Hepatic Precursor Research Articles

Hepatic glycerol flux after E. coli endotoxin administration.

Hepatic glycerol flux was examined in dogs after the administration of Escherichia coli endotoxin (0.4 mg/kg) to determine the contribution of the liver to the previously observed decline in the metabolic clearance rate of glycerol. Hepatic glycerol flux was estimated by determining hepatic arterial and portal venous blood flows with electromagnetic flow probes and by measuring arteriovenous difference of glycerol across the liver. Administration of endotoxin significantly decreased total hepatic blood flow (by approximately 20%) but did not alter hepatic arterial blood flow. Hepatic glycerol clearance decreased by 25-30% after endotoxin administration. Hepatic glycerol extraction also decreased. Under control conditions, 60% of the metabolic clearance rate of glycerol was attributable to the liver, whereas in the postendotoxin state approximately 72% of the glycerol clearance could be accounted for by hepatic clearance. Thus changes in transhepatic glycerol flux are only partially responsible for the previously observed alterations in glycerol clearance after endotoxin administration. Although hepatic glycerol clearance decreased, net hepatic glycerol, as well as lactate and alanine, uptake did not decrease, indicating that gluconeogenic precursor availability to the hepatocytes was not diminished. Hepatic glucose output was elevated after endotoxin administration. Changes in hepatic glucose output and gluconeogenic precursor uptake help explain the endotoxin-induced alternations in the fluxes of these metabolites.

Effect of ethionine on synthesis and methylation of ribosomal ribonucleic acid in regenerating rat liver

Ethionine, a hepatocarcinogen, was administered into rats 24 h before partial hepatectomy and immediately thereafter. Hepatic precursor ribosomal RNA (pre-rRNA) obtained 20 h after the operation of rats injected with ethionine and adenine resulted in methyl deficiency as judged by the incorporation of [ 3H]methyl group of S-adenosylmethionine into nuclear rRNA by partially purified rRNA methylase. The ethionine and adenine treatment causes methyl deficiency of nuclear rRNA at 2′-hydroxyribose sites of cytidine and uridine, but not at base sites. Although the ethionine and adenine treatment produced no significant change in total hepatic RNA synthesis in vivo assayed by the incorporation of labeled orotate, a one-third increase in nuclear rRNA synthesis as well as a one-third decrease in microsomal rRNA synthesis was found under the treatment. These results suggest that the undermethylation at 2′-hydroxyribose of pre-rRNA in liver nucleus, which is caused by ethionine and adenine administration into rats, causes an inhibition of the processing of nuclear pre-rRNA to cytoplasmic rRNA.

Studies on the production of low density lipoproteins by perfused livers from nonhuman primates. Effect of dietary cholesterol.

Nonhuman primates consuming diets containing cholesterol develop coronary artery atherosclerosis that we have found to be highly correlated with an increase in the size and cholesteryl ester content of plasma low density lipoproteins (LDL). The present studies were designed to determine whether the enlarged plasma LDL are produced directly by the liver of cholesterol-fed monkeys. African green monkeys were fed a diet containing 40% of calories as butter fat and either 0.16 mg cholesterol/kcal (control diet) or 0.78 mg cholesterol/kcal (test diet). The livers of these monkeys were perfused by recirculation with a lipoprotein-free medium for 4 h. The rate of accumulation of perfusate cholesterol was linear and greater in liver perfusates from test diet-fed vs. control diet-fed monkeys and was positively correlated with both the plasma cholesterol concentration and LDL size in the donor animal. All perfusate d less than 1.063 g/ml lipoprotein subfractions from livers of test diet-fed monkeys were enriched in cholesteryl ester severalfold over the corresponding subfractions from control diet-fed monkeys and contained only the larger form of apolipoprotein B typical of plasma LDL. However, the perfusate lipoproteins in the LDL density range did not have an average size or composition typical of LDL from plasma. Rather, they were relatively enriched in phospholipid and unesterified cholesterol and were deficient in cholesteryl esters. In addition, perfusate high density lipoproteins were discoidal particles. These data show that the enzyme lecithin:cholesterol acyltransferase (LCAT) was essentially inactive in these perfusates and, as a result, the dietary cholesterol-induced enrichment of perfusate d less than 1.063 g/ml lipoproteins with cholesteryl esters probably resulted from increased hepatic secretion of cholesteryl esters and not from modification of lipoproteins by LCAT during recirculating perfusion. In spite of this increase, enlarged cholesteryl ester-rich LDL were not found in the perfusate, suggesting that large molecular weight plasma LDL are not directly secreted by the liver but instead probably result from further intravascular metabolism of cholesteryl ester-enriched hepatic precursor lipoproteins.

