Catabolism In Liver Research Articles

In Vivo Fate of Hemopexin and Heme-Hemopexin Complexes in the Rat

The disposition in the rat of the plasma heme-binding protein hemopexin (Hx), as the native apoprotein and as its heme complex (HHx), has been studied using the residualizing protein label dilactitol-125I-tyramine (*I-DLT). The aim of this work was to identify the tissue sites of Hx uptake and catabolism, independent of heme binding, and to evaluate how heme loading affects Hx catabolism at these sites. *I-DLT-Hx had a circulating half-life of 1.2 days and was recovered in degraded form in comparable amounts in visceral (liver, kidney, spleen) and peripheral (skin, muscle) tissues, indicating a generalized diffuse catabolism of the protein throughout the body. The plasma half-life of *I-DLT-Hx injected as a preformed heme-Hx complex was the same as that of the apoprotein; however, injection of the complex resulted in about a twofold increase in hepatic degradation of Hx. The lack of an effect of heme on overall catabolism of the preformed HHx complex was consistent with the 1-h half-life of heme, injected as 14C-heme-Hx, in the circulation; however, as much as 20-fold more 14C-heme than Hx protein was recovered in liver from 14C-heme-Hx. The absolute amount of *I-DLT-Hx degraded in liver was significantly increased when heme was injected in excess of the heme binding capacity of circulating Hx, while 131I-DLT-albumin catabolism in liver was unaffected. Thus, depending on the physiological conditions studied, the data are consistent with a model in which, following hepatic uptake of heme from HHx, varying proportions of the protein are either returned to the circulation or degraded in the liver.

Action of Enterally Administered Ornithine α‐Ketoglutarate on Protein Breakdown in Skeletal Muscle and Liver of the Burned Rat

Several studies concerning burn patients have shown that supplementation of enteral nutrition with ornithine alpha-ketoglutarate (OKG) favorably modifies protein metabolism. Therefore, the effect of OKG administration on muscular and hepatic protein catabolism was evaluated in burned rats. Four groups of six rats were used. Two groups were scalded by immersion of the dorsum in water at 90 degrees C for 10 seconds and then starved for 24 hours. Controlled enteral nutrition was then administered in three boluses daily (Osmolite, 210 kcal/kg/d, 1.2 g N/kg/d); one group was supplemented with OKG (5 g/kg/d, ie, 0.68 g N/kg/d), while the other group received an equivalent amount of nitrogen in the form of glycine. One group of healthy control rats received Osmolite supplemented with glycine and the last group was fed ad libitum. The animals were killed after 2 days of nutrition. Protein catabolism was assessed in vitro by measuring the amount of valine (liver catabolism) and phenylalanine (muscle catabolism) released into the incubation medium of isolated tissues. Tissular and serum glutamine were also assayed. Burn injury induced muscle hypercatabolism without affecting hepatic catabolism. The administration of OKG limited both muscle weight loss and muscle protein hypercatabolism and significantly improved the muscle glutamine pool. These results demonstrate the nitrogen-sparing effect of OKG in muscle in hypercatabolic states.

Erythropoietin life span in rats with hypoplastic and hyperplastic bone marrows

The metabolic fate of erythropoietin (EPO) remains unknown. Urinary excretion does not appear to play a major role and liver catabolism has been shown to occur only after terminal sugars on the hormone have been removed. However, it has been proposed that EPO is eliminated by consumption in the bone marrow. In order to examine the extent of such consumption we measured the half-life of radioidinated recombinant EPO injected intravenously (IV) to rats with bone marrows suppressed by cyclophosphamide or hypertransfusion and marrows stimulated by phenylhydrazine or bleeding. The mean half-life or erythropoietin in normal rats was 179 +/- 16 min, with similar half-lives found in the other rats regardless of decreased or increased bone marrow activity. The results indicate that it is unlikely that erythroid activity determines EPO life span and catabolism.

