Deficiency of hepatic organic anion-binding protein, impaired organic anion uptake by liver and "physiologic" jaundice in newborn monkeys

The glutathione S-transferases: a group of multifunctional detoxification proteins

The physiological roles of the glutathione S-transferases, by whatever name, seem to result in detoxification. As is true of albumin, members of this group of proteins bind an enormous number of compounds that appear to have in common only hydrophobic topography; the binding of bilirubin is an example of a major function common to all higher species. If the ligand bears a sufficiently electrophilic center, it will be attacked by the nucleophile GSH; such compounds would be the substrates of the enzyme. And should such a ligand be extraordinarily reactive--as, for example, some of the epoxide carcinogens generated by the cytochrome P450-linked, mixed-function oxidases, or even 1-chloro-2,4-dinitrobenzene--then reaction may occur either with GSH or irreversibly with the transferase itself. By reason of the wide distribution and high intracellular concentration of these proteins, there appears to be sufficient enzyme for all three roles in detoxification.

Evolution of the activity of UGT1A1 throughout the development and adult life in a rat

Biliary excretion is the main route of disposal of bilirubin and impaired excretion results in jaundice, a well recognisable symptom of liver disease. Conjugation of bilirubin in the liver is essential for its clearance. The glucuronidation of bilirubin is catalysed by the microsomal UDP-glucuronosyltransferase UGT1A1. Patients with Crigler-Najjar syndrome type 1 and Gunn rats, mutant strain of the Wistar rats, bear an autosomal recessive disorder resulting in hyperbilirubinemia. The aim of this work is to add new data about activity of UGT1A1 during the perinatal period and adult life. The results showed that activity of UGT1A1 is detectable from day 22 of the gestation. After birth, activity of UGT1A1 gradually increases and reaches the levels of adult life. Furthermore, bilirubin azopigments have been separated and characterized by thin layer chromatography. We have found that concentration of samples by evaporation and ulterior storing at -20 degrees C seemed to be suitable for the maintenance of samples.

Developmental pharmacology and toxicology: biotransformation of drugs and other xenobiotics during postnatal development

The following conclusions can be drawn: Depending on reaction type, species (strain), organ (tissue) and sex (in rats and mice) both phase I and phase II ractions show developmental patterns with maximum activities in juvenile or young adult animals. In man drug disposition in newborns is also generally much lower than in children and young adults. Mammals with long gestation periods are born with considerable activities and adult values are reached earlier than in animals with short gestation periods. Highest activities (with very few exceptions) are observed in liver postnatally. With increasing age, in all laboratory animals and in man, a decline of biotransformation capacity is observed. Most of these reactions are inducible by inducers of the phenobarbital-, 3-methylcholanthrene- and tetrachlorodibenzo-p-dioxine-, steroid- or alcohol-type. Inducibility also depends on reaction type, species (strain), organ (tissue) and sex (in rats and mice). Inducibility begins in the earliest embryonic stage and reaches high rates in species with long gestation periods before birth, and at or after birth in species with short gestation periods. The beginning of high inducibility depends also on inducer types and the induced parameter. Inducible reactions can be increased in immature as well as in very old animals up to or above the level of adult animals. Isozymes may show different developmental patterns and inducibilities. Development and induction of isozymes catalyzing different reactions can be triggered in clusters. There is increasing evidence that basic biotransformation activities as well as their inducibilities by foreign compounds are essentially influenced by a kind of temporal genetic control during the whole life span. When we compared different monooxygenase reactions and their inducibility (Klinger et al., 1968) both the presence of P450 isozymes and their different inducibility as well as their different developmental pattern became evident, the direct proof of which was given 10 years later by the Nebert group (Atlas et al. 1977). Functional heterogeneity was demonstrated by differential development and inducibility by glucocorticoids also for UDP-glucuronosyl transferase (Wishart, 1978): in the rat the increased fetal glucocorticoid activity between days 17 and 20 of gestation triggers the surge to adult or higher than adult activities. This cluster of the so-called late fetal glucuronyltransferase group is distinctly different from the neonatal or postnatal cluster with peak values after birth and a pronounced inducibility by 3-methylcholanthrene, but not by steroids. These different transferase clusters can be differentiated by different substrates, too. To get an overview on the postnatal development of the most important phase I and phase II reactions and also for the heme biosynthetic pathway as a prerequisite for the P450 synthesis, a compilation of literature data similar to a score was constructed with the aim to recognize parallel developments, possible common control and regulation mechanisms, cp. Klinger et al. 1987, cp. also Figs. 1-3. Evidently different developmental patterns can be observed, direct connections or dependencies cannot be detected. Thus different concentrations resp. activities of different P450 forms during ontogenetic development are influenced by many factors on the transscriptional and posttransscriptional level. Reviews on concepts and therories of development and aging have been published by several authors respectively editors (cp. Klinger, 1996). But there is no convincing concept or proof on the molecular-biological basis of an internal clock which regulates and controls individual development, aging and death.

