[3H]tryptamine binding to reconstituted fraction of acidic lipids

The possible involvement of sulphatides (CS), phosphatidylserine (PS) and phosphatidylinositol (PI) in [3H]tryptamine binding to various reconstituted fractions of these acidic lipids was examined by Sephadex LH20 column chromatography. The results indicated that each of the four systems, PS, PS-CS, PS-PI and PS-CS-PI, had the same binding capacity for [3H]tryptamine, whereas other systems (CS, PI and CS-PI systems) had no binding capacity. Furthermore, competitive inhibition experiments revealed that among these four reconstituted systems, the PS-CS system exhibited the highest affinity for 5-methoxytryptamine. Kinetic studies suggested that at least two binding components (or sites) are implicated in the binding of [3H]tryptamine to the reconstituted system of PS and CS with apparent KD values of 3 and 10 nM. Displacement studies with various compounds indicated that only tryptamine and 5-methoxytryptamine inhibited the [3H]tryptamine binding to this fraction, while other indoleamine analogues and neurotransmitters had no effect. In addition, we subjected whole rat brain synaptic plasma membranes to treatment with several kinds of lipid-modifying reagents and examined the [3H]tryptamine binding capacities of the membranes by a radioreceptor-binding assay. [3H]Tryptamine binding was decreased by treatment with Azure A and phospholipase A2, while phospholipase D had no effect. All these observations led to the inference that PS and CS may be involved in the tryptamine binding activities as recognition sites.