Age-related changes in glucose metabolizing enzymes in spleen, thymus, and pulmonary lavage cells from F344 rats

Healthy male Fischer 344 rats were sampled at 6, 12, 18, and 24 months of age. There was no gross pathological evidence or deviations in body weight, hematology, or clinical chemistry that were indicative of disease. Mixed populations of thymus, spleen, and pulmonary cells were obtained for enzymatic analyses. Key enzymes from the hexose monophosphate shunt, glycolysis and the tricarboxylic acid cycle were evaluated to determine if there were tissue-specific or pathway-specific changes that occurred during aging. The enzyme responses among the tissues were not consistent during the aging process. Generally the activities of the glucose metabolizing enzymes in thymus and pulmonary lavage cells decreased with age whereas they increased in the spleen cells. Between 18 and 24 months enzymes representative of all three glucose metabolic pathways decreased in pulmonary lavage cells, whereas the decreases in thymic cells were mainly restricted to glycolytic enzymes. By contrast there were two- to ten-fold increases during aging in all of the splenic enzymes measured except malate dehydrogenase. The alterations in tissue enzyme activities probably reflected the changing cellular populations during aging, and in the thymic and pulmonary lavage cellular environment resulted in a loss of energy production by glucose oxidation, compared to the vigorous activity maintained in spleen.

Disposition of 4,4'-thiobis(6-t-butyl-m-cresol) in rats

The absorption, distribution, metabolism, and excretion of 14C-labeled 4,4'-thiobis(6-t-butyl-m-cresol) (TBBC) was studied in male rats. Oral treatment showed a dose-related decrease in the rate of absorption due to a dose-related increase in retention time in the stomach. TBBC was incompletely absorbed after oral treatment, although the rate of absorption was proportional to the dose once the compound reached the small intestine. TBBC was rapidly distributed throughout the body with the liver being the major tissue depot. Significant amounts of the compound were also present in blood, muscle, skin, and adipose tissue. TBBC was initially rapidly cleared from all tissues except adipose, although a small percentage of the total dose tended to persist in liver and skin. Over half of the compound was excreted the first day, primarily via the bile into the feces. Little TBBC-derived radioactivity appeared in the urine. Metabolites of TBBC were present in the tissues at early times after administration, but were rapidly excreted. The major metabolite(s) appeared to be glucuronide conjugates of the parent compound. Thus, TBBC, an important antioxidant in the rubber and plastic industries, would tend to accumulate in liver and lipid-rich tissues upon chronic exposure, which if by the oral route, could also result in direct damage to the gastrointestinal tract.

Hexabromonaphthalene contaminants of polybrominated biphenyls: chemical composition and disposition in the rat

Hexabromonaphthalene (HBN) has been identified as a toxic contaminant of Firemaster BP-6, a mixture of polybrominated biphenyls (PBBs) that is used commercially as a fire retardant and that was responsible for a major public health emergency in Michigan in 1974. Previously reported to be a single compound, the hexabrominated naphthalene derived from direct bromination of naphthalene was here shown by high-field nuclear magnetic resonance (NMR) to be a mixture of two closely related isomers, 1,2,3,4,6,7-HBN and 2,3,4,5,6,7-HBN, in a ratio of 60:40. The absorption, distribution, metabolism, and excretion of the HBN mixture was studied in the male Fischer-344 rat. [14C] HBN was incompletely absorbed after oral doses of 0.4 and 4.0 mumol/kg body weight. After iv treatment, fecal excretion accounted for 24% of the total dose by the end of d 1, 44% by d 3, and 62% by d 35. Urinary excretion was negligible. The excreted radioactivity was in the form of metabolite(s). Biliary excretion studies confirmed the nature of the excreted dose. The tissue distribution pattern showed accumulation and redistribution during early time points (0.5-3 h). By the end of the first day, 30.7% and 12.3% of the total dose remained in the liver and fat, respectively. All other tissues accounted for less than 5% of the total dose by d 1. The major tissue depots remained liver and adipose tissue, which contained 26% and 4.6% of the total dose, respectively, by d 35. This residual radioactivity was found to be unmetabolized HBN. Thus, over 60% of the dose of HBN is readily metabolized and excreted and was tentatively identified with the toxic isomer 1,2,3,4,6,7-HBN. The remainder, probably 2,3,4,5,6,7-HBN, was relatively nontoxic and extremely persistent, with the liver being the primary site of long-term accumulation.