Jaundice and breastfeeding

Optimal management of breastfeeding does not eliminate neonatal jaundice and elevated serum bilirubin concentrations. Rather, it leads to a pattern of hyperbilirubinemia that is normal and, possibly, beneficial to infants. Excessive frequency of exaggerated jaundice in a hospital or community population of breastfed infants may be a warning that breastfeeding policies and support are not ideal for the establishment of good breastfeeding practices. The challenge to clinicians is to differentiate normal patterns of jaundice and hyperbilirubinemia from those that indicate an abnormality or place an infant at risk.

Dichotomous development of the organic anion transport protein in liver and choroid plexus

Both adult liver and choroid plexus express the organic anion transport protein (oatp1) and transport [35S]bromosulfophthalein (BSP). Studies of the developing rat liver reveal that oatp1 mRNA and protein do not begin to be expressed until 15 days postnatal and are at adult levels by 30 days. Uptake of [35S]BSP follows the same time course. In contrast, neonatal rat choroid plexus expresses oatp1 mRNA and protein. When quantified on a weight basis, the uptake of [35S]BSP in choroid plexus is lower in the adult than at earlier stages of development. Although fluorescence confocal microscopy of adult rat choroid plexus shows that oatp is localized to the apical surface, facing the cerebrospinal fluid, this method reveals an intracellular localization of oatp1 in the neonate. Approximately 12 wk are required for the appearance of the adult pattern of distribution. Changes in the localization and activity of oatp1 during development could play an important role in the pathobiology of maturation of the liver and the central nervous system.

Influence of glutathione S-transferase B (ligandin) on the intermembrane transfer of bilirubin. Implications for the intracellular transport of nonsubstrate ligands in hepatocytes

To examine the hypothesis that glutathione S-transferases (GST) play an important role in the hepatocellular transport of hydrophobic organic anions, the kinetics of the spontaneous transfer of unconjugated bilirubin between membrane vesicles and rat liver glutathione S-transferase B (ligandin) was studied, using stopped-flow fluorometry. Bilirubin transfer from glutathione S-transferase B to phosphatidylcholine vesicles was best described by a single exponential function, with a rate constant of 8.0 +/- 0.7 s-1 (+/- SD) at 25 degrees C. The variations in transfer rate with respect to acceptor phospholipid concentration provide strong evidence for aqueous diffusion of free bilirubin. This finding was verified using rhodamine-labeled microsomal membranes as acceptors. Bilirubin transfer from phospholipid vesicles to GST also exhibited diffusional kinetics. Thermodynamic parameters for bilirubin dissociation from GST were similar to those for human serum albumin. The rate of bilirubin transfer from rat liver basolateral plasma membranes to acceptor vesicles in the presence of glutathione S-transferase B declined asymptotically with increasing GST concentration. These data suggest that glutathione S-transferase B does not function as an intracellular bilirubin transporter, although expression of this protein may serve to regulate the delivery of bilirubin, and other nonsubstrate ligands, to sites of metabolism within the cell.

Developmental pharmacology: ontogenic basis of drug disposition

Our understanding of clinical pharmacology in pediatrics, although limited, is continually increasing. This increase in our database reflects the supportive perspective of our colleagues, but more importantly, the recognition that specific pharmacokinetic and pharmacodynamic data in children are necessary for the provision of optimal care. In this review, we have focused on the many predictable influences of body maturation on drug disposition. These data underscore the importance of the many patient-specific variables that directly influence a patient's clinical response to drugs and, thus, the design of appropriate pharmacotherapeutic regimens.

