Transport and metabolism of pyridoxine in rat liver

Evidence, obtained with in situ perfused rat liver, indicated that pyridoxine is taken up from the perfusate by a non-concentrative process, followed by metabolic trapping. These conclusions were reached on the basis of the fact that at low concentrations (0.125 μM), the ³H of [³H]pyridoxine accumulated against a concentration gradient, but high concentrations (333 μM) of pyridoxine or 4-deoxypyridoxine prevented this apparent concentrative uptake. Under no conditions did the tissue water : perfusate concentration ratio of [³H]pyridoxine exceed unity.The perfused liver rapidly converted the labeled pyridoxine to pyridoxine phosphate, pyridoxal phosphate and pyridoxamine phosphate and released a substantial amount of pyridoxal and some pyridoxal phosphate into the perfusate. Since muscle and erythrocytes failed to oxidize pyridoxine phosphate to pyridoxal phosphate, it is suggested that the liver plays a major role in oxidizing dietary pyridoxine and pyridoxamine as their phosphate esters to supply pyridoxal phosphate which then reaches to other organs chiefly as circulating pyridoxal.     

In vivo metabolism of 4'-deoxypyridoxine in rat and man.

In rats 80 to 95% of 4'-deoxypyridoxine administered intraperitoneally, intravenously, intramuscularly, or subcutaneously was excreted in the urine within 7.5 hours. Orally administered deoxypyridoxine was also rapidly eliminated. Over one-half of the excreted material appeared as deoxypyridoxine-3-(hydrogen sulfate) and the remainder as unchanged deoxypyridoxine. Tissue concentrations of deoxypyridoxine 5'phosphate were comparable to those of pyridoxal 5'phosphate. In normal men about 50% of a single oral dose (3 to 7.5 mg/kg of body weight) appeared in the urine within 6 hours. 4'Deoxy-5-pyridoxic acid accounted for 50 to 100% of the excreted material. The remainder was unchanged deoxypyridoxine. No deoxypyridoxine-3-(hydrogen sulfate) was detected in human urine and no 4'-deoxy-5-pyridoxic acid was found in rat urine. Deoxypyridoxine 5'-phosphate was not detected in the urine of either species. The complexity of deoxypyridoxine metabolism indicated by these data suggests the use of caution in extrapolating data obtained with deoxypyridoxine to B6 metabolism in the absence of deoxypyridoxine, and particularly in extrapolating results from the rat to man. Synthesis for 4'-deoxypyridoxine-3-(ethyl carbonate), 4'-deoxypyridoxine 5'-acetate, 4'-deoxy-3-0-(2-sulfoethyl)-pyridoxine, and the metabolites are presented. These synthesis were facilitated by using ethylchloroformate conjugates and N-methylpiperazine hydrolysis to block and unblock the phenol group.     

13C and 31P NMR study of gluconeogenesis: utilization of 13C-labeled substrates by perfused liver from streptozotocin-diabetic and untreated rats

The metabolism of 13C-labeled substrates was followed by 13C and 31P NMR in perfused liver from the streptozotocin-treated rat model of insulin-dependent diabetes. Comparison was made with perfused liver from untreated littermates, fasted either 24 or 12 h. The major routes of pyruvate metabolism were followed by a 13C NMR approach that provided for the determination of the metabolic fate of several substances simultaneously. The rate of gluconeogenesis was 2-4-fold greater and beta-hydroxybutyrate production was 50% greater in liver from the chronically diabetic rats as compared with the control groups. Large differences in the distribution of 13C label in hepatic alanine were measured between diabetic and control groups. The biosyntheses of 13C-labeled glutathione and N-carbamoylaspartate were monitored in time-resolved 13C NMR spectra of perfused liver. Assignments for the resonances of glutathione and N-carbamoylaspartate were made with the aid of 13C NMR studies of perchloric acid extracts of the freeze-clamped livers. 13C NMR spectroscopy of the perfusates provided a convenient, rapid assay of the rate of oxidation of [2-13C]ethanol, the hepatic output of [2-13C]acetaldehyde, and the accumulation of [2-13C]acetate in the perfusate. By 31P NMR spectroscopy, carbamoyl phosphate was measured in all diabetic livers and an unusual P,P'-diesterified pyrophosphate was observed in one-fourth of the diabetic livers examined. Neither of these phosphorylated metabolites was detected in control liver. Both 13C and 31P NMR were useful in defining changes in hepatic metabolism in experimental diabetes.     

