Membrane fluidity and drug metabolism in liver microsomes of lean, ob/ob and db/db mice

Effects of experimental dicrocoeliosis on oxidative drug metabolism in hamster liver

The purpose of this investigation was to determine the effects of experimental dicrocoeliosis on the hepatic oxidative drug-metabolizing system in hamsters. Studies were carried out 80 and 120 days after infestation with an oral dose of 40 metacercariae of Dicrocoelium dendriticum. The parasitic pathology was ascertained by detection of the fluke eggs in faeces, increased serum alanine aminotransferase and aspartate aminotransferase activities, and postmortem and histological findings. Cytochrome P-450 concentration, aniline hydroxylase activity and ethoxycoumarin O-deethylase activity were significantly decreased in both groups of infected animals. Aminopyrine N-demethylase activity and erythromycin N-demethylase activity were only reduced 120 days after infection. Effects on drug metabolizing enzymes were unrelated to changes in the physical state of the microsomal membrane, as assessed by measurement of fluorescence polarization. The results of this study indicate that the capacity of the liver for handling drugs and xenobiotics may be impaired as a consequence of dicrocoeliosis.

Influence of nutrition on liver oxidative metabolism

The liver plays a major role in the disposition of the majority of drugs. This is due to the presence of several drug-metabolizing enzyme systems, including a group of membrane-bound mixed-function oxidative enzymes, mainly the cytochrome P450 system. Hepatic oxidative capacity can be assessed by changes in antipyrine metabolism. Different drugs and other factors may induce or inhibit the cytochrome P450-dependent system. This effect is important in terms of the efficacy or toxicity of drugs that are substrates for the system. Microsomal oxidation in animals fed with protein-deficient diets is depressed. The mixed-function oxidase activity recovers after a hyperproteic diet or the addition of lipids. Similar findings have been reported in patients with protein-calorie malnutrition, although results in the elderly are conflicting. Different studies have revealed that microsomal oxidation is impaired by total parenteral nutrition and that this effect is absent when changing the caloric source from carbohydrates to a conventional amino acid solution or after lipid addition, especially when administered as medium-chain/long-chain triglyceride mixtures. Peripheral parenteral nutrition appears to increase antipyrine clearance.

Antipyrine clearance in surgical patients maintained on hypocaloric peripheral parenteral nutrition

BACKGROUND

Antipyrine clearance (CLAP) constitutes a sensitive indicator of hepatic microsomal enzyme activity providing specific information on hepatic function. The purpose of this study was to evaluate the influence of hypocaloric peripheral parenteral nutrition on CLAP in patients receiving nutrition support after elective surgery.

METHODS

CLAP was measured in 15 patients before elective gastrointestinal surgery and 6 days after the surgery. Antipyrine (1 g) was administered orally, and CLAP was determined by the one-sample method. Subjects received a postoperative 786 kcal/d regimen providing 66 g of amino acid per day and 133 g of glucose per day for 5 days. Nutritional status was evaluated by anthropometric parameters. A control group of 15 patients received no postoperative hypocaloric peripheral parenteral nutrition but received conventional fluid therapy.

RESULTS

Mean CLAP was increased by 61% (0.66 +/- 0.06 mL/min.kg-1 body wt vs 0.41 +/- 0.05 mL/min.kg-1 body wt in the preoperative period; p < .001), and antipyrine half-life was reduced by 42% (10.9 +/- 1.0 hours vs 18.9 +/- 2.0 hours; p < .001) after 5 days of hypocaloric peripheral parenteral nutrition. No significant modification was shown among control patients in CLAP (0.54 +/- 0.07 mL/min.kg-1 body wt vs 0.46 +/- 0.05 mL/min.kg-1 body wt in the preoperative period) or in antipyrine half-life (14.0 +/- 1.4 hours vs 16.5 +/- 1.8 hours). No significant correlation was observed between CLAP changes and those for the nutritional status of the patients.

CONCLUSIONS

The results of our study indicate that oxidative drug-metabolizing capacity is increased in surgical patients maintained on hypocaloric peripheral parenteral nutrition. Clinicians should be conscious of the potential of this effect for altering the efficacy or toxicity of many therapeutic agents.