Human fetal liver fatty acid binding proteins. Role on glucose-6-phosphate dehydrogenase activity

Fatty acid binding proteins (FABPs) may play an important role in the transport and metabolism of fatty acids during human embryogenesis. Three fractions of FABP, namely, DE-I, DE-II and DE-III, having Mr 14,200 Da each and pI values 7.8, 6.9 and 5.4, respectively, have been detected in human fetal liver. These proteins were purified by heat and butanol precipitation of fetal liver supernatant as well as by gel filtration and ion-exchange chromatography. Fetal liver FABPs are immunochemically identical to each other. Concentrations of DE-I, DE-II and DE-III increase gradually from early gestation to term. DE-I is almost lipid-free, DE-II binds long-chain fatty acids nonspecifically and DE-III transports mainly arachidonic acid. DE-II and DE-III protect glucose-6-phosphate dehydrogenase, which furnishes NADPH for fatty acid synthesis, from the feed-back inhibition exerted by added palmitoyl-CoA and oleate. In the absence of exogenous inhibitors, this enzyme is stimulated by FABPs. DE-I has no effect on such inhibition. Thus, FABPs play a regulatory role in critical aspects of cellular physiology during human embryogenesis.

Ontogeny of hepatic fatty acid-binding protein immunoreactivity in human liver and intestinal tract

Liver and intestinal tissues of human fetuses at gestational ages between 6 and 30 weeks were immunostained with rabbit antibody against fatty acid-binding protein (FABP) isolated from rat liver, since the antibody crossreacted with human FABP of the hepatic type. FABP-immunoreactive hepatocytes were found in the liver as early as the 7th week of gestation, but not at the 6th week. The frequency of immunoreactive cells was about 80% throughout the gestational period examined. No immunoreactive cells other than hepatocytes were found in liver tissue. In the intestinal tract, ileal, colonic and vermiform appendicular FABP immunoreactivity was demonstrated at the 23rd week of gestation, and duodenal and jejunal immunoreactivity at the 26th week. Positive cells in the jejunum were very few at this stage, but numerous at the 30th week of gestation. The immunoreactive cells were primitive absorptive cells in intestinal villi, and no cryptic epithelial cells were positively immunostained. Thus, FABP immunoreactivity was considered to be a marker for hepatocytes at the early to late fetal stage, and for intestinal absorptive cells at mid- to late fetal stage.

Immunohistochemical studies on the distribution and frequency of fatty-acid-binding protein positive cells in human fetal, newborn and adult liver tissues

Human tissues from adult, newborn/infant, and fetal livers, and gall bladders were immunostained with rabbit antibodies against fatty-acid-binding protein (FABP) isolated from rat liver. These antibodies cross-reacted with FABP from human liver by a double immunodiffusion method. The antigenicity of FABP of human liver is well preserved in various fixatives including 10 per cent formaldehyde, 4 per cent paraformaldehyde, and Zamboni's and Bouin's solution. FABP immunoreactivities, moreover, could be detected similarly in both cryostat and paraffin sections. FABP of hepatic type was found in hepatocytes most frequently in the fetus (as early as the 7th week of gestation), moderately in the newborn/infant, and infrequently in adult liver tissues. The distribution pattern of the positive hepatocytes was uniform throughout the acini in fetuses, but periportal in adults. No immunoreactive cells were found in the gall bladder or in the extra- and intra-hepatic bile ducts. In this study, FABP was found in human hepatocytes in both fetal and postnatal livers. FABP, therefore, might be a useful marker substance in the investigation of the physiology and pathology of the human liver.

Studies on the relationship between glutathione S-transferase phenotype and bile acid binding by human liver cytosol

The possibility that the GST1 phenotype of human liver cytosol is a determinant of bile salt binding has been investigated by using equilibrium dialysis and gel-exclusion chromatography. Binding of bile salts was non-saturable and whereas the glutathione S-transferases did not appear to be major bile salt binders, other binding components with molecular weights of 35 000 and 11 000 were identified in both fetal and adult cytosols.

Breast-milk jaundice

The syndrome of breast-milk jaundice, which often results in cessation of breastfeeding, maternal anxiety, and guilt, may be increasing. Research to date on pregnanediol, increased lipase, and free fatty acids as the causes of breast milk jaundice is reviewed. Variations in current treatment are presented and nursing measures supportive of parents and continued breastfeeding are provided.

