Early labeling of bilirubin from glycine and delta-aminolevulinic acid in bile fistula dogs, with special reference to stimulated versus suppressed erythropoiesis

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.

Proof that Sn-protoporphyrin inhibits the enzymatic catabolism of heme in vivo. Suppression of 14CO generation from radiolabeled endogenous and exogenous heme sources

Sn-protoporphyrin (SnPP) suppresses generation of 14CO from hepatic heme labeled with delta-aminolevulinic acid (5-[14C]ALA) or from infused [14C]hemin in rats. SnPP administered 1 h before administration of 5-[14C]ALA virtually abolished the peak output of 14CO occurring 2-3 h after injection of this heme precursor, and during the succeeding 12 h reduced 14CO excretion by approximately 61% compared with controls. When [14C]hemin was infused, SnPP diminished 14CO excretion by approximately 50%. These findings, derived from experiments using radiolabeled endogenous and exogenous heme sources, establish conclusively that the synthetic metalloporphyrin SnPP inhibits the oxidative degradation of heme in the intact animal.

The contributions of erythropoietic and nonerythropoietic haem turnover to the early labelle peak of endogenous CO formation in man

The endogenous production of 14CO during the first week after administration of glycine-2-14C (early labelled peak, ELP) was determined in 13 healthy subjects. The subjects were studied during normal, suppressed or stimulated erythropoiesis induced by hypertransfusion and phlebotomy, respectively. The production of 14CO was calculated from determinations of the specific activity of the body CO store and the endogenous production of CO. The incorporation of labelled glycine into circulating red cell haemo;globin haem was calculated from the maximal specific activity of haemoglobin haem and the total amount of Hb. The average ELP was 73.0 X 10(3) disintegrations per min (dpm) and the incorportion of glycine into circulating haemoglobin haem 491 X 10(3) dpm in controls. The ELP and the incorporation of glycine increased by 86% and 92%, respectively, in phlebotomized subjects and decreased by 37% and 62%, respectively, in hypertransfused subjects. A linear regression equation was calculated for the relationship between ELP and erythropoiesis, expressed as the incorportion of glycine into circulating haemoglobin haem. The residual ELP remaining in the absence of erythropoiesis was 24.9 X 10(3) dpm calculated from data of all the subjects, and 32.8 X 10(3) dpm calculated from data of controls and hypertransfused subjects only. In conclusion, erythropoietic haem turnover was found to contribute about 60% and nonerythropoietic (mainly hepatic) haem turnover about 40% of the ELP in man.

Quantitation of ineffective erythropoiesis from the incorporation of [15N] delta-aminolaevulinic acid and [15N] glycin into early labelled bilirubin. II. Anaemic patients

The incorporation of [15N]delta-aminolaevulinic acid and [15N glycine into haemoglobin haem and early labelled bilirubin was measured in subjects with various haematological disorders. The clearance of [14C bilirubin was used to measure bilirubin production rate, and the magnitude of the various sources of bilirubin production and the percentage ineffective erythropoiesis were calculated. Ineffective erythropoiesis was found to be a major factor in the production of the anaemia in patients with the following disorders: megaloblastic anaemia associated with the Lesch-Nyhan syndrome, thalassaemia intermedia, sideroblastic anaemia, and the anaemia of chronic disorders. In three patients with iron-deficiency anaemia ineffective erythropoiesis was increased, but was of minor importance in the production of the anaemia, while in two patients with aplastic anaemia and one with macrocytosis of alcoholism there was no increase in ineffective erythropoiesis.

Quantitation of ineffective erythropoiesis from the incorporation of [15N] delta-aminolaevulinic acid and [15N] glycine into early labelled bilirubin. I. Normal subjects

The incorporation of [15N] glycine into early labelled bilirubin and haemoglobin haem was measured in four haematologically normal subjects, using the clearance of [14C] bilirubin to measure total bilirubin production rate. Hepatic haem turnover was calculated from the incorporation of [15N]=sigma-aminolaevulinic acid into early labelled bilirubin. From the exprimental data previously published data in normal subjects a method is derived for the quantitation of ineffective erythropoiesis which can be applied to similar studies in patients with haematological disorders.

