Hepatocyte, endoplasmic reticulum

If liver cells (hepatocytes) are isolated and grown in culture, drugs are exposed to a similar array of enzymes. Because CYP enzymes are bound to the internal membrane fraction of hepatocytes, the liver can be homogenized and a preparation of vesicles of the hepatocyte endoplasmic reticulum called microsomes can be incubated with drug molecules. This preparation suffers because many of the soluble Phase II conjugation enzymes that are found in the cellular cytoplasm are lost. An alternative method for measuring microsomal metabolism involves isolation of the so-called S9 fraction , which includes the cytosolic soluble conjugation enzymes. 

RBP is synthesized in the hepatocytes, where it picks up one molecule of retinol in the endoplasmic reticulum. Both its synthesis and its secretion from the hepatocytes to the plasma are regulated by retinol. In plasma, the... 

Many of the phase 1 enzymes are located in hydrophobic membrane environments. In vertebrates, they are particularly associated with the endoplasmic reticulum of the liver, in keeping with their role in detoxication. Lipophilic xenobiotics are moved to the liver after absorption from the gut, notably in the hepatic portal system of mammals. Once absorbed into hepatocytes, they will diffuse, or be transported, to the hydrophobic endoplasmic reticulum. Within the endoplasmic reticulum, enzymes convert them to more polar metabolites, which tend to diffuse out of the membrane and into the cytosol. Either in the membrane, or more extensively in the cytosol, conjugases convert them into water-soluble conjugates that are ready for excretion. Phase 1 enzymes are located mainly in the endoplasmic reticulum, and phase 2 enzymes mainly in the cytosol. 

Monooxygenases (MOs) Enzyme systems of the endoplasmic reticulum of many cell types, which can catalyze the oxidation of a great diversity of lipophilic xenobiotics, are particularly well developed in hepatocytes. Forms of cytochrome P450 constitute the catalytic centers of monooxygenases. 

This reaction helps transfer certain amino acids across the plasma membrane, the amino acid being subse-quendy hydrolyzed from its complex with GSH and the GSH being resynthesized from cysteinylglycine. The enzyme catalyzing the above reaction is 7-glu-tamyltransferase (GGT). It is present in the plasma membrane of renal mbular cells and bile ducmle cells, and in the endoplasmic reticulum of hepatocytes. The enzyme has diagnostic value because it is released into the blood from hepatic cells in various hepatobihary diseases. 

MTP is responsible for the transfer of TGs and cholesteryl esters from the endoplasmic reticulum (ER) to lipoprotein particles (VLDL in hepatocytes in the liver and chylomicrons in endocytes in the intestine) for secretion [52]. It is a heterodimer consisting of a unique large subunit essential for lipid transfer encoded by the mttp gene and a smaller subunit, the ubiquitous ER enzyme protein disulfide isomerase [53]. 

FIGURE 22-2 ATP-dependent uptake of calcium into endoplasmic reticulum and mitochondria as a function of extraorganellar Ca2+ concentration. As [Ca2+] j was raised, ATP-dependent Ca2+ uptake into the endoplasmic reticulum (ER) and mitochondria] pools increased. Data for ER uptake were determined as the amount of Ca2+ [45Ca2+] taken up by saponin-permeabilized hepatocytes after addition of ATP and in the presence of mitochondrial inhibitors. The curve for mitochondria] uptake was obtained by subtracting the ATP-dependent uptake in the presence of these inhibitors from that in the absence of inhibitors. For additional details consult Burgess et al. [31]. Stylized data taken from results originally reported in Burgess etal. [31]. (Redrawn and modified from reference [4] with permission of Landes Bioscience.)... 

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 phosphatidylcholine in bile is synthesised in the endoplasmic reticulum of the hepatocyte and must be transported to the canalicular membrane. One possibility involves the nonspecific phosphatidylcholine transfer protein but a mouse null for this protein did not show reduced phosphatidylcholine secretion into bile and there was no compensatory increase in other phospholipids transfer proteins. However, the plasma membrane would receive a ready supply of phospholipid by insertion of vesicles, and the MDR3 protein translocates this molecule from the inner leafiet to the outer surface where there is contact with bile acids, as suggested by Smit and colleagues. The role of this transporter is shown in Figure 2.2. 

The hepatocyte secretes biliary fluid into the bile canaliculi (dark green), tubular intercellular clefts that are sealed off from the blood spaces by tight junctions. Secretory activity in the hepatocytes results in movement of fluid towards the canalicular space (A). The hepatocyte has an abundance of enzymes carrying out metabolic functions. These are localized in part in mitochondria, in part on the membranes of the rough (rER) or smooth (sER) endoplasmic reticulum. 

Regions of the ER that have no bound ribosomes are known as the smooth endoplasmic reticulum (sER). In most cells, the proportion represented by the sER is small. A marked sER is seen in cells that have an active lipid metabolism, such as hepatocytes and Leydig cells. The sER is usually made up of branching, closed tubules. 

The 3 10 cells in the liver—particularly the hepatocytes, which make up 90% of the cell mass—are the central location for the body s intermediary metabolism. They are in close contact with the blood, which enters the liver from the portal vein and the hepatic arteries, flows through capillary vessels known as sinusoids, and is collected again in the central veins of the hepatic lobes. Hepatocytes are particularly rich in endoplasmic reticulum, as they carry out intensive protein and lipid synthesis. The cytoplasm contains granules of insoluble glycogen. Between the hepatocytes, there are bile capillaries through which bile components are excreted. 

Desulfurations. The reactions take place in the hepatocytes on the smooth endoplasmic reticulum. 

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