Isolation and characterization of a cDNA clone specific for avian vitellogenin II.

A clone for vitellogenin, a major avian, estrogen responsive egg yolk protein, was isolated from the cDNA library of estrogen-induced rooster liver. Two forms of plasma vitellogenin, vitellogenin I (VTG I) and vitellogenin II (VTG II), distinguishable on the basis of their unique partial proteolysis maps, have been characterized and their corresponding hepatic precursor forms identified. We have used this criterion to specifically characterize which vitellogenin protein had been cloned. Partial proteolysis maps of BTG I and VTG II standards, synthesized in vivo, were compared to maps of protein synthesized in vitro using RNA hybrid-selected by the vitellogenin plasmid. Eight major digest fragments were found common to the in vitro synthesized vitellogenin and the VTG II standard while no fragments were observed to correspond to the VTG I map. A restriction map of the VTG II cDNA clone permits comparison to previously described cDNA and genomic vitellogenin clones.

Effect of dichloroacetate on gluconeogenesis in vivo in the presence of a fixed gluconeogenic substrate supply to the liver

Administration of dichloroacetate (DCA) to conscious 48 h fasted dogs causes a reduction in the conversion of circulating alanine and lactate to glucose coincident with both a reduction in the load of alanine and lactate delivered to the liver and a decrease in the plasma insulin-glucagon ( I G ) molar ratio. To determine whether DCA inhibits gluconeogenesis independent of its effect on alanine and lactate levels, the drug was infused concurrently with alanine and lactate in order to fix the load of these gluconeogenic precursors reaching the liver. When the circulating levels of alanine and lactate were fixed, DCA increased the net hepatic uptake of alanine markedly (98 ± 26%) but increased lactate uptake only slightly (8 ± 7%). However, the fraction of these gluconeogenic precursors which were converted to 14C-glucose decreased. Since the net effect of these changes was to increase the overall conversion of alanine and lactate to glucose, it is evident that the increase in precursor uptake overrode the inhibition of the gluconeogenic process per se. This increase in hepatic precursor uptake may have been due to DCA stimulation of pyruvate dehydrogenase activity or the observed decline in the I G molar ratio. The observed increase in overall glucose production (33 ± 10%) was probably attributable to the increase in gluconeogenesis as well as an increase in glycogenolysis since 48 h fasted dogs still have some hepatic glycogen (15 mg/gm). By overcoming the peripheral effects of DCA on gluconeogenic precursor supply it becomes evident that DCA has: (1) a direct inhibitory action on intrahepatic gluconeogenesis, and (2) an effect on the pancreas which by virtue of a change in the I G molar ratio may offset the direct intrahepatic effect of the drug.

Regulation of vitamin K-dependent carboxylation.

Vitamin K-dependent carboxylation activity measured with pentapeptide substrate (Phe-Leu-Glu-Glu-Leu) gradually decreases upon in vivo injection of vitamin K to vitamin K-deficient rats. A decrease in pentapeptide carboxylation can also be observed by the in vitro addition of antibodies against prothrombin and other vitamin K-dependent proteins to the soluble system derived from vitamin K-deficient rat liver microsomes. In both cases, adding back in vitro partially decarboxylated vitamin K-dependent proteins or purified hepatic prothrombin precursor restores the level of pentapeptide carboxylation. After warfarin treatment, a 3-fold increase in carboxylation results, which can be abolished by giving cycloheximide along with the warfarin. However, the resulting decreased activity is restored by the in vitro addition of partially decarboxylated vitamin K-dependent proteins. These data are consistent with the hypothesis that (after warfarin treatment) increased peptide carboxylation is primarily due to activation of the system by precursor proteins, rather than synthesis of an increased amount of enzyme.

Evidence for a Common Hepatic Cholesterol Precursor Site for Cholic and Chenodeoxycholic Acid Synthesis in Man

SummaryBile fistula patients were administered [14C]cholesterol and labeled mevalonic acid by constant infusion (for 5 hr) or by intravenous pulse. Bile was collected at frequent time intervals. The specific activities of newly synthesized cholic and chenodeoxycholic acids were virtually identical at all time periods (up to 24 hr). These observations provide evidence for the presence of a common cholesterol substrate pool for primary bile acid synthesis in man. Both cholic and chenodeoxycholic acids appear to be synthesized via the 7α-hydroxy-cholesterol pathway.