Liver metabolic role

The main organ involved in PCB metabolism and excretion in fish is the liver. Metabolism of PCBs in fish liver homogenates has been demonstrated (29,30,32) and PCB metabolites are excreted into bile (25,28,34). What is not known is extent to which PCB metabolites excreted in bile are eliminated in feces. Also the role of kidneys, gills, intestine and skin in PCB elimination in fish has not been fully elucidated. The only study on urinary excretion of PCBs was in dogfish sharks and revealed that urine was not a major route of elimination (28). 

Liver Chemical Processing. For protection from dangerous accumulation of various toxins, the body has evolved methods to eliminate foreign substances. Potentially harmful chemicals can originate from myriad sources including foods, liquids, air, and, of course, medications. The liver s role is to inactivate these chemicals and to convert (metabolize) them to water-soluble forms (i.e., forms that dissolve in water rather than fat), which can more easily be filtered and eliminated by the kidneys. 

The metabolic roles and regulation of glucose-6-P phosphohydrolase activity have been considered in detail in reviews by Cahill et al. (6) and Ashmore and Weber (7). More recently, possible metabolically important roles for phosphotransferase activities of this enzyme in liver, kidney, and intestine have been described in reviews by the present author (9, 10)—who also considered a variety of regulatory features based on interaction of substrates, inhibitors, and activators with the multifunctional enzyme—and by Cohn et al. (11). 

Another indication of the difference between the metabolic roles of muscle, a consumer tissue and liver, a contributer tissue, is the enzyme glucose-6-phospha-tase. This enzyme is required for the production of neutral glucose. It is present in liver and kidney but not in strict consumer tissues, such as muscle and brain. Without glu-cose-6-phosphatase the glucose-6-phosphate cannot be converted to dephosphorylated glucose, which is necessary for... 

Kerr BM, Thummel KE, Wurden CJ. Human liver carbamazepine metabolism. Role of CYP3A4 and CYP2C8 in 10,11-epoxide formation. Biochem Pharmacol 1994 47 1969-1979. 

Liver tissue contains an array of cell types, but the preponderant one is the hepatocyte. It has an overall structure much like that of the cell in Fig. 1-3. The cells are arranged in long, branching columns of about 20 cells in a cross section around a central bile cannaliculus (channel). Into the cannaliculus the cells secrete bile. The liver is the main producer of urea (Chap. 15), stores glycogen (Chap. 11). synthesizes many of the amino acids used by other tissues (Chap. 3), and produces serum proteins, among many other metabolic roles. 

Lipoproteins are essential for the transport of lipids from the gut and liver to the tissues, and for lipid metabolism. Lipoproteins are spherical particles with a hydrophobic core, covered by a single layer of amphi-pathic molecules phospholipids, cholesterol and one or more apoproteins (of which ten have been isolated these are produced in the liver). The role of these protein coverings is twofold they solubilise hydrophobic lipids and contain cell-targeting signals. 

J. Liebig postulated the liver s role in intermediary metabolism bile production was described as one part of protein and carbohydrate metabolism, and the synthesis of urea in the liver was accepted. 

Decker CJ, Laitinen LM, Bridson GW, Raybuck SA, Tung RD, Chaturvedi PR. Metabolism of amprenavir in liver microsomes role of CYP3A4 inhibition for drug interactions. J Pharm Sci 1998 87(7) 803-7. 

Liddle C, Goodwin B. Regulation of hepatic drug metabolism Role of nuclear receptors PXR and CAR. Semin Liver Dis 2002 22 115-122. 

A contribution to the understanding of lipoprotein metabolism would be the definition of the metabolic role of the lysolecithin-rich d 1.21 lipoprotein. This protein-phospholipid complex may simply represent a degradation product of the high-density lipoprotein after removal of the other lipid moieties. On the other hand, it could be considered as a newly synthesized product from the liver on its way to further lipidation to form a more complete class of lipoproteins. Its possible identity with the apolipoprotein described by Roheim and Eder (1961 Roheim et al., 1964) remains to be established. 

---