Effects of phenobarbital on bilirubin metabolism and its response to phototherapy in the jaundiced Gunn rat

Jaundiced Gunn rats, treated with phenobarbital (60 mg per kg i.p. for 7 to 10 days) showed 25 and 36% decreases in mean plasma bilirubin levels in two experiments (p less than 0.01). Kinetic studies with tracer 14C-bilirubin revealed that there was no change in bilirubin turnover or total pool size due to phenobarbital, but a 49% increase in the hepatic pool and a 27% decrease in the cutaneous pool of bilirubin. The increase in the hepatic pool accounted for over 90% of the bilirubin lost from the plasma. Such pretreatment with phenobarbital did not alter the decline in plasma bilirubin or total bilirubin pool due to subsequent phototherapy. Phenobarbital followed by phototherapy produced a significantly greater reduction in plasma bilirubin levels than either treatment alone. These studies demonstrate that phenobarbital does decrease plasma bilirubin in Gunn rats primarily by shifting the pigment to the liver, and suggests that combined treatment with phenobarbital and phototherapy might be of value in patients with congenital hyperbilirubinemia due to glucuronyl transferase deficiency.

Fatty acids and the control of bilirubin levels in blood

The evidence that changes in the hepatic intracellular concentration of non-esterified fatty acids affect the clearance of bilirubin from blood by the liver is reviewed. Such a mechanism would in particular explain the rise in plasma bilirubin levels during fasting.

Modulation of the transport of bilirubin and asialoorosomucoid during liver regeneration

The normal rat hepatocyte divides approximately once per year but, following two thirds hepatectomy, rapid cellular replication occurs throughout the remaining liver remnant. Using a multiple indicator dilution technique, single-pass transport of 3H-bilirubin and 125I-asialoorosomucoid was studied in isolated perfused liver from 6 hr to 6 d after two thirds hepatectomy or sham surgery. Influx (k1), efflux (k2), and sequestration (k3) rates were quantitated by computer analysis. k1 for 3H-bilirubin fell by over 50% within 6 hr after two thirds hepatectomy and returned to normal 4 d later. k2 progressively decreased with a nadir at 2 d, and returned to normal by 4 d. k3 was transiently depressed, and became normal within 2 d. Although hepatic uptake of asialoglycoproteins has been thought to be irreversible, the experimental data required k2 and k3 parameters for best fit. Similar to results for 3H-bilirubin, the k1 of 125I-asialoorosomucoid was 20% of normal at 1 d after two thirds hepatectomy, and returned to normal by 6 d. Unlike results for 3H-bilirubin, there was a prolonged 50% reduction of k2 and k3 with return to normal by 6 d. The transport changes during regeneration are independent of reduced liver mass or changes in hepatic spaces of distribution. The fact that influx of both compounds reaches a nadir at the time of greatest cellular proliferation with subsequent return to normal suggests a "maturation" of liver cell function for restoration of these specific hepatocyte functions. Modulation of the hepatocyte receptor for desialylated glycoproteins may also be required for cellular recognition as a prerequisite for proliferative responses.

Immunohistological localization of ligandin in human tissues

An immunohistochemical localization of ligandin was undertaken in formalin fixed and paraffin wax embedded human tissues using the indirect immunoperoxidase (PAP) method and a monospecific antiligandin serum raised in rabbits. A substance reacting with this antiligandin serum was distributed diffusely in normal liver and selectively in kidney, intestine, testis, ovary and adrenal cortex. Small changes in the distribution and intensity of the reaction product were found in inflammatory conditions such as hepatitis, cholestasis, pyelonephritis and renal allograft rejection. Tissues which normally appear to contain abundant ligandin produce, as a general rule, easily demonstrate amounts of antiligandin reacting substance in the tumors and hyperplasias which arise from them.

