pictet Spengler cyclization to develop probes for the study of tetrahydroisoquinoline formation between biogenic amines and biogenic aldehydes. Side-chain fluorinated amines and amino acids. Ring-fluorinated imidazoles we previously prepared also have been very valuable as biochemical and pharmacological agents. We now have developed syntheses of side-chain fluorinated biologically important imidazoles, including beta-fluoro- and beta,beta-difluorohistamine, beta-fluorourocanic acid, and alpha,beta-difluorourocanic acid. We are extending this to the corresponding histidine derivatives, and biosynthetic precursors of histidine. Fluorinated cis-urocanic acids are being used to study the mechanism of cis-urocanic acid-mediated photo-immunosuprression. We also prepared beta,beta-difluorotryptamines, including beta,beta-difluoromelatonin. Studies include the use of these analogues as mechanistic probes of tryptamine N-acetyltransferase. Approaches to the corresponding tryptophans are being developed. MAO Inhibitors. Several inhibitors, some medicinally effective, of the important metabolic enzyme, monoamine oxidase (MAO), contain fluorine as an important feature that is critical to the inhibitory mechanism. Other inhibitors contain cyclopropane for the same purpose. We have prepared and are now studying a series of benzylamines and phenylethylamines that incorporate fluorinated cyclopropyl groups in the side chain as new MAO inhibitors to determine the combined effects of these two structural motifs.