Spontaneous Transfer of Retinoic Acid, Retinyl Acetate, and Retinyl Palmitate between Single Unilamellar Vesicles

M T P Ho,H J Pownall,J G Hollyfield

doi:10.1016/s0021-9258(19)84637-9

Abstract

The transfer of retinoic acid, retinyl acetate, and retinyl palmitate between single unilamellar vesicles was studied by resonance energy transfer. The retinoic acid transfers spontaneously between single unilamellar vesicles with a first order rate constant of 9.6 s-1 at 15 degrees C and pH 7.4. At 30 degrees C, the transfer rate was 3.5 times faster than that at 10 degrees C. At pH 7.4, the transfer rate was almost 2 orders of magnitude faster than that observed at pH 1.6. Increasing the concentration of NaCl decreased the retinoic acid transfer rate significantly. Retinyl acetate transfers with a rate constant of 0.15 s-1, but no spontaneous transfer of retinyl palmitate was observed over 60 min. The evidence supports the proposal that retinoic acid and retinyl acetate transfer between single unilamellar vesicles occur via the aqueous phase. In contrast, no spontaneous transfer of retinyl palmitate was observed. However, linear free energy relationships and the thermodynamic parameters for retinyl acetate transfer permit the calculation of rate constant for retinyl palmitate transfer.

Highlights

The transfer of retinoic acid,retinylacetate, and retinyl palmitate between single unilamellar vesicles was studied by resonance energy transfer
Increasing the concentration of NaCl decreased the retinoic acid transfer rate significantly.Retinyl acetate transfers witha rate constant of 0.15 s-’, butno spontaneous transfer of retinyl palmitate was observed over 60 min
The evidence supports the proposal that retinoic acid and retiny1 acetate transfer between single unilamellar vesicles occur via the aqueous phase

Summary

RESULTS

P E whenever they arein close proximity (for example, within sample was excited at 345 nm, and the decrease of fluorescence a t the samesingle unilamellar vesicle). The reaction was done at 15 "C in Tris/NaClbuffer, pH 7.4. Z is the observedfluorescence intensity a t time t. In this trace, the first order rate constant is 9.4. Spontaneoustransfer of retinoicacid, asmonitored by Effect of vesicle concentration on the transfer rate of retinoic acid fluorescence decrease at 520 nm after mixing of donor and between single unilamellar vesicles acceptor vesicles, appears to be a first order process (Fig. 2). Reactions took place a t 10 "C and pH 1.6. The experimental erroraslso increasedabout two times so that the concentration-independent nature of the ofSpTornatnasnferous

Retinoic acid

The timecourse of retinyl acetate transfer is illustratedin

DISCUSSION