[Indocyanine green kinetics in newborns with non-hemolytic hyperbilirubinemia (author's transl)]

The kinetic parameters of indocyanine green elimination from blood were determined after an intravenous load of the dye in a dosage of 2-4 mg per kg body weight in 22 newborns with a non-hemolytic hyperbilirubinemia. Since the uptake of indocyanine green by liver is selectively carried out and the dye is not further metabolised, these kinetic parameters serve as measures for the performance of hepatocellular elimination. 11 of these newborns were treated 5 days previously with 7.5 mg phenobarbital per kg body weight. Compared to the untreated group, the serum bilirubin concentration significantly decreased after treatment with phenobarbital and the parameters of elimination of the dye from blood changed as described by saturation-kinetics. The maximal elimination-rate Vmax and the Michaelis-Menten-constant Km were significantly higher in newborns treated with phenobarbital (71.1 muMol/l-min and 356.4 muMol/l) than in the untreated ones (23.4 muMol/l-min and 100.0 mutmol/l). Kinetic data of indocyanine green elimination gathered in newborns treated with and without phenobarbital support the hypothesis that cytoplasmatic proteins oliver should facilitate the uptake of organic anions inclusively bilirubin into the liver cell. A defiency of such transport proteins may be one of the causes of non-hemolytic hyperbilirubinemia in newborns.

Development of bilirubin transport and metabolism in the newborn rhesus monkey

Hepatic transport and metabolism of bilirubin have been examined in term, premature, and postmature newborn Macaca mulatta (rhesus) monkeys with and without prior phenobarbital treatment of pregnant mother and neonate. In untreated neonates a biphasic pattern of physiologic unconjugated hyperbilirubinemia has been observed. Phase I was characterized by a rapid increase in serum bilirubin concentration to 4.5 mg/dl by 19 hours and an equally rapid decline to 1.0 mg/dl by 48 hours of age. Phase II was characterized by a stable elevation at 1.0 mg/dl (four times greater than in the adult) from 48 to 96 hourse of age, followed by a decline to normal adult concentrations thereafter. An identical pattern was observed in 29 normal, term human neonates, but the duration of each phase was approximately three times as long as that in the monkey. Phase I hyperbilirubinemia appears to result from a sixfold increase in bilirubin load presented to the liver in the neonatal period, combined with marked deficieny in hepatic bilirubin conjugation, the rate-limiting step during Phase I. Hepatic uptake of bilirubin is not rate limiting during Phase I but may contribute to Phase II hyperbilirubinemia. An increased bilirubin load persists throughout the first 19 days of life in the monkey. Phase I physiologic jaundice in the monkey neonate was completely eliminated by prenatal maternal and neonatal administration of phenobarbital. A threefold enhancement of hepatic conjugation of bilirubin (glucuronyl transferase activity) during Phase I entirely accounted for the prevention of hyperbilirubinemia. The bilirubin load was unaffected by administration of phenobarbital. Whereas in control neonates the bilirubin load slightly exceeded hepatic bilirubin conjugating capacity and resulted in retention of bilirubin, in phenobarbital-treated neonates, hepatic conjugating capacity slightly exceeded that required for the bilirubin load. Administration of phenobarbital failed to alter Phase II hyperbilirubinemia and did not enhance either maximal hepatic uptake or excretion of bilirubin. Hepatic glucuronly transferase activity was increased threefold during Phase II and during the remainder of the neonatal period. Premature birth retarded maturation of hepatic glucuronyl transferase activity. In one phenobarbital-treated premature monkey neonate, there was no apparent response to treatment. Accelerated maturation of bilirubin uptake, conjugation, and excretion of bilirubin was observed in one postmature monkey neonate.

Developmental aspects of glutathione S-transferase B (ligandin) in rat liver

The postnatal development in male Sprague-Dawley rats of hepatic glutathione S-transferase B (ligandin) in relation to the other glutathione S-transferases is described. The concentration of glutathione S-transferase B in 1-day-old male rats is about one-fifth of that in adult animals. The enzyme reaches adult concentrations 4-5 weeks later. When assessed by substrate specificity or immunologically, the proportion of transferase B relative to the other glutathione S-transferases is high during the first week after birth. At this age, 67.5% of the transferase activity towards 1-chloro-2,4-dinitrobenzene is immunoprecipitable by anti-(transferase B), compared with about 50% in adults and older pups. Between the second and the fifth postnatal week, the fraction of transferase B increases in parallel fashion with the other transferases in hepatic cytosol. Neither L-thyroxine nor cortisol induce a precocious increase in glutathione S-transferase activity. Phenobarbital did induce transferase activity towards 1-chloro-2,4-dinitrobenzene and 1,2-dichloro-4-nitrobenzene in both pups and adults. The extent of induction by phenobarbital was a function of basal activity during development such that the percentage stimulation remained constant from 5 days postnatally to adulthood.