Comparison of the metabolic behavior of rat apolipoproteins A-I AND A-IV, isolated from both lymph chylomicrons and serum high density lipoproteins

1. 1. Rat apolipoprotein (apo) A-I and A-IV, isolated from both lymph chylomicrons and serum high density lipoproteins (HDL) were analyzed by isoelectric focusing. 2. 2. Lymph chylomicron apo A-I consisted for 81 ± 2% of the pro form and for 19 ± 2% of the mature form, while apo A-I isolated from serum HDL was present for 36 ± 4% in the pro form and for 64 ± 4% in the mature form. Apo A-IV also showed two major protein bands after analysis by isoelectric focusing. 3. 3. The most prominent component is the more basic protein that amounts to 80 ± 2% in apo A-IV isolated from lymph chylomicrons and to 60 ± 3% in apo A-IV isolated from serum HDL. 4. 4. Apo A-I (or apo A-IV), isolated from both sources (lymph chylomicrons or serum HDL), was iodinated and the radioactive apolipoproteins were incorporated into rat serum lipoproteins. 5. 5. The resulting labeled HDL was isolated from serum by molecular sieve chromatography on 6% agarose columns and injected intravenously into rats. 6. 6. No difference in the fractional turnover rate or the tissue uptake of the two labeled HDL preparations was observed, neither for apo A-I nor for apo A-IV. 7. 7. It is concluded that the physiological significance of the extracellular pro apo A-I conversion or the post-translational modification of apo A-IV is not related to the fractional turnover rate in serum or to the rate of catabolism in liver and kidneys.

The influence of starvation and tryptophan administration on the metabolism of phenylalanine, tyrosine and tryptophan in isolated rat liver cells.

Liver cells from fed Sprague-Dawley rats metabolized phenylalanine, tyrosine and tryptophan at rates consistent with the known kinetic properties of the first enzymes of each pathway. Starvation of rats for 48 h did not increase the maximal activities of phenylalanine hydroxylase, tryptophan 2,3-dioxygenase and tyrosine aminotransferase in liver cell extracts, when results were expressed in terms of cellular DNA. Catabolic flux through the first two enzymes was unchanged; that through the aminotransferase was elevated relatively to enzyme activity. This is interpreted in terms of changes in the concentrations of 2-oxoglutarate and glutamate. Cells from tryptophan-treated animals exhibited significant increases in the catabolism of tyrosine and tryptophan, but not of phenylalanine. The activities of tyrosine aminotransferase and tryptophan 2,3-dioxygenase were also increased, although the changes in flux and enzyme activity did not correspond exactly. These results are discussed with reference to the control of aromatic amino acid catabolism in liver; the role of substrate concentration is emphasized.

Changes in protein degradation in chickens due to an inflammatory challenge.

Tissue-specific changes in protein catabolism were examined in chicks 16 hr following an inflammatory challenge. It was determined that tyrosine was not catabolized or converted to phenylalanine in muscle, thymus, bursa, or spleen. Therefore, rates of tyrosine release from protein were used to estimate rates of protein catabolism in these tissues. Arginine was not catabolized to urea by chick liver; consequently, arginine release from liver protein was used to measure protein catabolism in this tissue. An injection of sheep red blood cells (SRBC) or Escherichia coli did not change rates of protein catabolism in liver or bursa as compared to saline-injected controls. SRBC significantly increased protein catabolism in muscle and spleen by 29 and 15%, respectively. E. coli resulted in significant increases in muscle, spleen, and thymus of 43, 30, and 34%, respectively. These changes in protein catabolism, together with known changes in protein synthesis, suggest that an inflammatory response to SRBC and E. coli result in increased protein accretion in the bursa and liver, and net protein loss from muscle.

Effects of dietary soy protein on liver catabolism and plasma transport of cholesterol in hypercholesterolemic rats

Sprague-Dawley rats were fed hypercholesterolemic regimens with either soy protein (S) or casein (HC) as the protein source. Lipid, lipoprotein composition as well as lecithin cholesterol acyltransferase activity (LCAT) were measured in plasma. Liver lipid levels and enzyme activities, cholesterol 7α-hydroxylase and acylCoA cholesterol acyltransferase (ACAT), were also determined. Plasma cholesterol levels were significantly reduced with the S as compared with the HC diet, but LCAT activity was not modified. Cholesterol 7α-hydroxylase, which increases in animals fed hypercholesterolemic diets as compared to controls, was unaffected when S was substituted for casein. In contrast, higher cholesterol ester levels and an elevated ACAT activity were observed in the liver of rats on the soybean diet. Substitution of casein with soy protein reduced cholesterol levels in all lipoprotein fractions but no changes were observed in apoproteins.

