Catabolism of photo-oxidized and desialylated hemopexin in the rabbit.

Following injection of rabbit 125I-asialohemopexin, more than 90% of the protein-bound 125I was removed from the circulation of rabbits within 12 min. The amount of asialoprotein in the catabolic compartment reached a peak concentration (75 to 85%) 12 min after injection and was completely eliminated from this compartment within 2 hours. The degradation products were excreted into the urine, with 50 to 70% of the 125I eliminated during the first 24 hours and 90 to 95% excreted by 48 hours. Analysis of these data indicated an apparent first order rate constant for uptake of asialohemopexin of 0.32 min-1, for catabolism of 0.020 min-1 and for excretion of 0.054 to 0.093 hour-1. The plasma distribution curves of 125I-hemopexin, after the first 24 hours, showed essentially no difference. Both proteins were catabolized with an average T1/2 of 25 to 26 hours and a similar fractional catabolic rate. Simultaneous injection of heme and 125I-hemopexin resulted in rapid removal and catabolism of the protein. In contrast, injection of heme had little if any effect on the plasma radioactivity curve of photoinactivated 125I-hemopexin.     

Relationship of Plasma-Free Amino Acids and Aminotransferase Expression in Nile Tilapia Oreochromis niloticus (Cichlidae)

Journal of Ichthyology - There is a relationship between concentrations of free amino acids in blood plasma in Nile tilapia and expression levels of hepatic enzymes involved in protein catabolism....     

House sparrows (Passer domesticus) increase protein catabolism in response to water restriction

Birds primarily rely on fat for energy during fasting and to fuel energetically demanding activities. Proteins are catabolized supplemental to fat, the function of which in birds remains poorly understood. It has been proposed that birds may increase the catabolism of body protein under dehydrating conditions as a means to maintain water balance, because catabolism of wet protein yields more total metabolic and bound water (0.155·H(2)O(-1)·kJ(-1)) than wet lipids (0.029 g·H(2)O(-1)·kJ(-1)). On the other hand, protein sparing should be important to maintain function of muscles and organs. We used quantitative magnetic resonance body composition analysis and hygrometry to investigate the effect of water restriction on fat and lean mass catabolism during short-term fasting at rest and in response to a metabolic challenge (4-h shivering) in house sparrows (Passer domesticus). Water loss at rest and during shivering was compared with water gains from the catabolism of tissue. At rest, water-restricted birds had significantly greater lean mass loss, higher plasma uric acid concentration, and plasma osmolality than control birds. Endogenous water gains from lean mass catabolism offset losses over the resting period. Water restriction had no effect on lean mass catabolism during shivering, as water gains from fat oxidation appeared sufficient to maintain water balance. These data provide direct evidence supporting the hypothesis that water stress can increase protein catabolism at rest, possibly as a metabolic strategy to offset high rates of evaporative water loss.     

HDLs in apoA-I transgenic Abca1 knockout mice are remodeled normally in plasma but are hypercatabolized by the kidney

Patients homozygous for Tangier disease have a near absence of plasma HDL as a result of mutations in ABCA1 and hypercatabolize normal HDL particles. To determine the relationship between ABCA1 expression and HDL catabolism, we investigated intravascular remodeling, plasma clearance, and organ-specific uptake of HDL in mice expressing the human apolipoprotein A-I (apoA-I) transgene in the Abca1 knockout background. Small HDL particles (7.5 nm), radiolabeled with (125)I-tyramine cellobiose, were injected into recipient mice to quantify plasma turnover and the organ uptake of tracer. Small HDL tracer was remodeled to 8.2 nm diameter particles within 5 min in human apolipoprotein A-I transgenic (hA-I(Tg)) mice (control) and knockout mice. Decay of tracer from plasma was 1.6-fold more rapid in knockout mice (P < 0.05) and kidney uptake was twice that of controls, with no difference in liver uptake. We also observed 2-fold greater hepatic expression of ABCA1 protein in hA-I(Tg) mice compared with nontransgenic mice, suggesting that overexpression of human apoA-I stabilized hepatic ABCA1 protein in vivo. We conclude that ABCA1 is not required for in vivo remodeling of small HDLs to larger HDL subfractions and that the hypercatabolism of normal HDL particles in knockout mice is attributable to a selective catabolism of HDL apoA-I by the kidney.     

Effect of insulin deficiency on low density lipoprotein metabolism in rabbits

These studies have been carried out in rabbits with alloxan-induced diabetes in order to see if insulin deficiency affects low density lipoprotein (LDL) catabolism. The results showed that plasma LDL-cholesterol was lower in diabetic rabbits, associated with a fall in the cholesterol to protein ratio of LDL particles. In addition, 125I-LDL disappeared more slowly from plasma of diabetic rabbits, leading to a significant reduction in fractional catabolic rate and a decrease in residence time of 125I-LDL. These data demonstrated that LDL composition and catabolism are greatly altered as a consequence of insulin deficiency.     