Purification and characterization of vitamin B6-phosphate phosphatase from human erythrocytes.

Human erythrocytes rapidly convert vitamin B6 to pyridoxal-P and contain soluble phosphatase activity which dephosphorylates pyridoxal-P at a pH optimum of 6-6.5. This phosphatase was purified 51,000-fold with a yield of 39% by ammonium sulfate precipitation and chromatography on DEAE-Sepharose, Sephacryl S-200, hydroxylapatite, and reactive yellow 86-agarose. Sephacryl S-200 chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that the enzyme was a dimer with a molecular mass of approximately 64 kDa. The phosphatase required Mg2+ for activity. It specifically catalyzed the removal of phosphate from pyridoxal-P, pyridoxine-P, pyridoxamine-P, 4-pyridoxic acid-P, and 4-deoxypyridoxine-P at pH 7.4. Nucleotide phosphates, phosphoamino acids, and other phosphorylated compounds were not hydrolyzed significantly nor were they effective inhibitors of the enzyme. The phosphatase showed Michaelis-Menten kinetics with its substrates. It had a Km of 1.5 microM and a Vmax of 3.2 mumol/min/mg with pyridoxal-P. The Vmax/Km was greatest with pyridoxal-P greater than 4-pyridoxic acid-P greater than pyridoxine-P greater than pyridoxamine-P. The phosphatase was competitively inhibited by the product, inorganic phosphate, with a Ki of 0.8 mM, and weakly inhibited by pyridoxal. It was also inhibited by Zn2+, fluoride, molybdate, and EDTA, but was not inhibited by levamisole, L-phenylalanine, or L(+)-tartrate. These properties of the purified enzyme suggest that it is a unique acid phosphatase that specifically dephosphorylates vitamin B6-phosphates.     

Metabolism of cysteinyl leukotrienes in non-recirculating rat liver perfusion. Hepatocyte heterogeneity in uptake and biliary excretion

1. The uptake, metabolism and biliary excretion of the cysteinyl leukotrienes LTC4, LTD4 and LTE4, were studied in a non-recirculating rat liver perfusion system at constant flow in both antegrade (from the portal to the caval vein) and retrograde (from the caval to the portal vein) perfusion directions. During a 5-min infusion of [3H]LTC4, [3H]LTD4 and [3H]LTE4 (10 nmol/l each) in antegrade perfusions single-pass extractions of radioactivity from the perfusate were 66%, 81% and 83%, respectively. Corresponding values for LTC4 and LTD4 in retrograde perfusions were 83% and 93%, respectively, indicating a more efficient uptake of cysteinyl leukotrienes in retrograde than in antegrade perfusions. The concentrations of unmetabolized leukotrienes in the effluent perfusate were 8-12% in antegrade and 2-4% in retrograde perfusions. [14C]Taurocholate extraction from the perfusate was inhibited by LTC4 by only 3%, suggesting that an opening of portal-venous/hepatic-venous shunts does not explain the effects of perfusion direction on hepatic LTC4 uptake. 2. Following infusion of [3H]LTC4 and [3H]LTD4, in the antegrade perfusion direction, about 80% and 87%, respectively, of the radiolabel taken up by the liver was excreted into bile. In retrograde perfusions, however, only 40% and 57%, respectively, was excreted into bile and the remainder was slowly redistributed into the perfusate, indicating that leukotrienes were taken up into a hepatic compartment with less effective biliary elimination or converted to metabolites escaping biliary excretion. The metabolite pattern found in bile was not affected by the direction of perfusion. Biliary products of LTC4 were polar metabolites (31-38%), LTD4 (27-30%), LTE4 (about 1%) and N-acetyl-LTE4 (3-4%) in addition to unmodified LTC4 (17-18%). 3. LTC4 was identified as a major metabolite of [3H]LTD4 in bile, amounting to about 20% of the total radioactivity excreted into bile. This is probably due to a gamma-glutamyltransferase-catalyzed glutamyl transfer from glutathione in the biliary compartment, as demonstrated in in vitro experiments. The presence of sinusoidal gamma-glutamyltransferase activity in perfused rat liver was shown in experiments on the hydrolysis of infused gamma-glutamyl-p-nitroanilide. 90% inhibition of this enzyme activity by AT-125 did not affect the metabolism of LTC4. 4. When [3H]LTE4 was infused in the antegrade perfusion direction, biliary metabolites comprised N-acetyl-LTE4 (24%) and polar components (60%).(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of chlorpromazine and methylprednisolone on perfusion preservation of rabbit livers

