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Lipid metabolism appears to be less zonated than carbohydrate and protein metabolism. Studies on the zonation of lipid metabolism have been centered in particular on fatty acid synthesis which, according to the concept of metabolic zonation, should be a predominantly perivenous process while fatty acid oxidation should be periportal. There are, however, conflicting data on the activity gradients of lipogenic enzymes as well as measurements of actual synthesis of fatty acid and very low density lipoprotein. Data obtained by microdissection show a 1.5- to 2-fold higher activity of acetyl-CoA carboxylase and citrate lyase in the perivenous zone in agreement with measurements of the actual rate of fatty acid synthesis in preparations of hepatocyte, enriched in periportal or perivenous cells. On the other hand, results obtained with the dual-digitonin-pulse perfusion technique demonstrate the opposite gradient in the form of a 2- to 3-fold higher specific activity of acetyl-CoA carboxylase in the periportal zone based on measurements of the acetyl-CoA carboxylase protein proper. This specific activity gradient, which applies to male and not female rats, disappears almost completely in the fasted-refed animal, were lipogenesis is strongly induced. In this review we attempt to rationalize these discrepancies in the results as methodological differences which in particular apply to the following parameters: (1) expression of results (reference substance); (2) selectivity of zonal sampling, and (3) differences in methodology of acetyl-CoA carboxylase measurements. It is concluded that these factors could account for the discrepancies, but further studies, in particular on the zonation acetyl-CoA carboxylase mRNA, are required in order to further understand the zonation of lipid metabolism and its possible role in the metabolic regulation of the liver.

Results of our conclusive study on urinary enzyme stability during sample storage are reported. We measured alanine aminopeptidase (AAP) and N-acetyl-beta-D-glucosaminidase (NAG) in morning urines from 9 healthy normal subjects immediately after collection and throughout a 1-year storage at -70 and -20 degrees C. AAP proved to be quite stable at -70 degrees C (99.2% of the basal value at the end of the year). NAG is partially preserved (84.1% of the basal value) at -70 degrees C, but significantly decreased (50.4%) at -20 degrees C.

The development of liver parenchyma starts from entodermal cells which grow out from the gut into the mesenchyma of the septum transversum. In the definitive organ this close association of epithelial cells (hepatocytes) and mesenchyma-derived nonparenchymal cells is maintained. The liver, and with it each hepatocyte, acts in two directions: the vascular poles of the hepatocytes serve in an ingestive sense, while at their biliary poles secretory functions are exerted. Hepatic microvascularization comprises two afferent vessels (arterial and portal terminal branches), the sinusoids and the terminal hepatic venule. Sinusoidal cells surround the capillaries but also have highly specialized functions with regard to filtration, phagocytosis, fat storage and defense. The autonomic innervation plays an important role in the regulation of metabolic functions. Above the cellular level the proper architecture of the liver parenchyma has been the object of controversial discussions for centuries. The concept of the liver lobule, the portal unit, the liver acinus and other structures are presented and discussed. Finally, the liver parenchyma is described as an irregular interdigitating system of regions related to the terminal blood vessels.

5'-Nucleotidase was measured in isolated fat cells from normal, hypothyroid and hyperthyroid rats. This was done to find out whether thyroid hormones had an effect on the production of adenosine by the fat cell. The results showed that 5'-nucleotidase is modified when the rats received injections of 3,3',5-triiodo-L-thyronine (T3). There was no change in the enzyme in hypothyroidism or when T3 was added to incubation of cells.

Acatalasemia was detected in 2 sisters of a Hungarian family. The pedigree of the family showed hypocatalasemia in the children of the patients and in 1 of their brothers, while the other members of the family had normal blood catalase activity. The biochemical characterization (catalase activity, electrophoretic migration, isoelectric point and enzyme stability) of the blood as well as tissue catalase of the acatalasemic patients yielded a catalase form which did not differ from normal.

6-Pyruvoyl tetrahydropterin synthase, the enzyme that catalyses the conversion of 7,8-dihydroneopterin triphosphate to 6-pyruvoyl tetrahydropterin, was purified 3,330-fold from human pituitary gland with an overall recovery of 30%. The native enzyme has a molecular mass of 68 kD and consists of four identical subunits of 16.5 kD. The pH optimum of the enzyme in Tris/HCl buffer is 7.5. The enzyme is dependent on Mg2+ and NADPH and has a Michaelis-Menten constant of 10 microM for its natural substrate, 7,8-dihydroneopterin triphosphate. The isoelectric point of the human enzyme is 4.3-4.6. The human pituitary gland enzyme is heat instable in contrast to the enzymes from human, rat and salmon liver, and Drosophila head. The amino acid composition showed remarkably high content of acidic amino acids Asp and Glu. The N-terminus was found to be blocked.

