Binding and immobilization of catecholamines by liposomes

Abstract

The polarization of the native fluorescence of dopamine and noradrenaline has been used to measure their binding and immobilization by liposomes suspended in aqueous buffers. Whereas both catecholamines are significantly immobilized by brain phosphatidyl serine and yeast phosphatidyl inositol, phosphatidyl ethanolamine and phosphatidyl inositol from brain are ineffective. Dopamine is immobilized to a greater degree than noradrenaline. The dissociation constants determined from modified Scatchard plots of the polarization data are 1.7 × 10⁻⁴ and 9.6 × 10⁻⁵ M for dopamine with yeast phosphatidyl inositol and brain phosphatidyl serine, respectively. Apomorphine binds to a hydrophobic region of phosphatidyl serine liposomes with a K_D value of 69 μM. It is suggested that a fraction of dopamine is complexed with membranous phosphatidyl serine in nerve terminals.