The isolated perfused rabbit liver was used to determine how continuous hypothermic perfusion affected liver function. Rabbit livers were perfused for 0, 24, 48, and 72 hr at 5 degrees C with the UW perfusate containing hydroxyethyl starch (5 g%) dissolved in a solution containing gluconate (80 mM), adenosine (5 mM), glutathione (3 mM), phosphate (25 mM), and additives as described previously, and they were used successfully for kidney preservation. At the end of preservation the livers were perfused in an isolated circuit with a Krebs-Henseleit solution with addition of 4 g% bovine serum albumin and 10 mM glucose at 38 degrees C for 120 min. Bile was collected from the cannulated common duct. Biliary excretions of indocyanine green and liver enzymes lactate dehydrogenase, aspartate aminotransferase, and alanine aminotransferase, were determined both in the cold perfusate and the normothermic perfusate. Livers were also studied after pretreatment of the donor with chlorpromazine (CPZ) and/or methylprednisolone (MP). Bile production (ml/120 min, 100 g liver) upon reperfusion produced the most interesting data and decreased from a control value of 10.3 +/- 2.6 to 9.3 +/- 1.0 (24 hr), 5.3 +/- 0.7 (48 hr), and 4.1 +/- 1.5 (72 hr). Enzyme release was not predictive of the degree of preservation-induced damage. Pretreatment of rabbits with a combination of CPZ/MP improved bile flow at 48 and 72 hr (8.3 +/- 3.0 and 7.0 +/- 1.3, P less than 0.05). Pretreatment with either drug alone also improved function after 72 hr of preservation (7.1 +/- 1.8, CPZ; 8.2 +/- 3.5, MP).(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibitory effects of pyridoxal phosphate, ascorbate and aminoguanidine on nonenzymatic glycosylation

Nonenzymatic glycosylation of serum albumin was studied in the presence of naturally occurring metabolites, pyridoxal, pyridoxal phosphate and ascorbate/dehydroascorbate, and a hydrazine compound, aminoguanidine. Pyridoxal, pyridoxal phosphate, ascorbate and dehydroascorbate, at concentrations of 0.1 mM or greater, significantly inhibited the nonenzymatic glycosylation of albumin. Aminoguanidine was the most potent inhibitor of nonenzymatic glycosylation and 54% or 85% inhibition occurred when 5 or 50 mM aminoguanidine, respectively, was present in the incubation mixture containing 20 mM glucose. A major effect of aminoguanidine was to lower the free glucose concentration in the incubation mixture by a direct reaction with glucose as judged by thin layer chromatography. The present studies suggest that vital metabolites such as pyridoxal phosphate and ascorbate may be potentially important in controlling glucose-induced nonenzymatic glycosylation of proteins. Pyridoxal phosphate forms a Schiff base with proteins as does glucose and therefore may be a preferable drug, over aminoguanidine which is a hydrazine, for inhibiting the effects of glucose-induced nonenzymatic glycosylation.     

Metabolism and excretion of exogenous adenosine 3':5'-monophosphate and guanosine 3':5'-monophosphate. Studies in the isolated perfused rat kidney and in the intact rat.

