Early-labeled peak of bile pigment in man. Studies with glycine-14C and delta-aminolevulinic acid-3H

Ineffective erythropoiesis and its treatment

The erythroid marrow and circulating red blood cells (RBCs) are the key components of the human erythron. Abnormalities of the erythron that are responsible for anemia can be distinguished into 3 major categories, that is, erythroid hypoproliferation, ineffective erythropoiesis, and peripheral hemolysis. Ineffective erythropoiesis is characterized by erythropoietin-driven expansion of early-stage erythroid precursors, associated with apoptosis of late-stage precursors. This mechanism is primarily responsible for anemia in inherited disorders like β-thalassemia, inherited sideroblastic anemias, and congenital dyserythropoietic anemias, as well as in acquired conditions like some subtypes of myelodysplastic syndromes (MDS). The inherited anemias due to ineffective erythropoiesis are also defined as iron loading anemias because of the associated parenchymal iron loading caused by the release of erythroid factors that suppress hepcidin production. Novel treatments specifically targeting ineffective erythropoiesis are being developed. Iron restriction through enhancement of hepcidin activity or inhibition of ferroportin function has been shown to reduce ineffective erythropoiesis in murine models of β-thalassemia. Luspatercept is a TGF-β ligand trap that inhibits SMAD2/3 signaling. Based on pre-clinical and clinical studies, this compound is now approved for the treatment of anemia in adult patients with β-thalassemia who require regular RBC transfusions. Luspatercept is also approved for the treatment of transfusion-dependent anemia in patients with MDS with ring sideroblasts, most of whom carry a somatic SF3B1mutation. While long-term efficacy and safety of luspatercept need to be evaluated both in β-thalassemia and MDS, defining the molecular mechanisms of ineffective erythropoiesis in different disorders might allow the discovery of new effective compounds.

Evaluation of abnormal liver tests

The routine use of serum biochemical tests allows for the detection of acute and chronic liver injury before the onset of symptoms. These tests consist of markers of hepatocellular injury (aminotransferases and APs); tests of liver metabolism (total bilirubin); and tests of liver synthetic function (serum albumin and PT). Noninvasive tests for assessment of liver fibrosis are promising tools for diagnosis and prognosis of patients with chronic liver disease. A comprehensive history, physical examination, and assessment of pattern of liver injury with additional laboratory and imaging testing establish the cause of hepatobiliary disease in most cases.

The origins and kinetics of bilirubin in healthy dogs, in comparison with man

The origins and the kinetics of unconjugated bilirubin were assessed in 25 healthy dogs. Bilirubin kinetics were measured by the analysis of the plasma disappearance of [3H]bilirubin in a two-compartment model. The relative bilirubin productions from erythrocyte haem degradation and early labeled bilirubin were determined by measuring the incorporation of [14C]glycine in erythrocyte haem and in fecal stercobilin. The incorporation of this relation into the model permitted the quantitation of the bilirubin production from erythrocyte destruction, ineffective erythropoiesis and the catabolism of hepatic haemoproteins. The contribution of the three bilirubin sources to the plasma concentration was derived from the calculated fraction reflux into the plasma of bilirubin produced in the liver from hepatic haemoproteins. Other calculated model-dependent and -independent parameters were plasma bilirubin clearance, hepatic bilirubin extraction efficiency, pool sizes, and the fractional transfer rates which reflect the hepatic uptake process, reflux from liver to the plasma, and the conjugating enzyme activity. In plasma of healthy dogs only unconjugated bilirubin was detected. It averaged 0.68 mumol/l, which is far below levels in man. This is probably due to the 20-fold higher hepatic clearance rate in dogs (median 32.2; range 21.6-43.9 ml/kg per min). In addition, fasting hyperbilirubinaemia could not be documented in the dog. The total bilirubin turnover was 14.9 (12.6-17.1) mumol/kg (median and 95% range), with 67 (60-70)% derived from erythrocyte degradation, 5.3 (4.7-5.5)% from ineffective erythropoiesis and 27.7 (24.5-35.3)% from hepatic haemoproteins. The figures for the plasma bilirubin turnover were 12.3 (10.3-14.2) mumol/kg per day, 79 (75-84)%, 6.3 (6.0-6.6)% and 14.8 (9.2-18.9)%, respectively. The presented model permits the simultaneous quantitation of both the origins and the kinetics of bilirubin. The application of this approach in pathological conditions is expected to provide better insight in the pathophysiology of acquired hyperbilirubinaemia.

