[44] Intermembranous transfer of retinoids

This chapter discusses the procedures of intermembranous transfer of retinoids. It is generally assumed that intermembranous hydrophobic ligand transfer proteins are reserved for compounds that undergo very slow transfer in the absence of a catalyst. It has been shown that nonesterified retinoids undergo rapid intermembranous transfer in the absence of catalytic transfer or exchange proteins. The rates of transfer are at least an order of magnitude faster than that for cholesterol. These results suggest that carrier protein assistance is not essential for catalyzing bulk transfer of the retinols(als) in the retina and retinal pigment epithelium. The intermembrane transfer techniques discussed in this chapter are of qualitative use for rapid transfer processes such as those found in the case of retinol and retinal. This is because the standard separation techniques are useful only when the half-times for transfer occur on a time scale of minutes, whereas the actual transfer rates for the retinol(al)s occur within seconds.