Isolated rat kidneys were perfused with a recirculating medium containing exogenous adenosine 3':5'-monophosphate (cyclic AMP) or guanosine 3':5'-monophosphate (cyclic GMP) at an initial concentration of 0.1 mM. Both cyclic nucleotides were rapidly removed from the perfusate. Urinary excretion accounted for about 20% and 40% of the respective cyclic AMP and cyclic GMP lost from the perfusate. The metabolism of the cyclic nucleotides was studied by 14C-labeled cyclic nucleotides in the perfusate. During 60 min, 30% of added cyclic [14C]AMP was metabolized to renal [14C]adenine nucleotides (ATP, ADP, and AMP) and 30% to perfusate [14C]uric acid. Similarly, 20% of cyclic[14C]GMP was metabolized to renal [14C]guanine nucleotides (GTP, GDP, and GMP) and 30% to perfusate [14C]uric acid. Urine contained principally unchanged 14C-labeled cyclic nucleotide. Addition of 0.1 mM cyclic AMP to the perfusate elevated the renal ATP and ADP contents 2-fold. Addition of 0.1 mM of either cyclic AMP or cyclic GMP to the perfusate also elevated the renal production of uric acid 2- to 3-fold. The production and distribution of metabolites of exogenous cyclic nucleotides were also studied in the intact rat. Within 60 min after injection, 3.3 mumol of either 14C-labeled cyclic AMP or cyclic GMP was cleared from the plasma. Kidney cortex and liver were the principal tissues for 14C accumulation. Urinary excretion accounted for about 20 and 45% of the cyclic [14C]AMP and cyclic [14C]GMP lost from the plasma, respectively. The 14C found in the kidney and liver was present almost entirely as the respective purine mono-, di-, and trinucleotides. The other principal metabolite was [14C]allantoin, found in the urine and, to a lesser extent, the liver. The urine contained mostly unchanged 14C-labeled cyclic nucleotide. Unlike the findings with the perfused kidney, [14C]uric acid was not a significant metabolite of the 14C-labeled cyclic nucleotides in these in vivo experiments.     

The effect of rejuvenation of aged erythrocytes on biochemical parameters in the perfused hind limb muscle preparation

(1) A systematic investigation was carried out into the use of time-expired erythrocytes in an isolated perfused skeletal muscle preparation. Comparisons were made between erythrocytes subjected to a process of 'rejuvenation' (Rennie and Holloszy (1977), Biochem. J. 168, 161-170) and untreated erythrocytes (controls). (2) The use of rejuvenated erythrocytes had no significant effect on concentrations of muscle ATP, phosphocreatine and lactate, nor fractional rates of muscle protein synthesis. However, muscle water concentrations were reduced when compared to controls. (3) There was an influx of K+ from the plasma into rejuvenated erythrocytes. This was accompanied by a substantial loss (17%) of intramuscular K+. There was also loss of K+ from control preparations but this amounted to approx. 1% of muscle content. (4) Erythrocyte fragility was greater in the control perfusate (6%, haemolysis) when compared to the medium with rejuvenated cells (1%, haemolysis). As a consequence of either erythrocyte storage, rejuvenation or haemolysis, plasma concentrations of phosphate, magnesium, calcium and potassium were significantly different from starting values, by as much as 300% in both groups, and varied throughout the study. (5) It is concluded that the use of rejuvenated erythrocytes does not confer any advantage in unexercised perfused skeletal muscle preparations. However, both types of erythrocyte induce changes in perfusate composition relative to starting or in vivo profiles.     