Metabolism of apolipoprotein A-IV in rat

The metabolism of apolipoprotein A-IV (apo-IV) has been investigated in the rat. In this animal species, apoA-IV is a major protein constituent of plasma HDL and lymph chylomicron. The apolipoprotein is also present in the lipoprotein-deficient fraction (LDF) of plasma and lymph. In vivo studies with the radioiodinated protein showed the apoA-IV does not exchange freely between HDL and LDF and that LDF apoA-IV had a faster catabolism than HDL apoA-IV. ApoA-IV in chylomicrons is a direct precursor of apoA-IV in plasma HDL but not of that in LDF. On the other hand lymph LDF apoA-IV is an important precursor of plasma LDF apoA-IV. Transfer of apoA-IV from plasma to lymph is negligible, and since most of apoA-IV in lymph is present in LDF, we speculate that LDF apoA-IV is the major apoA-IV secretory product of the intestine. Studies aimed at identifying the site of catabolism of apoA-IV utilizing either radioiodinated or [14C]sucrose labelled apoA-IV, gave results consistent with the view that the liver plays a major role. When tested, human apoA-IV behaved in vivo in rat as the autologous protein. These findings, together with others previously published (Ghiselli, G. et al. (1987) J. Lipid Res. 27, 813-827), support the conclusion that the plasma metabolism of apoA-IV is remarkably similar in rat and human. We speculate that in mammals the rapid plasma catabolism of apoA-IV is mediated by an efficient uptake by the liver.     

Metabolism of high-density lipoproteins in rainbow trout.

Trout high-density lipoproteins have been labelled with residualizing tracers for the lipid and protein moieties ([3H]cholesteryloleyl ether and 125I-tyramine-cellobiose, respectively). Plasma kinetics and tissue site of catabolism were determined for both tracers. The lipid tracer was cleared about twice as fast from the blood as the protein tracer (half lifes were 63.5 and 125.3 h, respectively). This selective removal of lipid from the lipoprotein was mainly accomplished by the higher liver uptake of the cholesteryl ether. The main catabolic site for HDL protein was kidney tissue. This data established the existence of differential HDL catabolism in a lower vertebrate, in which HDL is the dominant plasma lipoprotein. In addition, the findings confirm the importance of fish kidney as a major site of endocytosis of macromolecules, of both exogenous and endogenous origin.     

Protein utilization during starvation in fat and lean Svalbard ptarmigan (Lagopus mutus hyperboreus)

Summary Body protein sparing during starvation has been examined in fat and lean Svalbard ptarmigan. Protein utilization was determined from daily N excretion and from the rate of decrease in body mass. Changes in plasma concentrations of -hydroxybutyrate, free fatty acids, glucose, and uric acid were also recorded. When fat birds were starved for 15 days protein catabolism initially fell (phase I) and was thereafter kept low (phase II). This was evident from the temporal pattern in both N excretion and body mass loss. In two birds, N excretion eventually increased, revealing enhanced protein catabolism and thus a third phase of starvation. Changes in protein utilization were paralleled by changes in plasma uric acid. Approximately 9% of the energy demand was covered by breakdown of body protein during phase II. The importance of fat catabolism in providing energy was indicated by markedly elevated plasma levels of -hydroxybutyrate and free fatty acids. When lean birds were starved for 5 days there appeared to be no phase II. The temporal pattern of body mass loss indicated phase I and III but that of N excretion only phase III. The relative contribution of body protein to energy demand increased from 22% at day 2 to 41% at the end of starvation and was paralleled by increased plasma uric acid. When data from lean and fat birds were pooled, the changes in uric acid and N excretion were highly correlated (r=0.92, P<0.001), indicating that plasma uric acid is a reliable index of protein breakdown in starving Svalbard ptarmigan. In conclusion, starving fat Svalbard ptarmigan have a much greater capacity to spare body protein than lean birds. Fat birds effectively reduce protein catabolism and maintain this at a low level whereas starving lean birds increase protein catabolism.Abbreviations -OHB -hydroxybutyrate - BM body mass - BMR basal metabolic rate; dne daily nitrogen excretion - FFA free fatty acids - MR metabolic rate     

Effect of an anti-lipoprotein lipase serum on plasma triglyceride removal.

