Liposome based solubilisation of carotenoid substrates for enzymatic conversion in aqueous media.

C. Nacke, J. Schrader

Enzymatic conversions of strongly hydrophobic substrates can be conducted in non-denaturing aqueous reaction media if appropriate substrate delivery systems such as micelles or liposomes are applied. We investigated liposome based substrate delivery with regard to qualitative and kinetic effects of vesicle properties on the enzymatic reaction. The cleavage of highly hydrophobic carotenoid and xanthophyll substrates byArabidopsis thaliana carotenoid cleavage dioxygenase 1 (AtCCD1) was used as model reaction for the investigation. Conversions of the partly polar xanthophylls and the fully non-polar carotenes showed different responses to variations in the phospholipid composition of the delivery vesicles. Furthermore, the cellular reaction environment of AtCCD1 was imitated by incorporation of the galactolipid monogalactosyl-diacylglycerol (MGDG) into the liposome membranes. This led to an increase of the specific AtCCD1 activity towards the fully non-polar β-carotene by approximately 70% at MGDG shares between 8 and 20 mol% while the specific activity towards the more hydrophilic zeaxanthin decreased with increasing MGDG content. Liposome based systems proved to be less suitable for the delivery of amphiphilic and non-symmetric substrate molecules than micelles due to substrate orientation within the liposome membrane. The results represent a starting point for a systematic design of liposome based delivery of hydrophobic substrates in enzymatic conversions performed in aqueous media.

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