Endoplasmic reticulum, carboxylesterases

The microsomal fraction consists mainly of vesicles (microsomes) derived from the endoplasmic reticulum (smooth and rough). It contains cytochrome P450 and NADPH/cytochrome P450 reductase (collectively the microsomal monooxygenase system), carboxylesterases, A-esterases, epoxide hydrolases, glucuronyl transferases, and other enzymes that metabolize xenobiotics. The 105,000 g supernatant contains soluble enzymes such as glutathione-5-trans-ferases, sulfotransferases, and certain esterases. The 11,000 g supernatant contains all of the types of enzyme listed earlier. 

Satoh, H. et al., Human anti-endoplasmic reticulum antibodies in sera of patients with halothane-induced hepatitis are directed against a trifluoroacetylated carboxylesterase, Proc. Nat. Acad. Sci. USA, 86, 322, 1989. 

Three isoenzymes of carboxylesterase were purified from rat liver micro-somes and were named RL1, RL2, and RH1. These differ from each other in their response to hormone treatment, inducibility, substrate specificity, and immunological properties [75], It was shown that RL1, RL2, and RH1 resemble hydrolases p/ 6.2/6.4, pI 6.0, and pI 5.6, respectively. Enzyme RL2 was found to be identical to egasyn, a protein with esterase activity found in the endoplasmic reticulum [76], The role of egasyn is to stabilize glucuronidase (EC 3.2.1.31) by noncovalent binding to the microsomal membrane. 

The intracellular localization of carboxylesterases is predominantly microsomal, the esterases being localized in the endoplasmic reticulum [73] [79] [93], They are either free in the lumen or loosely bound to the inner aspect of the membrane. The carboxylesterases in liver mitochondria are essentially identical to those of the microsomal fraction. In contrast, carboxylesterases of liver lysosomes are different, their isoelectric point being in the acidic range. Carboxylesterase activity is also found in the cytosolic fraction of liver and kidney. It has been suggested that cytosolic carboxylesterases are mere contaminants of the microsomal enzymes, but there is evidence that soluble esterases do not necessarily originate from the endoplasmic reticulum [94], In guinea pig liver, a specific cytosolic esterase has been identified that is capable of hydrolyzing acetylsalicylate and that differs from the microsomal enzyme. Also, microsomal and cytosolic enzymes have different electrophoretic properties [77]. Cytosolic and microsomal esterases in rat small intestinal mucosa are clearly different enzymes, since they hydrolyze rac-oxazepam acetate with opposite enantioselectivity [95], Consequently, studies of hydrolysis in hepatocytes reflect more closely the in vivo hepatic hydrolysis than subcellular fractions, since cytosolic and microsomal esterases can act in parallel. 

The physiological functions of carboxylesterases are still partly obscure but these enzymes are probably essential, since their genetic codes have been preserved throughout evolution [84] [96], There is some evidence that microsomal carboxylesterases play an important role in lipid metabolism in the endoplasmic reticulum. Indeed, they are able to hydrolyze acylcamitines, pal-mitoyl-CoA, and mono- and diacylglycerols [74a] [77] [97]. It has been speculated that these hydrolytic activities may facilitate the transfer of fatty acids across the endoplasmic reticulum and/or prevent the accumulation of mem-branolytic natural detergents such as carnitine esters and lysophospholipids. Plasma esterases are possibly also involved in fat absorption. In the rat, an increase in dietary fats was associated with a pronounced increase in the activity of ESI. In the mouse, the infusion of lipids into the duodenum decreased ESI levels in both lymph and serum, whereas an increase in ES2 levels was observed. In the lymph, the levels of ES2 paralleled triglyceride concentrations [92] [98],... 

Trifluoroacetylchloride is very reactive, and therefore, if any of it escapes from the vicinity of the cytochromes P-450 in the smooth endoplasmic reticulum, it might be expected to react with those proteins in highest concentration. The 59-kDa polypeptide, a microsomal carboxylesterase, constitutes 1.5% of the total microsomal protein, and therefore it fulfils this... 

FIGURE 8.1 The carboxy-terminal amino acid residues of carboxylesterase enzymes from disparate species are abgned to show the conserved HXEL motif found among intracellular enzymes (shown in bold letters), and the dismpted versions of this retention motif found in the mouse and rat secreted carboxylesterase isoenzymes (alterations to the motif shown in itahcs). The capacity of the carboxy-terminal HXEL motif to act as an endoplasmic reticulum retention signal has been directly demonstrated. (From Medda and Proia, 1992.)... 

Medda, S. and Proia, R.L. 1992, The carboxylesterase family exhibits C-terminal sequence diversity reflecting the presence or absence of endoplasmic-reticulum-retention sequences, Fur. J. Biochem., vol. 206, no. 3, pp. 801-806. 

Robbi, M., Beaufay, H., and Octave, J.N., Nucleotide sequence of cDNA coding for rat liver pi 6.1 esterase (ES-10), a carboxylesterase located in the lumen of the endoplasmic reticulum. Biochemistry, 269, 451, 1990. 

Satoh H, Martin BM, Schulick AH, Christ DD, Kenna JG, Pohl LR (1989) Human anti-endoplasmic reticulum antibodies in sera of patients with halothane-induced hepatitis are directed against a trifluoroacetylated carboxylesterase. Proc Natl Acad Sci USA 86 322-326 Saxon AJ, Sloan KL, Reoux J, Haver VM (1998) Disulfiiam use in patients with abnormal liver function test results. J Clin Psychiatry 59 313-316 Schnyder B, Mauri-Hellweg D, Zanni M, Bettens F, Pichler WJ (1997) Direct, MHC-dependent presentation of the drug sulfamethoxazole to human alphabeta T cell clones. J Clin Invest 100 136-141... 

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