Biosynthesis and localization of rat liver microsomal carboxyesterase E1

One of the microsomal carboxyesterases, carboxyesterase E1, was purified from rat liver to homogeneity. Carboxyesterase E1 is a glycoprotein of high mannose type, and is composed of three identical subunits of 59 kDa each. It is very similar to "esterase pI 6.0" described by Menthein et al. (Arch. Biochem. Biophys. 200, 547-559 (1980)) in molecular weight, amino acid composition, and enzymic activities. Carboxyesterase E1 was found to be evenly distributed between rough and smooth microsomes. The content of the enzyme in microsomes was about 1.5% of total microsomal protein. It was exclusively located on the luminal side of microsomes, and was not detected immunologically in Golgi fractions or serum. In vitro translation of rat liver RNA by reticulocyte lysate showed that carboxyesterase E1 was synthesized preferentially on the bound ribosomes, as a precursor peptide larger than the peptide of the mature enzyme. Carboxyesterase E1 was solubilized from microsomes by treatment with low concentrations of detergents. However, it was not released from microsomes by treatment with a synthetic peptide which made the microsomal membrane permeable to soluble protein molecules. Carboxyesterase E1 is not a soluble luminal protein, and seems to be bound to the luminal surface of the membrane.