1995 Volume 10 Issue supplement Pages 138-141
Liver microsomal carboxylesterases (CES) function in the hydrolysis of a wide variety of endogenous and xenobiotic compounds, and play an important role in drug and lipid metabolism in many mammalian species. In the present study, twenty-seven CES isozymes were purified to electrophoretic homogeneity from liver microsomes of ten mammalian species and humans, and their physical, enzymological and immunological properties were compared with each other. The CES isozymes from various species examined here showed considerable similarities in physicochemical and immunochemical properties, but not similar in substrate specificities. λZap II library from mouse liver, and λgt11 libraries from human and rat liver were screened with antibodies raised to a purified liver CES isozymes. The deduced amino acid sequence of the clone possessed many structural characteristics that are highly conserved among rat RL1, RH1, RHlec, RS1, RS2, mouse ML1, MH1, MS1 and human HU1, including active site sequence(GESAGG, NKQEXG, GDHXD), and four cysteines which may be involved in the specific disulfide bond. It is well known that proteins which are retained in the endoplasmic reticulum (ER) lumen contain the retention signal at their carboxy terminal of the tetrapeptide. The five CES clones (RL1, RH1, RHlec, MH1, and HU1) also contained an ER-retention signal (HXEL). When clone was expressed in COS cells, the plasmid-coded protein was retained. The cells expressing CES is very high activity towards xenobiotic ester and amide. In conclusion, liver microsomal CES in mammals and humans are closely involved in drug and lipid metabolism in the endoplasmic reticulum, and it is noteworthy that the isozymes from various species examined here showed considerable similarities in amino acid sequences, but not similar in substrate specificities. These reasons may be, at least in part, due to the variances of substrate binding site, although the sequence is not clarified yet.