Effect of development on the functional and histological changes induced by bile-duct ligation in the rat

Secondary biliary cirrhosis in the rat can be induced by bile duct ligation; the aim of the present study was to investigate whether susceptibility to this injury depends on development. Rats aged 4, 7, 14 and 22 weeks were bile-duct ligated or sham operated. Four weeks later, stereologic analysis of the liver was performed and the volume fraction of parenchyma, bile ducts and connective tissue was determined. Microsomal function was assessed in vivo by the aminopyrine breath test and in vitro by determining the microsomal cytochrome P450 content and microsomal lipid composition. In addition, portal pressure was measured. The volume fraction of parenchyma decreased in an age-dependent fashion in bile-duct ligated rats from 64.0 +/- 11.2% in the youngest to 46.4 +/- 8.4% in the oldest age group. This decrease was compensated by an age-dependent increase in both ductular proliferation and fibrosis. Microsomal function both in vivo and in vitro showed an age-dependent deterioration. Microsomal cholesterol and some individual phospholipids showed age-dependent changes. Portal hypertension developed in all bile-duct ligated groups, but portal pressure was significantly lower in the oldest bile-duct ligated groups (16.0 +/- 2.6 cmH2O) compared with other bile-duct ligated groups (around 21 cmH2O). We conclude that susceptibility to the sequelae of chronic cholestasis depends on the stage of development in rats. In experiments using this model, the age of the rats should be explicitly stated.

Cytochrome P-455 nm complex formation in the metabolism of phenylalkylamines. XII. Enantioselectivity and temperature dependence in microsomes and reconstituted cytochrome P-450 systems from rat liver

Formation of metabolic intermediate (MI) complexes was studied with the enantiomers of amphetamine, 1-phenyl-2-pentanamine, N-hydroxyamphetamine, and 2-nitroso-1-phenylpropane (the C-nitroso analogue of amphetamine). Three different enzyme systems were used; liver microsomes from phenobarbital pretreated rats and two reconstituted systems containing the P450 2B1 and P450 2C11 forms of cytochrome P-450. Enantioselective complex formation in microsomes was shown for the amines and the nitroso compound, but not for the hydroxylamine. The highly purified P450 2B1 system formed the MI complex with all substrates tested, and the enantioselectivity observed with the microsomal system was reproduced. In the P450 2C11 system the nitroso compounds were completely inactive, whereas the enantiomers of N-hydroxyamphetamine still produced the complex at a high rate. Changes in temperature were shown to affect (R)-2-nitroso-1-phenylpropane more than its enantiomer. Both enantiomers showed biphasic Arrhenius plots for MI complex formation in microsomes (breaks around 22 degrees C), but the activation energies of the (R)-isomer were about five times higher than those of the (S)-isomer. A theory is presented which suggests different modes of interaction with the active site of P-450 to account for the different behaviour of the various substrates.

Urinary organic acid profiles in obese (ob/ob) mice: indications for the impaired omega-oxidation of fatty acids

As a means of generating an hypothesis to explain genetic obesity of the C57BL/6J ob/ob mouse, we used gas chromatography-mass spectrometry to compare the urinary organic acid profiles of obese (ob/ob) and lean (+/?) mice on both a chow and a chemically simplified diet. More than 60 peaks were found and quantified; 45 peaks were identified. No acid was excreted in greater amounts by lean mice and none was excreted exclusively by either lean or obese mice. When normalized to body weight (obese mice being 40% heavier) and to creatinine excretion (30% greater in obese mice), however, only the daily excretion of malate, 2-hydroxyglutarate, aconitate, 3-hydroxy-3-methylglutarate, oxalate, ethylmalonate, and 4-hydroxyphenylacetate were significantly greater in obese mice. When allowed to eat only an all-fat (Crisco) diet for 4 days, the excretion of adipate rose 10-fold in lean mice, but only threefold in obese mice. Adipate excretion by Zucker rats also increased on the Crisco diet, but was indistinguishable between lean and fatty rats, suggesting that omega-oxidation might be impaired in obese mice but not in fatty rats. This suggestion complements an earlier proposal that a comparative increase in ethylmalonate excretion, which was also characteristic of fatty Zucker rats, might be explained by an increased concentration of butyryl-CoA due to inadequate beta-oxidation. An impairment of omega-oxidation in the obese mouse may also explain why urinary 3-hydroxy-3-methylglutarate, which is derived from short chain products of beta-oxidation, is increased in obese mice but not in fatty rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of hyperalimentation on rat hepatic microsomal fluidity and function