The normal hepatic sinusoid is formed or lined by four different cell types, each with its specific phenotypic characteristics, functions and topography. Endothelial cells constitute the closed lining or wall of the capillary. They contain small fenestrations to allow the free diffusion of substances, but not of particles like chylomicrons, between the blood and the hepatocyte surface. This filtering effect regulates the fat uptake by the liver. Sinusoidal endothelial cells also have a pronounced endocytotic capacity which makes them an important part of the reticuloendothelial system. They are also active in the secretion of bioactive factors and extracellular matrix components of the liver. Recently, a zonal heterogeneity of the endothelial lining has been reported with regard to its filtering capacity (fenestration) and binding capacity for lectins and cells. Kupffer cells are intrasinusoidally located tissue macrophages with a pronounced endocytotic capacity. They are potent mediators of the inflammatory response by the secretion of a variety of bioactive factors and play an important part in the immune defense. A zonal heterogeneity has been established with regard to the endocytotic capacity and cytotoxic function. Pit cells are now known to represent a liver-associated population of large granular lymphocytes. They have the capacity to kill tumor cells and probably also play a role in the antiviral defense of the liver. In addition, pit cells may have a growth-regulatory function of the liver. They are known to be numerically more prominent in the periportal region, as is also the case for Kupffer cells. Fat-storing or Ito cells are present in the perisinusoidal space of Disse and are thought to represent the main hepatic source of extracellular matrix components. They are also the main site of vitamin-A storage. Fat-storing cells are more numerous in the periportal region than in the central region of the hepatic acinus. The periportal cells also store higher amounts of vitamin A. Sinusoidal cells may be considered to represent a functional unit at the border line between the hepatocytes or parenchymal cells and the blood. They participate in various liver functions and liver pathologies and our knowledge about this is growing. The heterogeneity of these cell types and possible cooperations between them and the hepatocytes may add to our understanding of liver functions.

Hydrolysis of extracellular ATP and other nucleoside phosphates by A-431 human epidermoidal carcinoma cells was studied. The hydrolysis of extracellular ATP by these cells required either Mg2+ or Ca2+, and either cation could be replaced by Co2+, Fe2+, or Mn2+. Nucleoside triphosphates (ATP, GTP, CTP, UTP, and dTTP), but not nucleoside diphosphates, were hydrolyzed by the cells with Km and Vmax values similar to those for ATP (0.9-1.1 mmol/l and 6-10 nmol Pi formed/10(6) cells, respectively). The hydrolysis of ATP was inhibited strongly by ATP-gamma S and AMPPNP, and weakly by AMPCPP and ADP-beta S, but not by AMPCPP or AMPCP. Since the hydrolysis of [gamma-32P]ATP was inhibited by all these nucleoside triphosphates, the binding site for ATP is presumed to be the same as that for the other nucleoside triphosphates. All these results indicate that ecto-ATPase activity associated with A-431 cells is due to ecto-nucleoside triphosphatase. The nucleotide specificity shown in the present study indicates that ecto-nucleoside triphosphatase associated with A-431 cells is a molecule different from P2-purinergic receptors which can be stimulated specifically with nucleoside phosphates like ATP, ADP, UTP, UDP, and GTP, but not by other nucleotides.

Ascorbic acid's function in animals is attributed in part to the ease with which it reduces potentially damaging components, such as reactive free radicals. After more than six decades of speculation and laboratory efforts, the mechanisms by which ascorbic acid is maintained in the useful, reduced state remain uncertain. Previous attempts to isolate the enzymes that reduce the partially and the fully oxidized metabolites of vitamin C are reviewed. Some speculation on why dehydroascorbate reductase (EC 1.8.5.1) has not been purified from animal tissues is presented.

Conflicting data have been published regarding the effects of phenobarbital treatment on bilirubin UDP-glucuronyltransferase activity in native liver microsomes. Recent evidence suggests that the bilirubin UDP-glycosyltransferase system faces the interior of microsomal vesicles, and that expression of its activities in sealed microsomes may be rate-limited by transport of UDP sugars across the membrane. These observations raise the possibility that the reported variability in the effects of phenobarbital may reflect differences in integrity of the membrane in microsomal preparations. We examined the effect of phenobarbital on bilirubin UDP-glucosyltransferase and the UDP-glucuronyltransferase activities towards bilirubin, 4-nitrophenol, and 1-naphthol using native rat liver microsomes with verified vesicle integrity. Phenobarbital-induced microsomes in which the membrane permeability barrier was eliminated by pretreatment with detergent displayed markedly higher UDP-glycosyltransferase activities towards all tested substrates compared with activities in similarly disrupted microsomes from untreated rats. In contrast, none of the transferase activities tested were significantly enhanced by phenobarbital treatment when the enzymic activities were assayed in sealed microsomes. Addition to the enzyme assay mixture of UDPGlcNAc, a presumed physiological activator of the UDP-glucuronyltransferases, failed to expose the enhanced UDP-glucuronyltransferase concentration in phenobarbital-induced sealed microsomes. Our findings are consistent with the idea that transport of UDP sugar across the membrane may be rate-limiting for expression of UDP-glycosyltransferase activities in sealed microsomes. Quantitative assessment of membrane integrity is an essential prerequisite in experiments designed to study the regulation of the microsomal UDP-glycosyltransferase system.