Heme catabolism in cultured hepatocytes: evidence that heme oxygenase is the predominant pathway and that a proportion of synthesized heme is converted rapidly to biliverdin

Heme oxygenase has been considered to be involved in the predominant pathway of heme degradation in vivo. However, alternative pathways involving cytochrome P-450 reductase, and lipid peroxidation, have previously been demonstrated in vitro, and studies with cultured rat hepatocytes were interpreted to show a majority of endogenous hepatic heme breakdown by non-heme oxygenase pathways. To clarify the pathway of heme breakdown in hepatocytes and the role of heme oxygenase in this process, cultured hepatocytes were pre-labelled with 5-[5-14C]aminolevulinate [( 14C]ALA). Radioactivity in heme, carbon monoxide, and bile pigments was measured for 8-24 h after the removal of [14C]ALA. In cultured chick embryo hepatocytes, which lack biliverdin reductase, the rate of production of biliverdin IXa was closely similar to the rate of catabolism of exogenous heme and radioactivity in carbon monoxide and biliverdin IXa was similar to the loss of radioactivity from endogenous heme. These results support the conclusion that heme breakdown occurred predominantly, if not solely, by heme oxygenase. Also, no evidence of non-heme oxygenase pathways was found in the presence of tin protoporphyrin, an inhibitor of heme oxygenase or mephenytoin, an inducer of both cytochrome P-450 and heme oxygenase. Similarly, in untreated cultured rat hepatocytes, radioactivity in carbon monoxide corresponded with loss of radioactivity in endogenous heme. In other experiments with chick hepatocyte cultures, rates of heme synthesis and breakdown were measured, and data were fitted to various models of hepatic heme metabolism. The results observed were consistent only with models in which an appreciable fraction (control cells, 17%, mephenytoin treated cells, 41%) of the newly synthesized heme was degraded rapidly to biliverdin.

Effect of ovulation on haem metabolism in rabbits

To investigate the origin of the cyclic changes in the rate of endogenous carbon-monoxide production (nCO) during the menstrual cycle, haem turnover was determined before and after chorion gonadotropic hormone-induced ovulation in six female rabbits. 14C-labelled delta-aminolevulinic acid and glycine were administered and the excretion rate of 14CO (A14CO) was measured for determination of hepatic and bone-marrow haem turnover, respectively. Carbon-monoxide production (nCO) was measured to estimate total haem turnover. After ovulation A14CO was increased significantly the first 2 h of the early labelled peak after 14C-ALA administration and was increased also at the first determination during the early peak after 14C-glycine but statistically not significantly. The total excretion of labelled CO during the period of the early labelled peak was not increased with any of these precursors for haem synthesis. On the other hand nCO was increased 34% (P less than 0.05) during the post-ovulation period. As the increase in 'unassigned' haem turnover was small and may be unaccompanied by a contemporary increase in bilirubin/CO production, it was concluded that the increase in nCO during the post-ovulation period essentially depends on increased destruction of circulating red cells in the rabbit.