Effect of fish oil diet on hepatic lipid metabolism in nonhuman primates: lowering of secretion of hepatic triglyceride but not apoB

African green monkeys were fed diets containing either 11% (by weight) fish oil or lard for 2.5 yr. To test the hypothesis that fish oil decreases hepatic secretion of triglyceride (TG) and apoB, livers from these animals were perfused with a fatty acid mixture [85% (w/w) oleate containing [14C]oleate and 15% n-3 containing [3H]eicosapentaenoic acid (EPA)] at a rate of 0.1 mumol fatty acid/min per g liver. Liver perfusate was sampled every 30 min during 4 h of recirculating perfusion. The concentration of triglyceride was similar for livers of animals of both groups and there was no difference between groups in the extent of incorporation of [3H]EPA or [14C]oleate into hepatic TG. While the secretion rate for the mass of TG was less in the fish oil-fed group (8.3 +/- 2.5 vs 18.3 +/- 4.4 mg/h per 100 g liver, P less than 0.05), the apoB secretion rate was similar (0.92 +/- 0.15 vs 1.01 +/- 0.13 mg/h per 100 g liver). Significantly less [3H]EPA was incorporated into secreted TG in the fish oil group (0.4 +/- 0.1 vs 1.0 +/- 0.1% infused dose/h; P less than 0.01). The rate of secretion of [14C]TG was similar for both groups (1.3 +/- 0.3 vs 1.4 +/- 0.1% infused dose/h for fish oil and lard groups, respectively). No significant diet-related differences in [3H]TG or [14C]TG fatty acid specific activity were observed for perfusate TG or hepatic TG. After perfusion, livers from fish oil-fed monkeys contained significantly more [3H]EPA in hepatic phospholipid than livers from lard-fed monkeys (19.5 +/- 1.8 vs 11.4 +/- 1.7% infused dose; P less than 0.01) although hepatic phospholipid mass concentrations were similar. The liver phospholipids of the fish oil group were enriched in n-3 fatty acid mass and were relatively depleted of oleate and linoleate. We conclude that although apoB secretion was unaffected, dietary fish oil significantly decreased hepatic TG secretion through relatively poor utilization of EPA for the synthesis of TG destined for secretion in VLDL; at the same time, increased incorporation of [3H]EPA into hepatic phospholipid accompanied the decreased incorporation into secreted TG and these events may be coupled.(ABSTRACT TRUNCATED AT 400 WORDS)     

A physical-chemical model for cellular uptake of fatty acids: prediction of intracellular pool sizes

If the uptake of fatty acids by liver is a physical, not a biological, process, then the size and location of the intrahepatic pool of fatty acids can be predicted from uptake rates and thermodynamic data. The purpose of the experiments in this paper was to test the accuracy of this idea. Rat livers were perfused with palmitate bound to albumin, and the total amounts of palmitate removed from the perfusate were measured at 3-s intervals. The intrahepatic pools of palmitate calculated from these data were 13.8 and 23.0 nmol/g of liver at ratios of palmitate/albumin (mol/mol) (afferent side) of 2/1 and 4/1, respectively, in the steady state. The intrahepatic pools of palmitate calculated from the distributions of palmitate between membranes, H2O, albumin, and fatty acid binding protein and the measured first-order rate constants for acyl-CoA ligases in mitochondria and microsomes were 12.1 and 34.6 nmol/g for perfusate ratios of palmitate/albumin of 2/1 and 4/1, in the steady state. Intrahepatic pools of palmitate measured after establishment of a steady-state rate of uptake were 15.0 and 31.8 nmol/g for these ratios of palmitate/albumin of 2/1 and 4/1.     

