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Retinyl esters are a major endogenous storage source of vitamin A in vertebrates and their hydrolysis to retinol is a key step in the regulation of the supply of retinoids to all tissues. Some members of nonspecific carboxylesterase family (EC 3.1.1.1) have been shown to hydrolyze retinyl esters.However, the number of different isoenzymes that are expressed in the liver and their retinyl palmitate hydrolase activity is not known. | Eur. J. Biochem. 269 4387-4398 2002 FEBS 2002 doi 10.1046 j.1432-1033.2002.03121.x Identification of microsomal rat liver carboxylesterases and their activity with retinyl palmitate Sonal P. Sanghani Wilhelmina I. Davis Natividad G. Dumaual Alan Mahrenholz and William F. Bosron Department of Biochemistry and Molecular Biology and of Medicine Indiana University School of Medicine Indianapolis USA Retinyl esters are a major endogenous storage source of vitamin A in vertebrates and their hydrolysis to retinol is a key step in the regulation of the supply of retinoids to all tissues. Some members of nonspecific carboxylesterase family EC 3.1.1.1 have been shown to hydrolyze retinyl esters. However the number of different isoenzymes that are expressed in the liver and their retinyl palmitate hydrolase activity is not known. Six different carboxylesterases were identified and purified from rat liver microsomal extracts. Each isoenzyme was identified by mass spectrometry of its tryptic peptides. In addition to previously characterized rat liver carboxylesterases ES10 ES4 ES3 the protein products for two cloned genes AB010635 and D50580 GenBank accession numbers were also identified. The sixth isoenzyme was a novel carboxylesterase and its complete cDNA was cloned and sequenced AY034877 . Three isoenzymes ES10 ES4 and ES3 account for more than 95 of rat liver microsomal carboxylesterase activity. They obey Michaelis-Menten kinetics for hydrolysis of retinyl palmitate with Km values of about 1 IM and specific activities between 3 and 8 nmol-min_1-mg_1 protein. D50580 and AY034877 also hydrolyzed retinyl palmitate. Gene-specific oligonucleotide probing of multiple-tissue Northern blot indicates differential expression in various tissues. Multiple genes are highly expressed in liver and small intestine important tissues for retinoid metabolism. The level of expression of any one of the six different carboxylesterase isoenzymes will regulate the metabolism of retinyl palmitate
