Saturable [D-Ala2, D-Leu5]-enkephalin transport into cholinergic synaptic vesicles

Incubation of cholinergic synaptic vesicles purified from the electric organ of Torpedo ocellata with radiolabeled [D-Ala2, D-Leu5]-enkephalin results in its accumulation in the vesicles. Kinetic analysis of the initial rates of uptake reveals temperature-dependent saturation kinetics which are best fitted by a single transport system (KT = 12 +/- 2 microM and Vmax = 0.85 +/- 0.24 pmol/mg protein per min). The specific rates and extents of [3H]-[D-Ala2, D-Leu5]-enkephalin uptake, like those of [3H]acetylcholine uptake, are highest in the purified synaptic vesicles fraction. These findings suggest that Torpedo cholinergic synaptic vesicles contain an opioid peptide transporter. The physiological significance of this transporter is discussed in view of the recent observation that Torpedo nerve terminals contain an endogenous enkephalin-like peptide and presynaptic opiate receptors.