Spermidine specificity

In addition to the glutamate and glycine sites on the NMDA receptor, there also exist polyamine sites which are activated by the naturally occurring polyamines spermine and spermidine. Specific divalent cation sites are also associated with the NMDA receptor, namely the voltage-dependent magnesium site and the inhibitory zinc site. In addition to the excitatory amino acids, the natural metabolite of brain tryptophan, quinolinic acid, can also act as an agonist of the NMDA receptor and may contribute to nerve cell death at high concentrations. 

Figure 9 summarizes the electrode responses toward a variety of DNA-binding substrates [14c]. For intercalators (quinacrine, acridine orange, and safranin) and groove binders (spermine and spermidine), a steep rise followed by a saturation of the concentration response curve is commonly observed. If one compares the specific concentration which gives a 50% response in the increment of the cathodic peak current (A/p ) for each substrate, a selectivity order of quinacrine acridine orange > spermine > spermidine > safranin can be estimated. The binding constants measured in aqueous media for the affinity reaction with ds DNA are as follows quinacrine, 1.5 x 10 (38 mM NaCl)... 

Several AFM studies examined the effect of buffer conditions on the formation of motifs with DNA molecules. For instance, it has been foimd that the multivalent cations induce the condensation of DNA molecules into higher ordered structures, including toroids and rods [122], More specifically, Zn and Mg ions induce the formation of DNA kinked and perfect circles, respectively [123] (Fig. 16). Also, higher concentrations of spermidine induce the formation of complex flower-shaped structures with single crossover points [122] and increased concentrations of ethanol lead to complex and looped structures [ 124] (Fig. 17). 

The enzymes involved in the polyamine metabolic pathway have been the subject of intensive study, and a number of specific inhibitors for these enzymes have been designed as potential antitumor or antiparasitic agents [166]. Thus, a-difluoromethylornithine, has become a clinically useful agent [167]. Most of the studies involving inhibitors of polyamine metabolism have focused on enzymes involved in the biosynthetic pathway. Recently, there has been considerable interest generated in the enzyme spermidine/spermine-hT -acetyltrans-ferase enzyme (SSAT), the rate-limiting step in the back conversion of polyamines. SSAT, in conjunction with polyamine oxidase (PAO), allows for reversal of the biosynthetic pathway and attenuation of the levels of individual polyamines. 

Three acyclic amines, dimethylamine (109), putrescine (111), and spermidine (110), have been isolated from the accessary sexual glands of the mature male desert locust, Schistocerca gregaria (Table VIII). In addition, A -pyrroline (12i) has been identified as a volatile emanating from the mature male locust colony (Table II). It is an oxidation product of putrescine and probably could be responsible for the maturation-accelerating effect observed to be specific to the mature male insect 106). 

Ornithine decarboxylase is specifically inhibited by the enzyme-activated inhibitor a-difluoromethyl-ornithine, which can cure human infection with Trypanosoma brucei (African sleeping sickness) by interfering with polyamine synthesis.243-2443 In combination with inhibitors of spermidine synthase or S-adenosylmethionine decarboxylase,245 it can reduce polyamine levels and growth rates of cells. Another powerful inhibitor that acts on both ornithine and adenosylmethionine decarboxylases is the hydroxy-lamine derivative l-aminooxy-3-aminopropane 246... 

FIGURE 4 Metabolic pathway of polyamines. Increased levels of spermidine, spermine, putrescine, acetylspermidine, and acetylspermine without a change of ornithine in AD pathology were observed. One theory suggests that the NMDA receptor excitotoxicity is caused by an excess of spermidine and spermine due to ornithine decarboxylase activity induced by plaque and/or tangle deposition in specific brain regions. Reproduced from Ref. (112). 

As for deaminase, the kinetic analysis suggests a partial mixed-type inhibition mechanism. Both the Ki value of the inhibitor and the breakdown rate of the enzyme-substrate-inhibitor complex are dependent on the chain length of the PolyP, thus suggesting that the breakdown rate of the enzyme-substrate-inhibitor complex is regulated by the binding of Polyphosphate to a specific inhibitory site (Yoshino and Murakami, 1988). More complicated interactions were observed between PolyP and two oxidases, i.e. spermidine oxidase of soybeen seedling and bovine serum amine oxidase. PolyP competitively inhibits the activities of both enzymes, but may serve as an regulator because the amino oxydases are also active with the polyamine-PolyP complexes (Di Paolo et al., 1995). 

A possible involvement of polyamines in the response has been suggested. Prolactin stimulated ornithine decarboxylase activity in Nb2 cells [109]. A specific inhibitor of this enzyme partially blocked the actions of lactogenic hormones on Nb2 cell proliferation [110], and addition of polyamines (putrescine, spermidine or spermine) restored normal growth. However, polyamines alone had no effect on Nb2 cell proliferation, suggesting that they are not the sole or major factors involved in mediating the actions of prolactin on cell growth. 

Figure 9.2. Metabolism of pyrrolizidine alkaloids (PAs) in Senecio vernalis. The substrates for alkaloid biosynthesis, putrescine and spermidine, are derived from primary metabolism. Homospermidine, synthesized by homospermidine synthase (HSS), is the first pathway specific intermediate. It is exclusively incorporated into the necine base moiety of senecionine A-oxide, the backbone structure of all PAs found in this Senecio species. During allocation from the roots as site of synthesis to the shoots, it is chemically modified to provide the species specific PA-pattem.

Experimental conditions for the reaction of N3P3C16 with spermidine (1 1) are detailed here, in order to point out some specific difficulties which arose in the synthetic... 

Acamprosate (calcium acetylhomotaurinate) has been postulated to act by restoring the alcohol-induced neurotransmission imbalance of inhibition-excitation inputs believed to underlie alcohol dependence (1,2). The molecular structure of acamprosate explains its specificity toward the basic molecular mechanisms involved in the pathophysiology of alcohol dependence. A competitive interaction has been described between spermidine and acamprosate, suggesting a specific binding site for acamprosate on A-methyl-D-aspartate receptors (3). 

Fig. 1). These include linear polyethylene and polypropylene imines of varying degree of polymerization (Pn = 10 to 20) in non-methylated (PEI and PPI) and methylated (PMEI and PMPI) forms, which were specifically synthesized. Hexaazadocosane, which may be regarded as a model for a PPI with Pn = 6, was prepared by Michael addition of acrylonitrile to spermidine [12]. In addition, a commercially available PEI with high molecular mass and with a branched polymer architecture was employed. 

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