Characteristics of acid lipase and acid cholesteryl esterase activity in parenchymal and non-parenchymal rat liver cells

1.(1) Parenchymal and non-parenchymal cells were isolated from rat liver. The characteristics of acid lipase activity with 4-methylumbelliferyl oleate as substrate and acid cholesteryl esterase activity with cholesteryl[1-14C]oleate as substrate were investigated. The substrates were incorporated in egg yolk lecithin vesicles and assays for total cell homogenates were developed, which were linear with the amount of protein and time. With 4-methylumbelliferyl oleate as substrate, both parenchymal and non-parechymal cells show maximal activities at acid pH and the maximal activity for non-parenchymal cells is 2.5 times higher than for parenchymal cells. It is concluded that 4-methylumbelliferyl oleate hydrolysis is catalyzed by similar enzyme(s) in both cell types.2.(2) With cholesteryl[1-14C] oleate as substrate both parenchymal and nonparenchymal cells show maximal activities at acid pH and the maximal activity for non-parenchymal cells is 11.4 times higher than for parenchymal cells. It is further shown that the cholesteryl ester hydrolysis in both cell types show different properties.3.(3) The high activity and high affinity of acid cholesteryl esterase from non-parenchymal cells for cholesterol oleate hydrolysis as compared to parenchymal cells indicate a relative specialization of non-parenchymal cells in cholesterol ester hydrolysis. It is concluded that non-parenchymal liver cells possess the enzymic equipment to hydrolyze very efficiently internalized cholesterol esters, which supports the suggestion that these cell types are an important site for lipoprotein catabolism in liver.

Regulation of glutamine catabolism in the perfused guinea-pig liver in relation to ureogenesis and gluconeogenesis

1. 1. L-Glutamine conversion into ammonia, urea and glucose by the perfused liver of 48 hr starved guinea-pigs was concentration dependent attaining the maximal rate at 4 mM. 2. 2. The activity of glutaminase I (EC 3.5.12), measured in isolated liver mitochondria was high enough to account for the observed rate of ammonia, urea and glucose formation by the perfused liver. Neither NH4C1 (5 mM) nor aminooxyacetate (0.5 mM) affected the rate of glutamine conversion into glutamate by isolated liver mitochondria. 3. 3. Gluconeogenesis and ureogenesis from glutamine was inhibited by octanoate, Dt-3-hydroxybutyrate, aminooxyacetate, ethanol and p-hydroxyphenylpyruvate while ammonia formation was stimulated by aminooxyacetate. 2,4-Dinitrophenol stimulated the rate of the formation of all three metabolites from glutamine. 4. 4. The major changes induced by aminooxyacetate, as determined in livers perfused with glutamine and stopped by freeze-clamping technique, consisted in a decrease in the content of ATP, aspartate and malate and in a slight increase in the content of glutamate. 5. 5. Glutamine is an effective precursor of phosphoenolpyruvate in isolated liver mitochondria. Its formation was inhibited by octanoate and by DL-3-hydroxybutyrate. 6. 6. The data are discussed in terms of regulation of glutamine catabolism in liver with emphasis on ureogenesis and gluconeogenesis.

Changes in Organ Growth with Feeding Pattern. The Influence of Feeding Frequency on the Circadian Rhythm of Protein Synthesis in the Rat

The effect of eating one large meal rather than several small meals per day on protein metabolism and the growth of individual organs was investigated in young male rats. Meal-eating did not affect the rate of protein catabolism in liver, kidney, small intestine, or spleen in vivo compared with continously fed control animals that consumed the same total amount of food. A circadian rhythm of protein synthesis was found in liver and kidney slices taken from normal rats killed at various times; starvation reduced the magnitude of protein synthesis but did not alter its cyclical nature. Consumption of the daily food all in one meal distorted the circadian rhythm, particularly when it was taken in the morning, and a morning meal increased the total 24 hour synthesis of protein in liver whereas an evening meal did not. Meal-feeding in the morning increased the weights of the liver, small intestine and tibia compared with continuously fed rats, but meal-feeding in the evening did not.