Hepatic organic anion uptake in the rat

The hepatic uptake of bilirubin (BR), indocyanine green (ICG), and sulfobromophthalein (BSP) was studied in 350 anesthetized Sprague-Dawley rats by determining the initial plasma disappearance rate (V) of various doses of unlabeled ICG, or of tracer quantities of [3H]BR or [35S]BSP injected into the jugular vein simultaneously with varying amounts of unlabeled BR or BSP. Similar studies were also performed involving the simultaneous injection of potential inhibitors of hepatic uptake. The results indicate that: (a) hepatic uptake determined by direct tissue measurement could be accurately estimated from the plasma disappearance data; (b) saturation of hepatic uptake with increasing dose was readily demonstrated for each of these three organic anions, and in each instance a plot of V versus dose took the form of a rectangular hyperbola analyzable in terms of Michaelis-Menten kinetics; (c) for BR, the saturable uptake process showed a Vmax more than 100 times the normal net transfer rate from plasma to bile; (d) hepatic uptake of BR, BSP, and ICG showed relatively selective, mutually competitive inhibition; glycoholic acid did not inhibit hepatic uptake of any of these substances; and (e) "counter-transport" could be demonstrated for each of the three test substances. These data are compatible with the existence of a carrier-mediated transport process for hepatic uptake of each of these three organic anions and clarify the relationship of hepatic BR uptake to its overall transport from plasma to bile.

Biliary excretion of xenobiotics

The biliary route is very important for the elimination of some foreign compounds from the body. For many of these compounds, an increase in the rate at which they are excreted into the bile will decrease their toxicity and vice versa. A number of factors which are known to alter the biliary excretion of xenobiotics, as well as the current concepts of the physiological mechanisms responsible for the excretion of foreign compounds, have been enumerated. However, much remains still to be understood; essentially nothing is known at the subcellular level about the biliary excretion of foreign compounds. It has recently been concluded that our knowledge of the biliary excretion of compounds is about 40 years behind that of the renal excretion mechanism.

Structural and functional studies of ligandin, a major renal organic anion-binding protein

Sephadex gel filtration of the 1000,000 g supernate of homogenates of rat kidney revealed binding of various organic anions (penicillin, Bromsulphalein [BSP], bilirubin, phenolsulfonphthalein [PSP], phlorizin, glutathione [GSH], p-amino hippurate (PAH), probenecid, conjugated bilirubin, and BSP-GSH) to a nonalbumin-containing protein fraction (Y), which precipated on addition of monospecific anti-rat liver ligandin (Y protein)-IgG, but not control IgG. Quantitatively similar organic anion binding was observed in vivo after injection of BSP, BSP-GSH, phlorizin, probenecid, conjugated bilirubin, PAH, or penicillin. The binding protein was purified to apparent homogeneity and is a basic protein (pI 8.9) of 44,000 daltons with two apparently identical subunits of 22,000 daltons. Monospecific antibody was produced against the renal protein. The results of binding studies in vivo and in vitro and phsicochemical, immunologic, structural, and binding site investigations indicate that the renal protein is identical to hepatic ligandin. Immunofluorescent studies utilizing anti-ligandin IgG previously localized ligandin in the kidney to all proximal tubular cells. By quantitative radial immunodiffusion, the concentration of renal ligandin was 31.2 plus or minus 2.2 mug/mg supernatant protein and was increased 160% above basal values by pretreatment of rats with tetrachloro-dibenzo-p-dioxin. Pretreatment with phenobarbital, DDT, or pregnene-16alpha-carbonitrile did not increase renal ligandin concentration but doubled hepatic ligandin concentration. Circular dichroism studies of renal ligandin revealed percent helical structure similar to hepatic ligandin and primary association contrasts were derived for BSP (10-6 M-1) and PAH, probenecid, and penicillin (10-3 M-1). Administration of BSP or probenecid simultaneously with [C14] penicillin resulted in increased plasma retention and reduced kidney and urinary bladder content of [14C] penicillin and a correlation coefficient of -0.8 between total kidney/plasma radioactivity and percent of protein-bound radioactivity bound to ligandin in the kidney. These studies indicate that renal and hepatic ligandin are identical. Their response to drugs and chemicals varies. Competitive binding between several organic anions for ligandin correlated with their renal uptake from plasma, which suggests that ligandin may function in the proximal tubular cell as a component of the renal organic anion transport system.

