Retinoylation of proteins in rat liver, kidney, and lung in vivo

Retinoylation (retinoic acylation) is a posttranslational modification of proteins occurring in a variety of cell types in vitro. This study was done to examine whether retinoylation occurs in vivo. We found that in retinol-deficient rats, radiolabeled retinol or retinoic acid was incorporated into the liver, kidney, and lung in a form that was not removed by extraction with CHCl3:CH3OH. About 98% of the radiolabeled retinoid was acid-soluble after digestion with proteinase K indicating that it was covalently bound to protein. About 50% of the retinoid covalently bound to liver and kidney protein was removed by mild hydrolysis with CH3OH-KOH. Methyl retinoate, all-trans-retinoic acid, and polar metabolites of retinoic acid accounted for essentially all of the retinoids released. We conclude that retinoylation of protein occurs in vivo primarily via the formation of an ester bond.