Monoamine transporter binding site affinity

The affinity (Kj values) observed for [ H]MDA and [ HJMDMA binding were similar to the effective doses (i.e., ED50 or K] values) of MDA and MDMA reported for various pre- and postsynaptic monoamine markers, such as serotonin and dopamine release (Johnson et al. 1986), monoamine transport (Steele et al. 1987), and multiple brain, ligand binding sites (Battaglia et al. 1988). 

Interestingly, we have recently identified a mutation of a tyrosine in the third intracellular loop of the hDAT that causes a major alteration in the conformational equilibrium of the transport cycle, and thus as such is comparable to mutants on G protein-coupled receptors causing constitutive isomerization of the receptor to the active state (66). Most importantly, this conclusion is based on the observation that mutation of the tyrosine completely reverts the effect of Zn2+ at the endogenous Zn2+ binding site in the hDAT (50,51) from potent inhibition of transport to potent stimulation of transport (Fig. 6). In the absence of Zn2+, transport capacity is reduced to less than 1% of that observed for the wild-type, however, the presence of Zn2+ in only micromolar concentrations causes a close to 30-fold increase in uptake (66). Moreover, it is found that the apparent affinities for cocaine and several other inhibitors are substantially decreased, whereas the apparent affinities for substrates are markedly increased (66). Notably, the decrease in apparent cocaine affinity was around 150-fold and thus to date the most dramatic alteration in cocaine affinity reported upon mutation of a single residue in the monoamine transporters (66). 

Using selective 5-HT- and DA uptake blockers and applying binding surface analysis Silverthorn et al. [98] demonstrated that the cocaine analog RTI-55 binds with equal affinities at two sites on the 5-HT transporter of membranes of whole rat brain minus caudate. Other 5-HT- and monoamine uptake blockers do not discriminate between these two binding sites. 

Serotonin, also known as 5-hydroxytryptamine (5-HT) is biosynthesized from tryptophan and is a neurotransmitter. Serotonin plays an important role in many behaviors including sleep, appetite, memory, and mood [52]. People with depressive disorders exhibit low levels of serotonin in the synapses. Protonated serotonin binds to a serotonin reuptake transporter protein, sometimes referred to as the serotonin transporter (SERT) and is then moved to an inward position on the neuron and subsequently released into the cjdoplasm. Selective serotonin reuptake inhibitors (SSRI) bind with high affinity to the serotonin binding site of the transporter. This leads to antidepressant effects by increasing extracellular serotonin levels which in turn enhances serotonin neurotransmission [53]. The SSRI class of antidepressants has fewer side effects than the monoamine oxidase inhibitors. 

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