Preferential hemolysis of immature erythrocytes in experimental iron deficiency anemia: source of erythropoietic bilirubin formation

Bilirubin-(14)C production was measured in rats transfused with labeled erythrocytes from animals with iron deficiency anemia, a condition associated with ineffective erythropoiesis. With labeled reticulocytes harvested 1 day after the administration of glycine-2-(14)C, conversion of hemoglobin-(14)C to bilirubin averaged 47.3% over the 3 days of observation; the corresponding value for reticulocytes from normal rats was only 1.7%. Findings were not altered by splenectomy. Bilirubin-(14)C production fell to 35.8% with iron-deficient cells harvested 3 days after glycine-(14)C administration, and declined further to a plateau averaging 25% with cells labeled 5, 7, 10, or 15 days earlier. The latter values still far exceed those for mature erythrocytes from normal animals.The findings indicate that experimental iron deficiency anemia is associated with hemolysis of red cells of various ages, but with preferential destruction of the youngest cells. Degradation of hemoglobin from reticulocytes is sufficient to account for a major fraction of the increase in erythropoietic bilirubin production found in this disorder, as has also been shown for physiologically regulated erythroid hyperplasia. However, the defect is quantitatively much more striking in experimental iron deficiency, and this and perhaps a similar defect in bone marrow cells appear to explain the decrease in net hemoglobin production that is characteristic of pathologic ineffective erythropoiesis.

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.

The nonerythropoietic component of early bilirubin

The early labeled bilirubin consists of two primary components. The more rapidly synthesized of the two is independent of erythropoiesis (nonerythropoietic), whereas the second fraction is related to red cell production (erythyropoietic). The present studies concern the origin of the nonerythropoietic component. The nonerythropoietic, early labeled bilirubin was studied in bile fistula rats with (delta ALA)-4-(14)C delta aminolevulinic acid and glycine-2-(14)C as precursors. That nephrectomy did not reduce the size of this component despite the large and rapidly turning over pool of renal heme suggests that this pool may be of minor importance in its production. Intoxication with lead to a level that reduced hepatic heme synthesis was associated with a decrease in early bilirubin formation. The synthesis of this bilirubin was assessed in animals with phenobarbital-induced heme protein and cycloheximide-suppressed protein synthesis. Rats pretreated with phenobarbital at a dose level of 60 mg/kg with induction of cytochrome P-450 synthesis showed a minor increase in early labeling when glycine-2-(14)C but not when delta ALA-4-(14)C was used as precursor. Rats given cycloheximide at a dose level that markedly reduced hepatic protein and cytochrome P-450 synthesis but allowed heme synthesis to continue at 60% of its pretreatment level synthesized normal or increased amounts of early bilirubin from delta ALA-4-(14)C. Allylisopropylacetamide intoxication caused little change in early bilirubin formation, whereas aminotriazole given at a time after maximal hepatic heme labeling produced a small but significant increase in the appearance of labeled bilirubin. These findings indicate that early bilirubin production is little influenced by increased hepatic porphyrin synthesis or by changes in the rapidly turning over heme protein P-450. A minimal increase attends catalase inactivation by aminotriazole. Normal or increased synthesis takes place in the presence of suppression of protein synthesis. This finding suggests that the nonerythropoietic early bilirubin may itself consist of two subcomponents. The first of these may arise from free tissue heme or its precursors, and the second may derive from the turnover of the heme proteins. The first subcomponent may serve as a regulatory mechanism for the removal of heme synthesized in excess of its protein acceptor. A composite scheme is proposed for the origin of the total early bilirubin from heme compartments in tissue and marrow.