Anti-lipoprotein lipase sera injected intravenously in roosters blocked quantitatively the catabolism of very low density lipoprotein (VLDL) triglyceride. Antibodies were produced in rabbits immunized with highly purified lipoprotein lipase (LPL, glycerol ester hydrolase, E C 3.1.1.3) prepared from chicken adipose tissue. Following anti-LPL serum injection there was a linear increase in plasma triglyceride concentration. The rate of entry of triglyceride in plasma was estimated from the rate of triglyceride accumulation in the plasma of animals injected with anti-LPL serum, or from the disappearance curve of biologically labelled VLDL. In instances where both measurements were conducted in the same animals there was very close agreement between the two procedures. Inhibition of VLDL triglyceride catabolism of anti-LPL serum provided a way to characterize newly secreted VLDL that exhibited a broad spectrum of particle sizes with a median of 625 A degrees. They contained 76.2 +/- 1.2% triglyceride and had a high ratio of free to ester cholesterol (2.46 +/- 0.45). In control VLDL samples there was 46.1% triglyceride, and the ratio of free to ester cholesterol was 1.19. The complete inhibition of triglyceride removal by an antiserum prepared against adipose tissue LPL demonstrates that the NaCl-inhibited, serum-activated lipase prepared by affinity chromatography on heparin-Sepharose and concanavalin A-Sepharose columns is the enzyme responsible in vivo for the catabolism of VLDL triglyceride. Further, the kinetics of triglyceride accumulation in the plasma provide evidence that the site of degradation of VLDL triglyceride is within the plasma compartment.     

Clofibrate treatment promotes branched-chain amino acid catabolism and decreases the phosphorylation state of mTOR, eIF4E-BP1, and S6K1 in rat liver

Leucine stimulates protein synthesis by modulating the mammalian target of rapamycin (mTOR) signaling pathway. We hypothesized that promotion of the branched-chain amino acid (BCAA) catabolism might influence the leucine-induced protein synthesis. Clofibric acid (an active metabolite of clofibrate) is known to promote the BCAA catabolism by activation of branched-chain alpha-keto acid dehydrogenase complex (BCKDC), the rate-limiting enzyme of the BCAA catabolism. In the present study, we examined the phosphorylation state of mTOR, eukaryotic initiation factor 4E-binding protein-1 (4E-BP1), and ribosomal protein S6 kinase 1 (S6K1) in liver of rats with or without activation of the BCKDC by clofibrate treatment. Clofibrate-treated rats were prepared by oral administration of clofibrate 5 h before sacrifice. In order to stimulate phosphorylation of components in the mTOR signaling pathway, rats were orally administered with leucine 1 h before sacrifice. Clofibrate treatment almost fully activated hepatic BCKDC and significantly decreased the plasma leucine concentration in rats without leucine administration, resulting in decreased mTOR and 4E-BP1 phosphorylation. Similarly, in rats administered with leucine, clofibrate treatment attenuated the predicted increase in plasma leucine concentration as well as the phosphorylation of mTOR, 4E-BP1, and S6K1. These results suggest that BCAA catabolism enhanced by clofibrate treatment has significant influences on the leucine-induced activation of translation initiation processes.     

Scavenger receptor BI facilitates hepatic very low density lipoprotein production in mice

Scavenger receptor BI (SR-BI) is a selective uptake receptor for HDL cholesterol but is also involved in the catabolism of apolipoprotein (apo)B-containing lipoproteins. However, plasma levels of apoB-containing lipoproteins increase following hepatic SR-BI overexpression, suggesting that SR-BI not solely mediates their catabolism. We therefore tested the hypothesis that hepatic SR-BI impacts on VLDL production. On day 7 following adenovirus (Ad)-mediated overexpression of SR-BI, VLDL-triglyceride and VLDL-apoB production rates were significantly increased (P < 0.001), whereas VLDL production was significantly lower in SR-BI knockout mice compared with controls (P < 0.05). In mice injected with AdSR-BI, hepatic cholesterol content increased (P < 0.001), microsomal triglyceride transfer protein activity was higher (P < 0.01) and expression of sterol-regulatory element binding protein (SREBP)2 and its target genes was decreased (P < 0.01). Conversely, in SR-BI knockout mice, microsomal triglyceride transfer protein activity was lower and expression of SREBP2 target genes was increased (P < 0.01). Finally, we demonstrate in vitro in isolated primary hepatocytes as well as in vivo that cholesterol derived from HDL and taken up via SR-BI into the liver can be resecreted within VLDL. These data indicate that hepatic SR-BI expression is linked to VLDL production, and within liver, a metabolic shunt might exist that delivers HDL cholesterol, at least in part, to a pool from which cholesterol is mobilized for VLDL production. These results might have implications for HDL-based therapies against atherosclerotic cardiovascular disease, especially with SR-BI as target.     