Toxicity and distribution of 2,3,7,8-tetrachlorodibenzofuran in male guinea pigs

The toxicity of 2,3,7,8-tetrachlorodibenzofuran (TCDF) was studied in male Hartley guinea pigs after single or multiple oral treatment. In each animal studied, adipose tissue, liver, and skin were the major depots of TCDF and accounted for 50-74% of the total dose. The whole-body half-life of TCDF in guinea pigs was estimated to be approximately 40 d. This slow clearance of TCDF by guinea pigs may explain the high toxicity of TCDF for this species even when very low repeated doses were administered. Intervals of 1, 2, or 4 wk between doses had little effect on TCDF lethality, but did have an effect on the pattern of toxicity. Treatment of mature animals with high single doses of TCDF (10 or 15 micrograms/kg body weight) resulted in the immediate loss of weight and the eventual death of all treated animals within 2-4 wk after the loss of approximately 35% of their initial body weight. Treatment of immature animals with low multiple doses totaling cumulative doses of between 4 and 12 micrograms/kg resulted in the death of 75% of these animals, with the deaths occurring between 7 and 19 d after the initial appearance of quantitative toxic symptoms (loss of weight); however, weight loss was less dramatic following repeated low doses than after acute high doses. Concentration of TCDF in adipose tissue was generally proportional to dose of TCDF. With increasing time after dosage, there was a shift in distribution from adipose tissue to liver.

Distribution and excretion of 2,3,6,2',3',6'- and 2,4,5,2',4',5'-hexachlorobiphenyl in senescent rats

The disposition of two symmetrical [14C]hexachlorobiphenyls (HCBs), 2,3,6,2',3',6'-HCB (236) and 2,4,5,2',4',5'-HCB (245), was studied in 24-month-old male Sprague-Dawley rats after iv treatment. Because body composition changes with age, complete dissections were performed on all rats to determine the size of the skin and adipose tissue depots. More than 50% of 236 was metabolized and excreted via the bile into the feces within 2 days. In contrast, 245 redistributed from the liver, muscle, and skin to adipose tissue where it accumulated without being metabolized. Only 2% of the total dose of 245 was excreted primarily in the feces within 21 days. The data obtained in this study were compared to results previously obtained from 2- to 3-month-old rats in this laboratory (Matthews and Tuey, 1980, Toxicol. Appl. Pharmacol. 53, 377-388). Although the general pattern of HCB disposition did not change with age, i.e., metabolism and excretion of 236 versus persistence of 245, there were differences in the rates of elimination and in the tissue levels. There was enhanced metabolite retention in the muscle, skin, and adipose tissue of older animals which suggested an age-related decrease in tissue clearance. The large volume of adipose tissue in these older Sprague-Dawley rats could in part explain this observation. In general, there were few changes in decay rates from tissues or in biliary excretion. Age had a greater effect on the disposition of the persistent 245 than on the metabolizable 236. Thus, changes in body composition seemed to play a major role in age-related changes in the distribution and excretion of polychlorinated biphenyls.

Physiological model for the pharmacokinetics of 2,3,7,8-tetrachlorodibenzofuran in several species

A flow-limited physiological model was developed to describe the time course of 2,3,7,8-tetrachlorodibenzofuran (TCDF) in the blood and tissues of rats, mice, and monkeys. The liver showed the greatest tendency to concentrate the material with tissue-to-blood distribution coefficients ranging from 30 in the monkey to 130 in the mouse. TCDF was also concentrated in the fat with tissue-to-blood distribution coefficients between 25 and 40 in all species. TCDF was eliminated by metabolism followed by excretion primarily to the feces. Urinary excretion was a minor route of elimination in all species. Metabolism was modeled as a linear process occurring in the liver. Intrinsic metabolic clearances ranged from 0.45 ml/min/kg in the monkey to 2.8 ml/min/kg in one species of mice. Fecal excretion of TCDF-derived radioactivity can be simulated with a series of well-mixed compartments which receive input of metabolites in the bile.

Age-related changes in mutagen activation by rat tissues

Age-related changes in drug metabolism of the liver, lung and kidney of adult female Long-Evans rats were determined by measuring changes in mutagen formation. Activation of aflatoxin B1 (AFB1), 2-aminofluorene (AF) and 2-acetylaminofluorene (AAF) to mutagenic derivatives was assayed using the Ames Salmonella test system. The promutagens were incubated with tissue fractions from rats ranging in age from 2.5 to 25 months. With all three compounds, hepatic, renal and pulmonary activation was lower in the senescent than in the young adult animals. The largest decrease, however, occurred prior to middle-age, i.e. before 9-13 months. In liver and kidney, little change was detectable between the middle-aged and the old (20-25 months) animals. However, pulmonary metabolism in the oldest animals was slightly higher than in the extracts from the middle-aged rats. The observed decline in mutagen activation may thus be a function of maturation rather than senescence.

