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When the terminal amino group in the side chain of primaquine was blocked with an ethoxyacetyl group shown in 2, or eliminated by oxidative deamination to carboxylic acid 3, the antimalarial effect was markedly reduced in a screening assay which measures tissue schizonticidal activity. The optical isomers 1Aand 1Bof primaquine had similar antimalarial potency to the racemic mixture but 1Bappeared less toxic. The 5-phenoxy-substituted analogue 4, belonging to a new class of antimalarials, showed similar potency in the assays to either 1Aor 1Bbut seemed less cytotoxic than (±)-primaquine. Compounds 1Aand 1Bwere found to be competitive inhibitors of human monoamine oxidase (MAO) A and B ( Kirange 103–225 μM), but 4showed 10–30-fold greater competitive inhibition of MAO A ( Ki= 6.8 μM) and 40–90-fold greater non-competitive inhibition of MAO B ( Ki= 2.3 μM).

Bacterially expressed synapse-specific clathrin assembly protein, AP-3 (F1-20/AP180/NP185/pp155), bound with high affinity both inositol hexakisphosphate (InsP6) (Kd= 239 nM) and diphosphoinositol pentakisphosphate (PP-InsP5) (Kd= 22 nM). The specificity of this ligand binding was demonstrated by competitive displacement of bound [3H]InsP6. IC50values were as follows: PP-InsP5= 50 nM, InsP6= 240 nM, inositol-1,2,4,5,6-pentakisphosphate (Ins(1,2,4,5,6)P5) = 2.2 μM, inositol-1,3,4,5,6-pentakisphosphate (Ins(1,3,4,5,6)P5) = 5 μM, inositol-1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) > 10 μM, inositol-1,4,5-trisphosphate (Ins(1,4,5)P3) > 10 μM. Moreover, 10 μM inositol hexasulfate (InsS6) displaced only 15% of [3H]InsP6. The physiological significance of this binding is the ligand-specific inhibition of clathrin assembly (PP-InsP5> InsP6> Ins(1,2,4,5,6)P5); Ins(1,3,4,5,6)P5and InsS6did not inhibit clathrin assembly. We also observed high affinity binding of InsP6to purified bovine brain AP-3. We separately expressed the 33-kDa amino terminus and the 58-kDa carboxyl terminus, and it was the former that contained the high affinity inositol polyphosphate binding site. These studies suggest that specific inositol polyphosphates may play a role in the regulation of synaptic function by interacting with the synapse-specific clathrin assembly protein AP-3.