The identity of glutathione S-transferase B with ligandin, a major binding protein of liver

Evidence is presented that ligandin, an intracellular protein involved in the binding of such anions as bilirubin, indocyanine green, and penicillin, is identical to glutathione S-transferase B (EC 2.5.1.18), an enzyme catalyzing the conjugation of glutathione with such electrophiles as 1-chloro-2,4-dinitrobenzene, 1,2-dichloro-4-nitrobenzene, iodomethane, ethacrynic acid, and bromosulfophthalein. The proteins, isolated by distinct methods, have the same specificity for substrates and for ligands, react in identical fashion to antibody produced against ligandin, bear entirely similar physical characteristics and amino acid composition, and are both induced in response to phenobarbital. Indocyanine green, one of the ligands that is not effective as a substrate, was shown to competitively inhibit the conjugation reaction. It is suggested that specificity is directed toward compounds with electrophilic sites.

Fatty acid-binding protein in small intestine. Identification, isolation, and evidence for its role in cellular fatty acid transport

A soluble fatty acid-binding protein (FABP), mol wt approximately 12,000 is present in intestinal mucosa and other tissues that utilize fatty acids, including liver, myocardium, adipose, and kidney. This protein binds long chain fatty acids both in vivo and in vitro.FABP was isolated from rat intestine by gel filtration and isoelectric focusing. It showed a reaction of complete immunochemical identity with proteins in the 12,000 mol wt fatty acid-binding fractions of liver, myocardium, and adipose tissue supernates. (The presence of immunochemically nonidentical 12,000 mol wt FABP in these tissues is not excluded.) By quantitative radial immunodiffusion, supernatant FABP concentration in mucosa from proximal and middle thirds of jejuno-ileum significantly exceeded that in distal third, duodenum, and liver, expressed as micrograms per milligram soluble protein, micrograms per gram DNA, and micrograms per gram tissue. FABP concentration in villi was approximately three times greater than in crypts. Small quantities of FABP were present in washed nuclei-cell membrane, mitochondrial and microsomal fractions. However, the amount of FABP solubilized per milligram membrane protein was similar for all particulate fractions, and total membrane-associated FABP was only about 16% of supernatant FABP. Intestinal FABP concentration was significantly greater in animals maintained on high fat diets than on low fat; saturated and unsaturated fat diets did not differ greatly in this regard.The preponderance of FABP in villi from proximal and middle intestine, its ability to bind fatty acids in vivo as well as in vitro, and its response to changes in dietary fat intake support the concept that this protein participates in cellular fatty acid transport during fat absorption. Identical or closely related 12,000 mol wt proteins may serve similar functions in other tissues.

Sites of competition in the selective hepatic uptake of rifamycin-SV, flavaspidic acid, bilirubin, and bromsulphthalein

In rats both rifamycin-SV and flavaspidic acid impaired the disappearance of bromsulphthalein (BSP) from plasma. The addition of rifamycin-SV to rat liver supernatant containing BSP did not displace BSP from ligandin or Z protein unless it was added in very high concentration. In similar studies in vitro with flavaspidic acid, BSP was displaced from Z protein even at low concentrations of flavaspidic acid, whereas there was only a minor effect on the binding to ligandin. The intravenous administration of rifamycin to Gunn rats raised the plasma bilirubin concentration slightly after 30 minutes whereas flavaspidic acid was without significant effect. At 30 minutes the amount of bilirubin in the liver was markedly reduced by rifamycin and moderately depressed by flavaspidic acid. Although both the rifamycin and flavaspidic acid lowered the bilirubin in cell sap, the main effect of flavaspidic acid appeared to be on binding to Z protein. Rifamycin reduced the amount bound to both ligandin and Z protein. It is suggested that flavaspidic acid acted on the hepatic uptake of bilirubin and BSP predominantly by competing for binding to Z protein. Rifamycin-SV acted at a different site, probably blocking uptake at the plasma membrane.