Purification and characterization of acidic form of glutathione S-transferase in human fetal livers: high similarity to placental form

Ontogeny of hepatic and renal systemic clearance pathways in infants: part I

Dramatic developmental changes in the physiological and biochemical processes that govern drug pharmacokinetics and pharmacodynamics occur during the first year of life. These changes may have significant consequences for the way infants respond to and deal with drugs. The ontogenesis of systemic clearance mechanisms is probably the most critical determinant of a pharmacological response in the developing infant. In recent years, advances in molecular techniques and an increased availability of fetal and infant tissues have afforded enhanced insight into the ontogeny of clearance mechanisms. Information from these studies is reviewed to highlight the dynamic and complex nature of developmental changes in clearance mechanisms in infants during the first year of life. Hepatic and renal elimination mechanisms constitute the two principal clearance pathways of the developing infant. Drug metabolising enzyme activity is primarily responsible for the hepatic clearance of many drugs. In general, when compared with adult activity levels normalised to amount of hepatic microsomal protein, hepatic cytochrome P450-mediated metabolism and the phase II reactions of glucuronidation, glutathione conjugation and acetylation are deficient in the neonate, but sulfate conjugation is an efficient pathway at birth. Parturition triggers the dramatic development of drug metabolising enzymes, and each enzyme demonstrates an independent rate and pattern of maturation. Marked interindividual variability is associated with their developmental expression, making the ontogenesis of hepatic metabolism a highly variable process. By the first year of life, most enzymes have matured to adult activity levels. When compared with adult values, renal clearance mechanisms are compromised at birth. Dramatic increases in renal function occur in the ensuing postpartum period, and by 6 months of age glomerular filtration rate normalised to bodyweight has approached adult values. Maturation of renal tubular functions exhibits a more protracted time course of development, resulting in a glomerulotubular imbalance. This imbalance exists until adult renal tubule function values are approached by 1 year of age. The ontogeny of hepatic biliary and renal tubular transport processes and their impact on the elimination of drugs remain largely unknown. The summary of the current understanding of the ontogeny of individual pathways of hepatic and renal elimination presented in this review should serve as a basis for the continued accruement of age-specific information concerning the ontogeny of clearance mechanisms in infants. Such information can only help to improve the pharmacotherapeutic management of paediatric patients.

Differential sensitivity of children and adults to chemical toxicity. I. Biological basis

Children, particularly neonates, can be biologically more sensitive to the same toxicant exposure on a body weight basis than adults. Current understanding of the rates of maturation of metabolic capability and evidence from case examples on pharmaceuticals, drugs of abuse, environmental contaminants, and dietary and endogenous agents indicate that human infants up to approximately 6 months of age are typically--but not always--more sensitive to chemical toxicity than adults. For most chemicals, the immaturity of infant biotransformation, elimination, and other physiologic systems usually produces higher blood levels for longer periods. There is metabolic capacity for most tested substances in the newborn, although it is quite low and immature for some chemicals. For some chemicals, unique metabolic pathways not available in the adult human can also be utilized by the newborn. The newborn's metabolic capacity rapidly matures and, by about 6 months of age, children are usually not more sensitive to chemical toxicity than adults. By then, most metabolic systems are reasonably mature, becoming almost completely capable by 1 year of age. In many cases children are less sensitive than adults. Whether children are at greater risk from chemical exposures is another question. Risk depends on both inherent sensitivity and exposure conditions. If chemical exposure levels remain below those capable of overwhelming a child's metabolic detoxification systems and producing toxicity, children will be at no greater risk than are adults. Children of all ages are still developing so even if they are exposed to chemicals at levels below those of adults, they may be at greater risk than adults. However, as long as those exposure levels are still below those required to produce toxicity, children will not be at greater risk.

Allele-specific monoclonal antibodies against glutathione S-transferase Mu1-1

IgG1 class mouse monoclonal antibodies (MAbs) were produced against human glutathione S-transferase Mu1-1 (GSTMu1-1). Eight MAbs of 16 are able to recognize only the native form of the enzyme; 4 MAbs bind to native and denaturated enzyme, and the remaining 4 can bind only to partially denatured antigen in direct ELISA or Western blot. The antibodies recognizing the native form of the enzyme bind to six different epitopes. Three overlapping epitopes are responsible for specific binding of MAbs to different allelic variants of GSTMu1-1. Three allele-specific antibodies, 2E1, 11F12, and 7D11, bind to GSTM1a monomer and the other two, 1H8 and 3H10, recognize GSTM1b monomer.

Glutathione S-transferase mediated detoxification and bioactivation of xenobiotics during early human pregnancy

Glutathione S-transferase (GST) with activity towards CDNB as a substrate from human intrauterine conceptual tissues (HICT) at 6-10 weeks of gestation was purified approximately 200-fold by GSH coupled Sepharose 4B affinity chromatography. The preparations of affinity purified HICT-GST were used in all the experiments. There was no significant difference in specific activity of GST towards CDNB among 6-, 8- and 10-week-old HICT. GST specific activity decreased with an increase in the gestational age using p-nitrobenzyl chloride and 3,4-dichloro-1-nitrobenzene as the second substrate. Similarly, the lower the gestational age, the higher was the degree of GST mediated ethylene dibromide (EDB) conjugation with GSH. Covalent binding of [14C]EDB to DNA and protein was greater in the presence of HICT-GST at 6 weeks gestation than that at 10 weeks gestation. These results suggest that HICT possess a significant amount of GST capable of detoxification or activation of xenobiotics during the critical organogenesis period.

A novel model to assess developmental toxicity of dihaloalkanes in humans: Bioactivation of 1,2-dibromoethane by the isozymes of human fetal liver glutathione S-transferase

Glutathione S-transferase (GST) isozymes from human fetal liver (16-18 weeks gestation) were purified by affinity chromatography followed by ion-exchange high performance liquid chromatography (HPLC). The purified isozymes were used to investigate toxicity of 1,2-dibromoethane(EDB) in an in vitro model of rat embryos in culture as passive targets. At least five isozymes of GST were found in the human fetal liver. Two anionic forms [pI values 5.5 (P-2) and 4.5 (P-3)] and one basic form [pI value 8.7 (P-6)] were clearly separated. The presence of two near-neutral forms was also identified. All the isozymes of the human fetal liver GSTs tested metabolized EDB (specific activities were 2.1, 7.0, and 2.0 mumol of GSH consumed/min/mg protein for P-2, P-3, and P-6 isozymes, respectively). Covalent binding of EDB to DNA and protein was 144% and 212% higher, respectively, with the P-3 anionic isozyme when compared to the P-6 basic isozyme of GST. No covalent binding to either protein or DNA was observed with the P-2 isozyme. EDB bioactivation by the GST isozyme P-3 (15 units; 1 unit = 1 nmol of GSH consumed/min) resulted in toxicity to cultured rat embryos. Significant reductions of crown rump length, yolk sac diameter, and the composite score of morphological parameters (Brown and Fabro method) were observed. The central nervous system, optic and olfactory systems, and the hind limb were most significantly affected. The results of this investigation suggest that EDB may be classified as a suspected developmental toxicant in humans.