Binding of 3-hydroxybenzo[a]pyrene to bovine hemoglobin and albumin

Previous studies examined the bioavailability and first-pass biotransformation of 3-hydroxy[(3)H]benzo[a]pyrene ([(3)H]-3-OHBaP) in an isolated perfused catfish intestinal model. This work showed that 3-OHBaP, or a metabolite formed in intestine, bound covalently to blood protein. In this study, the blood adducts were characterized in vitro by incubating bovine ferric hemoglobin or albumin with [(3)H]-3OHBaP under various conditions. Incubation of 2 microM [(3)H]-3-OHBaP with hemoglobin for 1 h resulted in 7.49 pmol bound/mg protein, while albumin binding was 1.37 pmol/mg protein. Mild acid hydrolysis released only 5% of the radioactivity from 3-OHBaP-hemoglobin adducts. After gel filtration, the 3-OHBaP-hemoglobin adducts were examined by HPLC analysis. A single peak of radioactivity was detected at the same retention time as the heme component of hemoglobin. Unbound 3-OHBaP was oxidized to BaP-3,6-dione during incubation with ferric hemoglobin. Treatment of hemoglobin with ascorbic acid decreased the formation of hemoglobin adducts by 33%, while hydrogen peroxide treatment increased adduct formation by 44%. Incubation of [(3)H]-BaP-3-beta-D-glucuronide (BaP-3G) with hemoglobin and beta-glucuronidase resulted in greater binding to hemoglobin than incubation with [(3)H]-3-OHBaP alone. The hemoglobin adduct obtained from [(3)H]-BaP-3G also co-migrated with heme. These results indicate that an oxidative process is involved in formation of the heme adduct and that 3-OHBaP or BaP-3G might be a precursor of the bound metabolite.     

Plasma protein catabolism by the perfused rat liver. The effect of alteration of albumin concentration and dietary protein depletion

1. The isolated perfused rat liver was used to study degradation rates of plasma albumin, transferrin and fibrinogen. 2. Constant fractional rates were observed for all three proteins even when the albumin concentration was drastically increased by the addition of large amounts to the perfusate pool. 3. Livers taken from rats deprived of dietary protein for 14-18 days showed greatly diminished fractional catabolic rates for albumin when perfused with blood from similarly deprived animals. 4. These rates could be restored to near-normal values by adding albumin or by perfusing with blood from normally fed rats. 5. These findings are consistent with the theory of pinocytosis as a step in the degradation of plasma proteins by hepatic parenchymal cells.     

Inflammation causes tissue-specific depletion of vitamin B<Subscript>6</Subscript>

Previously we observed strong and consistent associations between vitamin B₆ status and several indicators of inflammation in patients with rheumatoid arthritis. Clinical indicators, including the disability score, the length of morning stiffness, and the degree of pain, and biochemical markers, including the erythrocyte sedimentation rate and C-reactive protein levels, were found to be inversely correlated with circulating vitamin B₆ levels. Such strong associations imply that impaired vitamin B₆ status in these patients results from inflammation. In the present study we examined whether inflammation directly alters vitamin B₆ tissue contents and its excretion in vivo. A cross-sectional case-controlled human clinical trial was performed in parallel with experiments in an animal model of inflammation. Plasma and erythrocyte and pyridoxal 5'-phosphate concentrations, urinary 4-pyridoxic acid excretion, and the activity coefficient of erythrocyte aspartate aminotransferase were compared between patients and healthy subjects. Adjuvant arthritis was induced in rats for investigating hepatic and muscle contents as well as the urinary excretion of vitamin B₆ during acute and chronic inflammation. Patients with rheumatoid arthritis had low plasma pyridoxal 5'-phosphate compared with healthy control subjects, but normal erythrocyte pyridoxal 5'-phosphate and urinary 4-pyridoxic acid excretion. Adjuvant arthritis in rats did not affect 4-pyridoxic acid excretion or muscle storage of pyridoxal 5'-phosphate, but it resulted in significantly lower pyridoxal 5'-phosphate levels in circulation and in liver during inflammation. Inflammation induced a tissue-specific depletion of vitamin B₆. The low plasma pyridoxal 5'-phosphate levels seen in inflammation are unlikely to be due to insufficient intake or excessive vitamin B₆ excretion. Possible causes of decreased levels of vitamin B₆ are discussed.     