Total parenteral nutrition with an amino acid-glucose solution has previously been shown to decrease rat hepatic drug metabolism compared with drug metabolic activity observed in rats receiving the same solution enterally and chow-fed animals. Because changes in membrane fluidity and lipid composition are reported to influence activity of a number of liver enzymes, effects of parenteral and enteral nutrition on hepatic microsomal membrane fluidity and lipid composition were assessed and compared with hepatic mixed-function oxidase activity. Both parenteral and enteral hyperalimentation produced a significant decrease in microsomal membrane fluidity (fluorescence anisotropy = 0.155 +/- 0.003 in both experimental groups versus 0.129 +/- 0.003 for microsomes from chow-fed animals). However, meperidine demethylase activity was significantly decreased compared with chow-fed experiments only in hepatic microsomes from parenterally hyperalimented animals, whereas ethoxyresorufin deethylase activity was significantly reduced only in the enteral-nutrition group. Inclusion of lipid in the parenterally administered hyperalimentation solution normalized microsomal membrane fluidity and lipid profile to those of chow-fed animals but did not increase hepatic meperidine demethylation. Both parenteral and enteral nutrition produce significant changes in physical state and lipid composition of rat hepatic microsomal membranes, but these changes are not responsible for the altered hepatic drug metabolism observed during hyperalimentation.

Purification and comparison of liver microsomal flavin-containing monooxygenase from normal and streptozotocin-diabetic rats

Liver microsomal flavin-containing monooxygenase (FMO) activity towards thiobenzamide is two-fold increased in streptozotocin diabetic (insulin deficient) rats and mice and, to a lesser degree in congenital insulin resistant Ob/Ob mice. No difference in thermal stability appears between microsomal FMOs from both normal and diabetic rats. FMO has been purified to homogeneity from these two sources, with a 50-fold increase of specific activity. Their apparent molecular weight is respectively 50,000 and 49,000 and a discrete modification appears in the HPLC profiles of tryptic peptides from purified FMOs. They appear immunochemically very similar and present in equal quantity in microsomal membranes from both normal and diabetic rats, so that the increased activity cannot be ascribed to an increased concentration of the enzyme protein.

Liposomes as membrane model for study of lipid peroxidation

This article describes the properties, production and characterization of liposomes with special reference to their use as membrane model for the study of lipid peroxidation. It presents briefly the methods that can be used for the assay of liposomal lipid peroxidation and brings out the special advantages these liposomes provide in elucidating the mechanism of lipid peroxidation by different physical and chemical agents. Studies involving liposomal lipid peroxidation by different agents and the consequent changes in the structure and function of liposomal membrane have been reviewed briefly.

Fetal and adult human liver differ markedly in the fluidity and lipid composition of their microsomal membranes

The fluidity and lipid composition of the human hepatic microsomal membrane were studied in 11 livers from 16- to 21-week-old fetuses and in 5 adult livers, and compared with those of fetal and adult rat liver microsomes. Membrane fluidity was analyzed by measurement of fluorescence polarization using the fluorophore 1,6-diphenyl-1,3,5-hexatriene. The lipid apparent microviscosity (eta) of human fetal liver microsomes was 2.17 +/- 0.13 poise, as compared with 1.08 +/- 0.08 poise in adult liver (p less than 0.001). Similar differences in fluidity were found between fetal and adult rat liver microsomes. The more "fluid" adult microsomes had higher phospholipid/cholesterol and phosphatidylcholine/sphingomyelin molar ratios than those of the more "rigid" fetal microsomes. The degree of unsaturation of the adult microsomal lipids was much higher than that of the fetal lipids. The ratios of unsaturated/saturated fatty acids in microsomal lipids highly correlated with the eta values obtained for the combined group of fetal and adult human livers, suggesting that the developmental increase in degree of unsaturation of the microsomal lipids is a major determinant of the increased fluidity of adult as compared with fetal liver microsomes. These differences in fluidity and lipid composition between fetal and adult human liver microsomes may be a critical factor in the regulation of hepatic microsomal drug and carcinogen metabolizing enzyme activity, and could so determine the extent of toxicity and teratogenicity of drugs and/or their metabolites in the developing human fetus.

