MECHANISTIC STUDIES ON THE ETHYL-ESTERIFICATION OF ACITRETIN BY HUMANLIVER PREPARATIONS IN-VITRO

Studies have been performed with human liver microsome preparations in vitro, to investigate the reaction mechanisms involved in the convers ion of acitretin to the corresponding ethyl ester, etretinate. The res ults indicate that: Three fresh samples of human liver, which had been stored in liquid nitrogen for up to 8 months, all produced traces of etretinate (5.8 +/- 0.8 ng/ml) in the presence of ethanol but not when the acitretin was added in acetone, or when the sample was denatured by preheating. Studies with pooled human liver microsomes, to identify the cellular location of the enzymes and the co-factors involved in t his esterification, indicate a primary requirement for both ethanol an d CoA + ATP with a secondary potentiation in the presence of an NADPH regenerating system. A possible explanation for these finding is that the microsomal ligase enzymes form an intermediate ester between CoA a nd acitretin, which is then trans-esterified by the ethanol. The low f ormation with CoA + ATP may indicate that second stage of this process occurs spontaneously, with the NADPH potentiation suggesting that it could also be mediated enzymically.