Thermodynamic parameters of the binding of retinol to binding proteins and to membranes

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Abstract
Retinol (vitamin A alcohol) is a hydrophobic compound and distributes in vivo mainly between binding proteins and cellular membranes. To better clarify the nature of the interactions of retinol with these phase which have a high affinity for it, the thermodynamic parameters of these interactions were studied. The temperature-dependence profiles of the binding of retinol to bovine retinol binding protein, bovine serum albumin, unilamellar vesicles of dioleoylphosphatidylcholine, and plasma membranes from rat liver were determined. It was found that binding of retinol to retinol binding protein is characterized by a large increase in entropy (T .DELTA. S.degree. = +10.32 kcal/mol) and no change in enthalpy. Binding to albumin is driven by enthalpy (.DELTA.H.degree. = -8.34 kcal/mol) and is accompanied by a decrease in entropy (T .DELTA. S.degree. = -2.88 kcal/mol). Partitioning of retinal into unilamellar vesicles and into plasma membranes is stabilized both enthalpic (.DELTA.H.degree. was -3.3 and -5.5 kcal/mol, respectively) and by entropic (T.DELTA.S.degree. was +4.44 and +2.91 kcal/mol, respectively) components. The implications of these finding are discussed.