Biosynthesis of 5-aminolevulinic acid and heme from 4,5-dioxovalerate in the rat

We previously demonstrated an alternate pathway for the biosynthesis of 5-aminolevulinic acid (ALA) in bovine liver mitochondria and of tetrapyrroles in suspensions of rat hepatocytes (1980. J. Biol. Chem. 255: 3742; 1981. Proc. Natl. Acad. Sci. USA. 78: 5335). This pathway involves a transamination reaction that incorporates the intact 5-carbon skeleton of 4,5-dioxovaleric acid (DOVA) into ALA. We investigated this alternate pathway in vivo by the intraperitoneal injection of DOVA into rats. Incorporation of DOVA and [5-14C]DOVA into urinary ALA and hepatic and erythroid heme was quantified and compared with the incorporation of [4-14C]ALA and [2-14C]glycine into heme. Within 3 h of injection of 175 mumol of DOVA, urinary ALA excretion increased 2.4-fold over controls. After injection of [5-14C]DOVA, 0.11% of the radioactivity was recovered as urinary ALA, which quantitatively accounted for the 2.4-fold increase in ALA excretion. After the injection 175 mumol of [5-14C]DOVA, 0.14% of the radioactivity was recovered after 3 h as hepatic heme. The injection of 1.75 mmol of [2-14C]glycine or 175 mumol of [4-14C]ALA resulted in recovery of 0.2 and 3.4%, respectively, of the radioactivity as hepatic heme after 3 h. These doses of radiolabeled DOVA, glycine, and ALA were injected into rats with phenylhydrazine-induced anemia. Recovery of radioactivity after 3 h as splenic (erythroid) heme was 0.35% for DOVA, 0.072% for glycine, and 0.25% for ALA. These studies establish that the intact 5-carbon skeleton of DOVA can be incorporated into ALA and heme in vivo.

The production of 14CO following the administration of 2-[14C] glycine in normal subjects determined by an oxygen washout technique

The endogenous production of 14CO during the first week after administration of 2-[14C]glycine, i.e. the early peak of 14CO derived mainly from erythropoietic and hepatic haem turnover, was measured in six normal subjects. Samples from the body CO store were collected by washout of CO with oxygen breathing, CO concentration of expired gas was determined with an infrared CO-meter and, for determination of 14C activity, CO washed out was oxidized to CO2 in a trapping system and absorbed in ethanolamine. The endogenous production of CO was measured with a rebreathing technique and the production of 14CO was calculated. The specific activity and total amount of circulating red cell haemoglobin haem was determined to calculate the potential late peak of 14CO production. The method for determination of 14CO expired was demonstrated to be specific and reproducible and no haemolysis induced by the sampling procedure was detected. The average production of 14CO was 6.94 nCi/24 h the day after injection of 100 muCi of labelled glycine and decreased gradually to 1.08 nCi/24 h on day 8. The early labelled peak was 32.9 +/0 8.1 nCi (mean +/- SD) and the potential late labelled peak 221 +/- 30 nCi. The early peak represented 13.0 +/- 2.9% of the calculated total production of labelled CO, a figure significantly lower than erythropoietic and hepatic haem turnover calculated from total bilirubin or CO production and red cell life span, probably due to the hepatic component being underestimated by the radiolabelled precursor method.

Quantitative studies of the delivery of hepatic-synthesized bilirubin to plasma utilizing -aminolevulinic acid-4- 14 C and bilirubin- 3 H in man

After the simultaneous intravenous administration of unconjugated bilirubin-(3)H and delta-aminolevulinic acid-4-(14)C, the plasma disappearance curves of unconjugated bilirubin-(3)H and the plasma appearance curves of biosynthesized unconjugated bilirubin-(14)C have been defined in seven patients, three of whom had acute intermittent porphyria (AIP). The incorporation of (14)C into plasma unconjugated bilirubin, derived by an analysis which involves deconvolution of the two plasma curves, varied between 13.1 and 23.5% (mean 19.3%) of the injected dose in the nonporphyric patients and between 5.4 and 13.6% (mean 8.3%) of the injected dose in the porphyric patients. In five of the patients, the stercobilin-(14)C specific activity in a pooled specimen of feces was measured, enabling the following further values to be calculated: (a) the total (14)C radioactivity incorporated into bilirubin (21.0 and 25.3% [mean 23.2%] of the injected dose in two of the nonporphyric patients and between 8.5 and 25.3% [mean 14.2%] of the injected dose in the porphyric patients), and (b) the proportion of hepatic synthesized bilirubin delivered directly to plasma in the unconjugated form (between 0.520 and 0.904; mean for nonporphyric patients 0.712; mean for porphyric patients 0.614). The results demonstrate that a large proportion of bilirubin derived from hepatic hemes passes through the plasma in the unconjugated form before conjugation and secretion into bile.