Mono- and diacylglycerol lipases in spinal cord of lean and obese Zucker rats

1. Genetically obese Zucker rats (fa/fa) contain 2-3 times higher activities mono- and diacylglycerol lipases in their spinal cords than their lean littermates. 2. When rats were exercised (1 hr daily, 5 days/week) on a treadmill for 6 months, there was a decrease of about 30% (P less than 0.05) in the activities of mono- and diacylglycerol lipases in lean rats but not in obese animals. 3. High activities of lipases in Zucker obese rats may be related to the elevated levels of beta-endorphin present in these animals. 4. The activities of arylsulfatase, beta-N-acetylhexosaminidase and alkaline phosphatase, tested to check the stability of spinal cord extracts, were similar in lean and obese rat spinal cords.

The kinetics of 4-nitrophenol conjugation by perfused livers and hepatic microsomes from streptozocin-induced diabetic rats

The formation of both glucuronide and sulphate conjugates of 4-nitrophenol is deficient in perfused livers from male diabetic rats. Experiments with 'native' hepatic microsomes demonstrated that the defect in glucuronidation is due to a decrease in the maximal velocity of the reaction. There is no alteration in the affinity of the glucuronyltransferase for 4-nitrophenol. Non-linear regression analysis of the 4-nitrophenol liver perfusate concentrations showed that the elimination follows saturable Michaelis-Menten kinetics. Clearance values in 'native' microsomal preparations and in perfused livers were calculated and found to be similar in both systems. This provides evidence that glucuronyltransferase is 'native' in the intact liver.

Modification of fatty acid composition in adipocyte plasma membranes by an oral treatment with a new antidiabetic agent, AS-6, in genetically obese diabetic mice, db/db

Male 12-week-old C57BL/KsJ db/db mice were treated for 1 week with a dietary admixture of an experimental antidiabetic agent, AS-6 (4-O-carboxymethylascochlorin, 0.1%). The fatty acid composition of the adipose tissue and its plasma membranes in the treated mice was compared with that in untreated db/db mice and their lean littermates. The results indicate that, when compared with the lean, the db/db adipose tissue and its plasma membrane are extremely rich in nonessential fatty acids, and AS-6 treatment modifies the fatty acyl composition only in the membranes in which 16:1 and 18:1 increase and C18 decreases.

Hepatic delta 6-desaturase activity in lean and genetically obese ob/ob mice

Hepatic delta 6-desaturase activity is primarily located in the mitochondrial fraction in mice. Both delta 6- and delta 5-desaturase activities are increased in the liver of young (6-week-old) obese mice. The increase in hepatic delta 6-desaturase activity in obese mice does not occur until weaning. Neither restriction of food intake nor hyperinsulinaemia normalize hepatic delta 6-desaturase activity of obese mice. Both cold acclimation and tri-iodothyronine (30 micrograms/day per kg) decreased hepatic delta 6-desaturase activity of obese mice to levels observed in lean mice, whereas the increase in activity in obese mice was still maintained after the induction of hypothyroidism.

Glucuronic acid conjugation by hepatic microsomal fractions isolated from streptozotocin-induced diabetic rats

The hepatic glucuronidation of 1-naphthol and 4-nitrophenol (3-methylcholanthrene inducible substrates of glucuronyltransferase, GT 1) was found to be deficient in freshly prepared untreated "native" microsomes from streptozotocin-induced diabetic male rats. The defect was not observed in female rats. Moreover, the glucuronidation of 1-naphthol and 4-nitrophenol was higher in "native" microsomes from male control rats than in those from female controls. This sex difference in the glucuronidation of the GT 1 substrates was abolished by detergent activation of the transferase enzyme in vitro. Streptozotocin treatment did not alter the glucuronidation of paracetamol or phenolphthalein (phenobarbitone inducible substrates for GT 2). This diabetes-induced defect in the glucuronidation of GT 1 substrates was abolished by insulin treatment of the animals and was diminished or completely abolished by detergent activation of the transferase enzyme in vitro. Increased membrane constraint is proposed as the mechanism responsible for the transferase defect. 3-Methylcholanthrene induction abolished the streptozotocin-induced defect in 4-nitrophenol glucuronidation, whereas phenobarbitone did not. This is attributed to the differential effect of these inducers on the microsomal membrane.