Changes in hepatic microsomal membrane fluidity with age

There are changes in the mixed function oxidase enzymatic activities of rat hepatic microsomal membranes with age. However, the protein components of the mixed function oxidase system do not appear to change with age. The purpose of this study was to detect possible changes in the fluidity of the lipid component of the microsomal membrane with age. Hepatic microsomes were isolated by differential centrifugation from uninduced, male CFN rats aged 3, 12 and 26 mo. The microsomal membrane fluidity was measured using electron paramagnetic resonance after incorporation of a 5-nitroxide stearic acid spin label into the membrane. The order parameter S decreased with age from 0.586 +/- 0.003 (3 mo) to 0.581 +/- 0.002 (12 mo) to 0.569 +/- 0.003 (26 mo) at 30 degrees C. This indicated an increase in membrane fluidity with age. In membranes labeled with the 16-nitroxide stearic acid, a similar increase in membrane fluidity with age was observed. The order parameter of microsomal membranes from 3 and 26 mo rats was measured over the temperature range 10 degrees to 31 degrees C in steps of 0.9 degrees C. A plot of the log of S versus the reciprocal temperature revealed a phase transition at 24 degrees C in membranes from 26 mo rats, but no phase transition was observed in 3 mo old rats in this temperature range. The change in fluidity of the hepatic microsomal membrane with age may account for some of the observed changes in membrane-bound mixed function oxidase activities with age.

Increased production of mutagenic metabolites of carcinogens by tissues from senescent rodents

Activation of the procarcinogens benzo(a)pyrene and 2-fluorenamine by liver homogenates (S9) prepared from senescent male CFN rats and C57BL/6J mice resulted in an enhanced production of mutagenic metabolites when compared to young rodents, as indicated by an enhancement of the induced reversion frequency in a Salmonella typhimurium bioassay. Similar results were observed when carcinogen activation was mediated by purified hepatic microsomes, indicating that the age-related differences in carcinogen activation did not result from aging changes in carcinogen metabolism involving non-microsomal mechanisms. The metabolites of many procarcinogens are thought to be the ultimate carcinogens in mammals. Therefore, the present findings are consistent with the hypothesis that some fraction of the markedly increased of neoplasia observed in senescent mammals is a result of age-related alterations in the metabolism of chemical carcinogens.

Inhibition of 2-fluorenamine-induced mutagenesis in Salmonella typhimurium by vitamin A

Vitamin A alcohol (retinol) completely inhibited the mutagenicity of the carcinogen 2-fluorenamine (2-FA) in Salmonella typhimurium TA98 when the mutagen was activated by liver microsomes from CFN rats. The mutagenicity of 2-FA activated by 9,000Xg rat liver supernatant S9 was inhibited by retinol to a lesser degree. The decline in the number of his+ revertants was not an artifact due to bacterial killing, inasmuch as retinol was not toxic to the bacteria at levels that totally inhibited mutagenesis by 2-FA. Mutagenesis induced by adriamycin, an antibiotic that does not require metabolic activation for mutagenic potential, was unaffected by vitamin A. These results indicate that retinoids inhibit the metabolic activation of some chemical carcinogens to forms that can interact with DNA. Our findings are consistent with the hypothesis that retinoids may exert anticancer activity by inhibiting carcinogen activation, thereby inhibiting tumor induction. In addition to the more widely accepted role of retinoids in modulating the proliferation of epithelially derived neoplasms.

Senescent changes in rodent hepatic epoxide metabolism

Age-related alterations in epoxide metabolism were examined in subcellular fractions of liver from 3-, 12- and over 24-month-old male rats and mice. Using styrene oxide as the substrate, glutathione-S-transferase activity remained unchanged while the activity of epoxide hydrase increased with age in both species. The microsomally-mediated binding of benzo[a]pyrene to DNA was also increased in the old animals. Thus, senescent rodents retain or increase their ability to metabolize epoxides. The effect on epoxide metabolism of pretreatment of the senescent rodents with polychlorinated biphenyls was also examined. Glutathione-S-transferase activity was induced only in old animals. However, epoxide hydrase activity, while inducible in all age groups of rats, increased only in young mice. Therefore, there is an age-related difference in response of epoxide metabolizing enzymes to polychlorinated biphenyl treatments between rats and mice.

Presence of a high-molecular-weight form of catalse in enzyme purified from mouse liver

Ultracentrifugation studies of purified mouse hepatic catalase revealed that 5-7% of the total material consists of a form with a higher molecular weight than the bulk of the catalase. The two components were separated by sucrose-gradient centrifugation. Polyacrylamide-gel electrophoresis (in borate buffer) demonstrated that high-molecular-weight catalase is enriched in a more slowly migrating component, and sodium dodecyl sulphate/polyacrylamide gel-electrophoresis demonstrated that the molecular weight of the subunits of the high-molecular-weight material is identical with that of the subunits of the major form. These results suggest that high-molecular-weight catalase consists of subunits that are not markedly distinct from those present in the normal catalase tetramer.

Pregnenolone-16alpha-carbonitrile-inducible cytochrome P450 in rat liver

The progesterone derivative, pregnenolone-16alpha-carbonitrile, induces the synthesis of a unique cytochrome P450 peptide in hepatic microsomes of male rats. This form, detectable both spectrally and electrophoretically, is distinct from the cytochrome P450 components synthesized in response to barbiturates or polycyclic hydrocarbons, but may be identical to the major peptide in control microsomes.