The measurement of the synthetic rate of bilirubin from hepatic hemes in patients with acute intermittent porphyria

A new method for the direct measurement in vivo of the synthetic rate of bilirubin from hepatic hemes is proposed. This method depends on the application of the labeled precursor-product relationship to the hepatic pool of porphobilinogen, which is a common precursor of both urinary porphobilinogen and hepatic-synthesized bilirubin. The hepatic pool of porphobilinogen is labeled by means of an intravenous injection of delta-aminolevulinic acid-4-(14)C. The proportion of total bilirubin production which is derived from hepatic hemes is calculated from the ratio of the mean (14)C specific activities of stercobilin and porphobilinogen estimated in pooled specimens of feces and urine, respectively. The method can be most readily applied to patients with acute intermittent porphyria, as the appreciable quantities of prophobilinogen in the urine of these patients greatly facilitate the measurement of porphobilinogen-(14)C specific activity. In three patients with acute intermittent porphyria, values obtained for the synthetic rate of bilirubin from hepatic hemes were 20.7, 15.8, and 13.3% of total bilirubin production.

Photocatabolism of labeled bilirubin in the congenitally jaundiced (Gunn) rat

To elucidate the mechanism by which phototherapy reduces serum bilirubin, studies were performed on the catabolism of labeled bilirubin in homozygous jaundiced Gunn rats before, during, and after a period of exposure to 1700 foot candles of daylight fluorescent light. Following equilibration with the body pool of an intravenously administered tracer dose of (3)H- or (14)C-bilirubin, radioactive and diazo reactive compounds were excreted in the bile at a slow, steady rate and plasma specific activity declined semilogarithmically. Subsequent exposure to light caused a marked increase in the biliary excretion of radioactive and diazoreactive compounds. Fecal and urinary radioactivity increased also but remained minor fractions of the total excreted radioactivity. After extinguishing the lights, these variables reverted gradually to control values. Spectral and chromotographic analysis of the excreted pigments and their azopigments demonstrated that the increased biliary radioactivity during phototherapy consisted of two roughly equal fractions: (a) unconjugated bilirubin, excreted at rates comparable to the output of conjugated bilirubin in the bile of normal nonjaundiced rats; and (b) water-soluble bilirubin derivatives, chromatographically identical with those found in Gunn rat bile under control lighting conditions but different from the products of photodecomposition of bilirubin in vitro. In some animals, phototherapy produced little decline in plasma bilirubin despite comparable acceleration of bilirubin catabolism. This was attributed tentatively to increased synthesis of early labeled bilirubin in these animals.

Hemolysis of "stress" reticulocytes: a source of erythropoietic bilirubin formation

The formation of bilirubin-(14)C was measured in rats given transfusions of red blood cells containing (14)C-labeled hemoglobin heme. Per cent conversion of hemoglobin-(14)C to bilirubin was 4 times greater with transfusion of "stress" reticulocytes from rats responding to hemorrhage than with normal reticulocytes from unstimulated donors. When the increased number of labeled reticulocytes produced by hemorrhaged donors was also considered, the total magnitude of labeled bilirubin formation was almost 20 times higher with stress as compared to normal reticulocytes. The findings were not influenced by splenectomy of either donor or recipient rats, iron loading of donors, or bleeding of recipients. However, bilirubin-(14)C formation fell off progressively as studies were performed at longer intervals after erythroid stimulation. Total bilirubin-(14)C formation in rats transfused with stress reticulocytes was compared to the production of early-labeled bilirubin from all potential sources in intact rats bled according to the same schedule used in the transfusion experiments. It is estimated that degradation of hemoglobin from sress reticulocytes accounts for virtually the entire rise in erythropoietic bilirubin formation from 24 to 96 hr after glycine-2-(14)C administration, but that additional sources make a major contribution before that time. These findings are consistent with the concept that destruction of immature erythroid cells in the peripheral blood, and probably in the bone marrow, accompanies the physiologic response to erythroid stimulation.