Acid-base and respiratory properties of a buffered bovine erythrocyte perfusion medium

Current research in organ physiology often utilizes in situ or isolated perfused tissues. We have characterized a perfusion medium associated with excellent performance characteristics in perfused mammalian skeletal muscle. The perfusion medium consisting of Krebs-Henseleit buffer, bovine serum albumin, and fresh bovine erythrocytes was studied with respect to its gas-carrying relationships and its response to manipulation of acid-base state. Equilibration of the perfusion medium at base excess of -10, -5, 0, 5, and 10 mmol X L-1 to humidified gas mixtures varying in their CO2 and O2 content was followed by measurements of perfusate hematocrit, hemoglobin concentration, pH, Pco2, Cco2, Po2, and percent oxygen saturation. The oxygen dissociation curve was similar to that of mammalian bloods, having a P50 of 32 Torr (1 Torr = 133.3 Pa), Hill's constant n of 2.87 +/- 0.15, and a Bohr factor of -0.47, showing the typical Bohr shifts with respect to CO2 and pH. The oxygen capacity was calculated to be 190 mL X L-1 blood. The carbon dioxide dissociation curve was also similar to that of mammalian blood. The in vitro nonbicarbonate buffer capacity (delta [HCO3-] X delta pH-1) at zero base excess was -24.6 and -29.9 mmol X L-1 X pH-1 for the perfusate and buffer, respectively. The effects of reduced oxygen saturation on base excess and pH of the medium were quantified. The data were used to construct an acid-base alignment diagram for the medium, which may be used to quantify the flux of nonvolatile acid or base added to the venous effluent during tissue perfusions.     

Synthesis of antithrombin III and alpha-1-antitrypsin by the perfused rat liver

Livers isolated from control or turpentine-injected rats were perfused for 3 h with human red cells suspended in Krebs-Henseleit solution containing bovine serum albumin, dextran, glucose, heparin, cortisol, insulin, a mixture of 20 amino acids and [³H]leucine. Changes in the concentrations of antithrombin III and α-1-antitrypsin were evaluated by rocket immunoelectrophoresis using specific antisera, and incorporation of the ³H radioactivity into the total protein, albumin, antithrombin III and α-1-antitrypsin in the perfusate was measured. The results indicate that both antithrombin III and α-1-antitrypsin are synthesized in the liver. Local inflammation induced in the liver donors moderately stimulated the synthesis of α-1-antitrypsin but it affected only marginally that of antithrombin III.     

Synthesis of antithrombin III and alpha-1-antitrypsin by the perfused rat liver.

Livers isolated from control or turpentine-injected rats were perfused for 3 h with human red cells suspended in Krebs-Henseleit solution containing bovine serum albumin, dextran, glucose, heparin, cortisol, insulin, a mixture of 20 amino acids and [3H]leucine. Changes in the concentrations of antithrombin III and alpha-1-antitrypsin were evaluated by rocket immunoelectrophoresis using specific antisera, and incorporation of the 3H radioactivity into the total protein, albumin, antitrhombin III and alpha-1-antitrypsin in the perfusate was measured. The results indicate that both antithrombin III and alpha-1-antitrypsin are synthesized in the liver. Local inflammation induced in the liver donors moderately stimulated the synthesis of alpha-1-antitrypsin but it affected only marginally that of antithrombin III.     

The conversion of orotic acid into uridine 5′-monophosphate by isolated perfused normal and regenerating rat livers

The release of (14)CO(2) from [7-(14)C]orotic acid was measured in isolated perfused normal and regenerating rat livers. With some limitations, the release of (14)CO(2) from [7-(14)C]orotic acid can be used to estimate UMP synthesis in perfused livers. Isolated perfused livers rapidly pick up labelled orotic acid added to perfusate and convert most of it into UMP. Perfused regenerating livers produce approx. 2.5 times as much UMP/g of liver as do perfused normal livers. However, the absolute amount of orotic acid converted into UMP is higher in perfused normal livers than in perfused regenerating livers. Perfused regenerating livers do not differ in their orotic acid uptake and UMP synthesis from livers of comparable size in which regeneration is not taking place. The total amount of orotic acid taken up by the liver (rather than the rate of uptake) and the size of the liver appear to be the determining factors in UMP production. The results suggest that the decrease in liver size caused by partial hepatectomy may be in itself sufficient to account for an increase in the flow of metabolites in the pyrimidine pathway at the early stages of liver regeneration.     

Quantification of the hepatic contribution to the catabolism of high density lipoproteins in rats.