Disordered branched chain amino acid catabolism in pancreatic islets is associated with postprandial hypersecretion of glucagon in diabetic mice

Dysregulation of glucagon is associated with the pathophysiology of type 2 diabetes. We previously reported that postprandial hyperglucagonemia is more obvious than fasting hyperglucagonemia in type 2 diabetes patients. However, which nutrient stimulates glucagon secretion in the diabetic state and the underlying mechanism after nutrient intake are unclear. To answer these questions, we measured plasma glucagon levels in diabetic mice after oral administration of various nutrients. The effects of nutrients on glucagon secretion were assessed using islets isolated from diabetic mice and palmitate-treated islets. In addition, we analyzed the expression levels of branched chain amino acid (BCAA) catabolism-related enzymes and their metabolites in diabetic islets. We found that protein, but not carbohydrate or lipid, increased plasma glucagon levels in diabetic mice. Among amino acids, BCAAs, but not the other essential or nonessential amino acids, increased plasma glucagon levels. BCAAs also directly increased the intracellular calcium concentration in α cells. When BCAAs transport was suppressed by an inhibitor of system L-amino acid transporters, glucagon secretion was reduced even in the presence of BCAAs. We also found that the expression levels of BCAA catabolism-related enzymes and their metabolite contents were altered in diabetic islets and palmitate-treated islets compared to control islets, indicating disordered BCAA catabolism in diabetic islets. Furthermore, BCKDK inhibitor BT2 suppressed BCAA-induced hypersecretion of glucagon in diabetic islets and palmitate-treated islets. Taken together, postprandial hypersecretion of glucagon in the diabetic state is attributable to disordered BCAA catabolism in pancreatic islet cells.     

The catabolism of plasma glycoproteins in normal and injured rats

The catabolism of (14)C-labelled plasma glycoprotein in rats was studied after injecting homologous plasma protein labelled in the N-acetylglucosamine and sialic acid moieties. In normal animals the catabolism was approximately described by a four-compartment model. The fractional rate of catabolism of the plasma-protein amino sugar was found to be 0.0305hr.(-1), corresponding to the degradation of 2.75mumoles/hr. The (14)C label was eliminated from the animals largely as carbon dioxide with a small proportion appearing in the urine. Freely circulating amino sugars or glycopeptides did not appear in the plasma as a result of the catabolic processes, and there was no evidence that the protein-bound amino sugars were reutilized in biosynthetic processes. A study of the distribution of (14)C label in the carcasses of animals 24hr. after injection provided evidence that the gastrointestinal tract accounted for 25-38% of the total catabolic pool; the lungs, kidneys, spleen and liver also appeared to contribute to catabolism. Studies were conducted with rats that had been treated with turpentine to induce an inflammatory reaction; the results could not be analysed kinetically, since the metabolism of plasma proteins in these animals did not appear to be in a steady state. The injected plasma protein disappeared from the intravascular pool more quickly than in normal animals, but there were no significant differences in the rates of excretion of the (14)C label.     

Histidine degradation enzymes in rat liver: Induction by high protein intake

Summary High protein dietary content stimulates urea formation in ureotelic animals but does not exert almost any effect on ammonia production from L-amino acids in vitro. L-histidine and L-threonine are the only amino acids which are most actively deaminated by ureotelic animals fed on a high protein diet.All the steps of L-histidine metabolism have been studied: it has been found that both the histidine transaminase pathway and the histidase pathway are stimulated. Glutamic acid is also a product of histidine catabolism through the histidase pathway, but its catabolism is unaffected by the dietary protein content. These data suggest the existence of independent mechanism controlling the catabolism of the two amino acids.     

PXR agonism decreases plasma HDL levels in ApoE⁎3-Leiden.CETP mice

Pregnane X receptor (PXR) agonism has been shown to affect multiple steps in both the synthesis and catabolism of HDL, but its integrated effect on HDL metabolism in vivo remains unclear. The aim of this study was to evaluate the net effect of PXR agonism on HDL metabolism in ApoE?3-Leiden (E3L) and E3L.CETP mice, well-established models for human-like lipoprotein metabolism. Female mice were fed a diet with increasing amounts of the potent PXR agonist 5-pregnen-3β-ol-20-one-16α-carbonitrile (PCN). In E3L and E3L.CETP mice, PCN increased liver lipids as well as plasma cholesterol and triglycerides. However, whereas PCN increased cholesterol contained in large HDL-1 particles in E3L mice, it dose-dependently decreased HDL-cholesterol in E3L.CETP mice, indicating that CETP expression dominates the effect of PCN on HDL metabolism. Analysis of the hepatic expression of genes involved in HDL metabolism showed that PCN decreased expression of genes involved in HDL synthesis (Abca1, Apoa1), maturation (Lcat, Pltp) and clearance (Sr-b1). The HDL-increasing effect of PCN, observed in E3L mice, is likely caused by a marked decrease in hepatic SR-BI protein expression, and completely reversed by CETP expression. We conclude that chronic PXR agonism dose-dependently reduces plasma HDL-cholesterol in the presence of CETP.