Studies of bilirubin kinetics in normal adults

This report describes studies of bilirubin kinetics in 13 healthy young adults. The plasma content of unconjugated bilirubin-(14)C was determined at frequent intervals for 24-30 hr after the intravenous injection of a tracer dose of unconjugated isotopic bilirubin. Fecal and urinary radioactivity were measured for 7 days. During this time cumulative recovery averaged 96% of the injected dose. The plasma curves were processed by digital computer. For the 30 hr experimental period, a sum of three exponentials, with average half-times of 18, 81, and 578 min, was required to describe the data. Using the plasma curve integral method, the hepatic bilirubin clearance (47 +/-10 ml/min, mean +/-SD), the bilirubin production rate (3.8 +/-0.6 mg/kg per day), and the mean red blood cell life span (101 +/-13 days) were calculated directly from the parameters of this function. To gain further insight into the metabolism of unconjugated bilirubin, the data were also used to determine the parameters of a multicompartmental model. In the model proposed, plasma unconjugated bilirubin exchanges with two additional pools one of which is thought to represent extrahepatic extravascular, and the other intrahepatic unconjugated bilirubin. Bilirubin is eliminated from the system via the proposed intrahepatic pool. From the data and the model, pool sizes and exchange rates between compartments were calculated, and the liver: plasma concentration gradient estimated. These studies provide a detailed analysis of the kinetics of unconjugated bilirubin in a healthy normal population and are intended to serve as a reference point for studies of abnormal states.

Effect of sodium phenobarbital on bilirubin metabolism in an infant with congenital, nonhemolytic, unconjugated hyperbilirubinemia, and kernicterus

Sodium phenobarbital and various hormones, compounds capable of hepatic enzyme induction, were given to an infant boy with congenital, nonhemolytic, unconjugated, hyperbilirubinemia and severe kernicterus for prolonged periods between the ages of 2 and 25 months to determine their effect on serum bilirubin concentrations. Phenobarbital, 5 mg/day orally, on two occasions decreased serum bilirubin concentrations approximately threefold over a period of 30 days. Withdrawal of phenobarbital after the first study resulted in a gradual (30 days) return of serum bilirubin to pretreatment levels. The lower serum bilirubin concentrations observed when phenobarbital therapy was reinstituted were maintained for 61 days on 2.5 mg/kg per day of the drug. Orally administered L-triiodothyronine, 0.05-0.1 mg/day for 71 days, intramuscular human growth hormone, 1 mg/day for 21 days, and testosterone propionate, 0.1 mg/day for 9 days, did not decrease serum bilirubin levels below lowest control values of 18 mg/100 ml.Bilirubin-(3)H was administered twice before and once with bilirubin-(14)C during phenobarbital therapy to study the kinetics of bilirubin metabolism. Results of the first and second control studies and of the bilirubin-(3)H and bilirubin-(14)C phenobarbital studies, respectively, were as follows: total body bilirubin pools, 200, 184, 73, and 72 mg; half-lives, 111, 84, 37, and 39 hr; and turnover, 30, 37, 33, and 31 mg/day. The data show that the approximate threefold decrease in serum bilirubin concentration and total body pool resulted from a comparable decrease in bilirubin half-life without a significant change in turnover. In vitro histological (electron microscopy) and enzymological studies of liver obtained by surgical biopsies before and during phenobaribtal administration showed that both the hepatocyte content of agranular endoplasmic reticulum (AER) and the ability of liver homogenate to conjugate p-nitrophenol were significantly increased during phenobarbital treatment. The observations suggest that phenobarbital affects bilirubin metabolism by the induction of an enzyme(s) with a slow rate(s) of degradation (or rapid rate of degradation with limited capacity).