Abnormal antioxidant defence in some tissues of congenitally obese mice

The concentration of lipoperoxides (estimated as thiobarbituric acid-reactive material) and some components of the antioxidant defence system have been compared in various tissues of lean and congenitally obese mice. NADPH-stimulated lipoperoxide generation in vitro was significantly higher in microsomes (microsomal fractions) prepared from obese hepatic tissue than lean. Plasma, liver and brain lipoperoxide concentration was significantly higher in obese mice. In blood derived from obese mice the concentration of non-enzymic antioxidants including caeruloplasmin and vitamin A was higher, but hepatic retinol concentration was lower in these animals. In all the tissues assayed the glutathione peroxidase activity against H2O2 was less than its activity against cumene hydroperoxide. Assayed with either substrate, glutathione peroxidase activity was significantly higher in the brain and blood of obese mice than their lean counterparts. Conversely, liver glutathione peroxidase was decreased in obese animals, representing 43% of the activity of the lean-mouse liver enzyme against H2O2 and 81% of the cumene hydroperoxide-reducing activity. The liver of obese mice had significantly less, and the kidneys more, oxidized glutathione than the corresponding tissues of lean mice. Further investigations on hepatic tissue indicated that glutathione reductase activity was lower in the obese animals, but there was no significant difference between glucose-6-phosphate dehydrogenase activity in obese and lean mice.

Structural changes of liver microsomes in rat during neonatal life: influence on the glucuronidation rates of various substrates

The Vmax of the membrane bound UDP-glucuronosyltransferase (UDP-GT) towards group 1 substrates (4-nitrophenol, 2-naphthol) was particularly higher in young rats than in adults. On the contrary, activity towards group 2 substrates such as borneol or testosterone was not detectable in fetus liver. The developmental pattern of UDP-GT was related to changes in lipid composition of microsomes, namely in the content in lysophosphatidylcholine which rose at birth. The phospholipid-cholesterol molar ratio also increased 2 fold from the 16th day of fetal life to the 4th day after birth. Measurement of the steady state anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) as well as determination of the order parameter S and the rotation cone angle of the fluorescent probe strongly suggested an increase in membrane fluidity in rat liver microsomes during ontogenesis.

Reconstitution studies on the involvement of radiation-induced lipid peroxidation in damage to membrane enzymes

The effect of radiation on the drug-metabolizing enzyme system of microsomes, reconstituted with liposomes of microsomal phospholipids, NADPH-cytochrome P-450 reductase and cytochrome P-450, was examined to elucidate the role of lipid peroxidation of membranes in radiation-induced damage to membrane-bound enzymes. The reconstituted system of non-irradiated enzymes with irradiated liposomes showed a low activity of hexobarbital hydroxylation, whereas irradiated enzymes combined with non-irradiated liposomes exhibited an activity equal to that of unirradiated controls. Irradiation of liposomes caused a decrease in cytochrome P-450 content by destruction of the haem of cytochrome P-450 and also inhibited the binding capacity of cytochrome P-450 for hexobarbital. The relationship between radiation-induced lipid peroxidation and membrane-bound enzymes is discussed.

Induction of drug metabolizing enzymes in the liver of diabetic mice

The effects of two classical inducers, phenobarbital and 3-methylcholanthrene, have been tested on some liver microsomal drug-metabolizing enzymes (monooxygenases and phase II enzymes) and on benzo(a)pyrene metabolism in genetically (ob/ob) and chemically (streptozotocin) diabetic mice. 1) In ob/ob mice, the basal activities and the inducibility of phase I and phase II enzymes, as well as the electrophoretic pattern of microsomal proteins, were not notably different from those of similarly treated lean mice. 2) A possibly common form of cytochrome P 450 present both in microsomes from steptozotocin-diabetic non-induced mice and in those from phenobarbital-treated non-diabetic mice could explain the increased "phenobarbital-like" enzyme activities in chemically diabetic animals. 3) The increase of monooxygenase activities produced by streptozotocin treatment is partially depressed by 3-methylcholanthrene, probably as a result of the dilution of "phenobarbital-like" cytochrome P 450 forms by 3-methylcholanthrene-induced cytochrome P 448. 4) The increased formation of the most carcinogenic metabolites of benzo(a)pyrene, and the slight decrease of phase II conjugation enzyme activities, may add their deleterious effects in 3-methylcholanthrene-induced streptozotocin-diabetic animals.