Study of the developmental pattern of heme catabolism in liver and the effects of cobalt on cytochrome P-450 and the rate of heme oxidation during the neonatal period.

The comparative development patterns of heme oxidation andof cytochrome P-450 dependent drug oxidation in rat liver were examined. High levels of heme oxygenase activity were present in whole embryo preparations at day 13 of gestation. At birth this enzyme activity in liver was approximately equal to that of normal adult liver. In the immediate postnatal period the rate of hepatic heme oxidation increased sharply, reaching levels 3-5 times normal during the first week postpartum. Thereafter, this enzyme activity progressively decreased and returned to normal adult levels by the 28th postpartum day. The development of microsomal heme oxidation and of P-450 dependent drug oxidation exhibited reciprocal patterns, with the latter being at low levels of activity during the immediate postnatal period and reaching adult activity only 4 or more wk after birth. Cobalt injected into pregnant animals or in to nursing mothers did not induce heme oxygenase in the fetus or suckling neonate. However, when treated directly with the metal, 4-day old neonates exhibited a small induction response of this enzyme; and the inducibility of heme oxygenase increased gradually to fully adult levels by the end of the 4th postpartum week. Cobalt at all postnatal developmental stages was capable of diminishing hepatic contents of total microsomal heme and P-450; however this effect of the metal was small in the immediate period after birth and increased progressively with maturation. These findings demonstrate that the patterns of development of hepatic capacity for carrying out the oxidation of heme and the P-450 dependent oxidation of drugs are different and thus provide further evidence that these microsomal enzyme systems are distinct from each other and under separate regulatory mechanisms. The degree of induction response for hepatic heme oxygenase evoked by the trace metal, cobalt, was also shown to have developmental determinants as did the susceptibility of hepatic cytochrome P-450 to degradation by this metal. The very high levels of hepatic heme oxygenase activity which characterize neonates during the first week of life indicate that over-production of bilirubin contributes significantly to the mechanism of neonatal jaundice.

The effect of phenobarbital, 3-methylcholanthrene, 3,4-benzpyrene, and pregnenolone-16α-carbonitrile on microsomal heme oxygenase and splenic cytochrome P-450

Inducers of the microsomal electron transport system (cytochrome P-450 and NADPH-cytochrome P-450 reductase), including phenobarbital, 3-methylcholanthrene, 3,4-benzpyrene, and pregnenolone-16α-carbonitrile, were tested for their effects on hepatic and splenic heme oxygenase activity and splenic cytochrome P-450 content in male Holtzman rats. Phenobarbital treatment caused no induction of heme oxygenase or splenic cytochrome P-450. 3-Methylcholanthrene administration produced a significant decrease in hepatic and splenic heme oxygenase activity but with no discernible change in splenic cytochrome P-450. 3,4-Benzpyrene treatment caused a decrease in splenic heme oxygenase specific activity, no change in hepatic heme oxygenase, and in only one experiment marked induction of splenic cytochrome P-450. Pregnenolone-16α-carbonitrile caused slight but variable decreases in hepatic and splenic heme oxygenase activity but was the most potent inducer of splenic cytochrome P-450 tested. These results suggest that although the elements of the microsomal electron transport system are involved in oxidative heme catabolism in liver and spleen, the process is regulated in a unique manner as compared with the metabolism of other xenobiotics and polycyclic hydrocarbons.