Isolated rat livers were perfused for four hours in a recirculating system containing washed rat erythrocytes. Biologically screened radioiodinated rat high density lipoproteins (1.090 < d < 1.21 g/ml) were added to the perfusate with different amounts of whole serum to supply unlabeled rat high density lipoproteins. The protein moiety of the lipoprotein contained more than 95% of the radioiodine. The fraction of apolipoprotein mass degraded during the perfusion was quantified by the linear increment of non-protein-bound radioiodine in the perfusate, corrected for the increment observed during recirculation of the perfusate in the absence of a liver. The small amount of (131)I secreted into bile was added to calculate the fractional catabolic rate. The fractional catabolic rate ranged from 0.22 to 0.63% per hour in 12 experiments and was inversely related to the size of the perfusate pool of high density apolipoprotein. The absolute catabolic rate of high density apolipoprotein (fractional catabolic rate x pool size) in three livers in which the concentration of rat HDL in the perfusate approximated that in intact rats was 69.5 +/- 10.4 micro g hr(-1) (mean +/- SD). The rate of disappearance of cholesteryl esters of rat high density lipoproteins (labeled biologically by injecting donor rats with [5-(3)H]mevalonic acid) from the liver perfusate did not exceed that of the apoprotein component. These rates were compared with catabolic rates for rat high density lipoproteins in intact rats. Fractional catabolic rate in vivo, obtained by multicompartmental analysis of the disappearance curve of (131)I-high density apolipoprotein from blood plasma, was 11.9 +/- 1.3% hr(-1) (mean +/- SD). Total catabolic rate in vivo (fractional catabolic rate x intravascular pool of high density apolipoprotein) was 986 +/- 145 micro g hr(-1) (mean +/- SD). The results suggest that only a small fraction of high density lipoproteins in blood plasma of rats is degraded directly by the liver.-Sigurdsson, G., S-P. Noel, and R. J. Havel. Quantification of the hepatic contribution to the catabolism of high density lipoproteins in rats.     

Intrahepatic conversion of a glutathione conjugate to its mercapturic acid. Metabolism of 1-chloro-2,4-dinitrobenzene in isolated perfused rat and guinea pig livers

Because of the low hepatic activity of gamma-glutamyl-transferase in the rat, the liver is generally considered to play only a minor role in the degradation of glutathione conjugates, a limiting step in mercapturic acid formation. Recent findings indicate, however, that the liver has a prominent role in glutathione catabolism, particularly in species other than rat. To examine the contributions of liver to mercapturic acid biosynthesis, mercapturate formation was compared in isolated perfused livers from rats and guinea pigs dosed with either 0.3 or 3.0 mumol of 1-chloro-2,4-dinitrobenzene (CDNB). Chemically synthesized glutathione conjugate, mercapturic acid, and intermediary metabolites of CDNB were used as standards in the high performance liquid chromatography analysis of bile and perfusate samples. Biliary excretion accounted for almost all of the recovered metabolites. A marked species difference was observed in the pattern of CDNB metabolism. Rat livers dosed with 0.3 mumol of CDNB excreted 55% of total biliary metabolites as the glutathione conjugate and 8.2% as the mercapturic acid, whereas guinea pig livers excreted only 4.8% as the glutathione conjugate and 47% as the mercapturate. Mercapturic formation was also dose-dependent, with a larger fraction formed at the 0.3- versus the 3.0-mumol dose (8.2 versus 3.7% in the rat; 47 versus 19% in the guinea pig). Hepatic conversion of the glutathione conjugate to the mercapturic acid was markedly inhibited in both species after retrograde intrabiliary infusion of acivicin, an inhibitor of gamma-glutamyltransferase activity. These findings provide direct evidence for intrahepatic biosynthesis of mercapturic acids. Thus, glutathione conjugates synthesized within hepatocytes are secreted into bile and broken down to cysteine conjugates; the latter are then presumably reabsorbed by the liver, N-acetylated to form the mercapturic acid and re-excreted into bile.