Recent studies of the urobilin problem

Microbial conversion of bilirubin to urobilins in vitro and in vivo

No urobilins are formed from bilirubin in germ-free rats. To isolate and investigate the strains of intestinal microorganisms responsible for this transformation, a suitable test medium was adopted. The strength of the medium and a rather high initial pH were found to be of importance. In this medium, suspensions of rat faeces and a single strain, Cl. ramosum (G62), converted bilirubin to urobilins. Cultivations of Cl. ramosum (G62) together with E. coli significantly enhanced the conversion, whereas addition of 4 other bacterial strains was without the influence. The highest in vitro formation of the urobilins was about 10% of the bilirubin present. When the 6 strains investigated in vitro were established in EXG rats, the in vivo conversion of bilirubin to urobilins was found to be about 15%, compared to 70% in CONV rats.

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.

Carbon monoxide production associated with ineffective erythropoiesis

The rate of endogenous carbon monoxide production ( Vco), determined by the closed rebreathing system technique, was elevated above the normal range in four of five patients studied with ineffective erythropoiesis (four patients with primary refractory anemia, one with thalassemia). The mean molar ratio of Vco to Vheme (rate of circulating heme catabolism, determined from (51)Cr red cell survival curves) was 3.0 +/- 0.6 (SE), indicating that most of the CO originated from sources other than circulating erythrocyte hemoglobin, in contrast to previous findings in patients with hemolytic anemia, where Vco paralleled Vheme closely.After administration of glycine-2-(14)C to these patients, endogenous CO was isolated by washout of body CO stores at high pO(2) or by reacting peripheral venous blood samples with ferricyanide. The CO was then oxidized to CO(2) by palladium chloride and trapped for counting in a liquid scintillation spectrometer. "Early labeled" peaks of (14)CO were demonstrated which paralleled "early labeled" peaks of stercobilin and preceded maximal labeling of circulating heme. Production of "early labeled" (14)CO in patients with ineffective erythropoiesis was greatly increased, up to 14 times that found in a normal subject. The increased Vco and "early (14)CO" production shown by these patients are presumably related mainly to heme catabolism in the marrow. The possibility exists that hepatic heme and porphyrin compounds may also contribute significantly to Vco, as suggested by the finding of a high Vco in an additional patient with porphyria cutanea tarda.

On the existence and structure of a new urobilin of molecular weight 592

Images

The sources of bile pigment in the rat: studies of the "early labeled" fraction

Images