Retinyl ester

A retinyl ester is an ester formed at the alcohol group of retinol. The retinyl portion is attached through oxygen to an acyl group; different attached groups produce different members of the class. The name therefore does not identify one unique molecular formula or molar mass.^[ref]

Write a retinol alcohol schematically as $\mathrm{Ret}-\mathrm{OH}$, where Ret represents its carbon framework. The corresponding ester has the arrangement $\mathrm{Ret}-\mathrm{O}-\mathrm{C}(=\mathrm{O})-\mathrm{R}$. Varying $R$ changes the attached acyl group without requiring a change in the retinyl framework. This is a structural comparison, not a claim that a free carboxylic acid supplies every biological esterification reaction.

For example, lecithin retinol acyltransferase uses phosphatidylcholine as an acyl donor. Its products include retinyl esters that can be stored or used in subsequent retinoid pathways.^[ref] An assay of total ester therefore needs an additional identity measurement if the question concerns a particular ester, geometric isomer or downstream substrate.

An all-trans-retinyl ester is a retinyl ester whose retinyl-chain double bonds have trans geometry. The prefix specifies that part of the molecule; the attached acyl group can still vary. A sample described only by this class name is consequently not a completely specified individual compound.^[ref]

Two independent structural choices are involved. Changing the attached acyl group can alter the molecular formula while leaving retinyl-chain geometry unchanged. Changing one retinyl double bond from trans to cis instead changes the geometry without necessarily changing the molecular formula. Esterification and cis-trans isomerism must therefore be distinguished.

The distinction has a concrete role in the visual cycle. A 2005 expression experiment showed that RPE65 converts an all-trans-retinyl ester substrate into 11-cis-retinol, provided the cells also expressed the substrate-supplying lecithin retinol acyltransferase.^[ref] The observed product differs in both ester status and retinyl geometry. Measuring only total retinoid would conceal those chemically important changes.