Protein Turnover in Skeletal Muscle. II. The Effect of Starvation and a Protein-Free Diet on the Synthesis and Catabolism of Skeletal Muscle Proteins in Comparison to Liver

1. Rates of synthesis and catabolism of liver and muscle sarcoplasmic and myofibrillar protein have been measured in young control, starved and protein (deprived) rats using [14C]Na2CO3 to label protein. 2. Half-lives for synthesis of 1·35, 2·8 and 7·2 days for liver, sarcoplasmic and myofibrillar proteins respectively were obtained, whilst half-lives for catabolism were 1·55, 3·6 and 15·6 days in each case in the control animals. 3. The protein free diet for 3 days caused a small decrease in the rate of synthesis of liver and muscle proteins. The catabolic rate of liver protein was increased by 20% whilst there was a smaller increase in the catabolic rate of myofibrillar proteins. 4. Starvation for 3 days caused a 20% reduction in the rate of liver protein synthesis whilst there were greater reductions in muscle protein synthesis. The catabolic rate of liver protein was only slightly increased whereas there was a 75% increase in the rate of myofibrillar protein breakdown. 5. The total amount of protein synthesis and catabolism in liver and the two muscle protein fractions over the first 3 days of the three regimes were calculated. Muscle protein turnover, in particular myofibrillar, was shown to be very sensitive to dietary protein and/or calorie deficiency. 6. These results are discussed in terms of the mobility and therefore importance of muscle protein metabolism in the economy of the whole animal.

Major pathways of glycine and serine catabolism in rat liver

The metabolism of glycine and serine in rat liver was studied with mitochondria, cell homogenates and liver slices. It was shown that the most significant pathway of glycine catabolism in rat liver is the direct cleavage of glycine to form methylene-THF, CO 2and ammonia, followed by the further oxidation of methylene-THF to CO 2, possibly by the sequential action of methylene-THF dehydrogenase, cyclohydrolase and 10-formyl-THF: NADP + oxidoreductase in liver mitochondria. Liver mitochondria, as well as liver homogenates and liver slices, actively catabolized serine and the patterns of serine catabolism in the liver homogenate and liver slice systems were essentially similar to those observed with liver mitochondria. A greater amount of 14CO 2 was obtained from serine-3- 14C than from serine-1- 14C in any system tested, and this difference was more distinct when relatively low concentrations of serine were employed. In the liver homogenate system, as well as in the liver slice system, the rate of 14CO 2 formation from serine-3- 14C was much higher than that from pyruvate-3- 14C. However, the activity of serine catabolism in the homogenate system was somewhat higher than in the mitochondrial system, although the soluble fraction alone catalyzed very little 14CO 2 formation from either serine-1- 14C or -3- 14C. In the homogenate system, the β-carbon of serine was converted to CO 2 even more rapidly than the carboxyl carbon of glycine. This seemed to be due to the participation of serine hydroxymethyltransferase, methylene-THF dehydrogenase and other enzymes related to one-carbon metabolism in the soluble liver fraction; these enzymes would increase the production of glycine, as well as the conversion of the β-carbon of serine to some one-carbon compound which may be oxidized to CO 2 in mitochondria. It was assumed that serine catabolism in liver under physiological conditions would proceed mainly by way of preliminary cleavage to methylene-THF and glycine and their subsequent oxidation to yield CO 2 as the final product in mitochondria. Serine can also be catabolized by serine dehydratase in the soluble liver fraction and through other minor pathways.

Dietary and hormonal control of enzymes of amino acid catabolism in liver.

A high glucose diet fed ad libitum decreased the activities in rat liver of histidase, serine dehydratase, homoserine dehydratase, ornithine transaminase, mitochondrial aspartate transaminase and cytoplasmic alanine transaminase, as compared to normal fed animals.Casein hydrolysate or tryptophan alone given to protein deplted rats increased the hepatic levels of serine dehydratase, alanine transaminase and tyrosine transaminase. Histidase and homoserine dehydratase were increased by casein hydrolysate but not by tryptophan. Aspartate transaminase was increased by tryptophan, but not by casein hydrolysate.Glucose administered intragastrically partly or totally counteracted the induction of serine dehydratase, tyrosine transaminase, cytoplasmic alanine transaminase by cortisol of glucagon administration to protein depleted rats. The similar induction of ornithine transaminase and cytoplasmic aspartate transaminase by cortisol was also counteracted by glucose.