Lipid composition in liver and brain of genetically obese (ob/ob), heterozygote (ob/+)and normal (+/+) mice

Lipid composition was studied in liver and brain of normal (+/+), heterozygote (ob/+) and obese (ob/ob) mice. It was found that this genetic defect is expressed differently in the lipid composition of these organs. Cholesterol is increased in liver but strongly decreased in brain of obese animals. Phosphatide fatty acid composition is modified in liver and not in brain. In contrast, phospholipids and total ganglioside sialic acid are affected similarly in both organs. Although clinically normal, heterozygote (ob/+) mice already show an abnormal lipid composition in liver and brain. The potential importance of these results is presented.

Differential effects of chronic ethanol feeding on cytochrome P-448- and P-450-mediated drug metabolism in the rat

The effects of chronic ethanol feeding on cytochrome P-448- and P-450-mediated drug metabolism have been studied both in vivo and in vitro in the rat, using caffeine, phenacetin, antipyrine and aminopyrine as test substrates. N-Demethylation of aminopyrine (P-450 mediated) was increased both in vivo and in vitro in rats after chronic ethanol feeding (P less than 0.05) whereas in vivo N-demethylation of caffeine and O-dealkylation of phenacetin (P-448 mediated) were unchanged in the same animals. N-Demethylation of antipyrine was increased by both phenobarbital and 3-methylcholanthrene pretreatment and by chronic ethanol feeding (P less than 0.05), possibly due to cytochrome P-450 induction. Furthermore, the Michaelis affinity constants, Km, for hepatic microsomal aminopyrine N-demethylase and antipyrine N-demethylase were lower in chronic ethanol-fed animals (P less than 0.05), suggesting a qualitative change in the enzymes resulting in greater substrate affinity. These findings suggest a differential effect of chronic ethanol feeding on the induction of cytochrome P-450- and cytochrome P-448 mediated drug metabolism, with a greater effect on the former microsomal system.

The effect of dietary methyl branched-chain fatty acids on aspects of hepatic lipid metabolism in the rat

1. Rats were fed to appetite on a stock laboratory diet or on diets consisting of the stock diet and in addition 50 or 200 g triolein/kg, 50 g palmitic acid/kg or 50 g/kg of a concentrate mixture of methyl branched-chain fatty acids (Me-BCFA) which had been prepared from sheep adipose triacylglycerols. 2. No differences could be detected in the delta 9-desaturase activity or fatty acid synthetase activity of liver preparations from rats which had been fed on either the stock diet, the 50 g palmitic acid/kg or the 50 and 200 g triolein/kg diet; the palmitic acid diet was therefore taken as the control diet in subsequent experiments. 3. Rats consuming the 50 g Me-BCFA/kg diet exhibited a marked reduction in the capacity of their liver microsomes for delta 9-desaturation when compared with animals receiving the control diet. The delta 6-desaturase activity also showed an inhibitory trend with the Me-BCFA diet. 4. Microsomal omega-oxidation of fatty acids, mitochondrial succinate oxidation and the activity of cytosolic fatty acid synthetase (FAS) were unaffected by the ingestion of the Me-BCFA mixture compared with the diet which included palmitic acid. 5. There were no differences in the plasma concentration of thyroxin, insulin and glucagon between animals fed on the diets containing palmitic acid or the Me-BCFA. 6. For a given concentration of fatty acids the Me-BCFA had a greater inhibitory effect when added to incubations of liver microsomes from rats fed on the standard diet than did the addition of palmitic acid. 7. The observations in vivo and in vitro strongly suggested that the Me-BCFA were having a specific inhibitory effect on the desaturation reaction.

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.