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J B Schenkman

Publications and source records attributed to J B Schenkman.

At least 91 records · Page 5Linked to original sources
PubMed1976

Kinetic studies of benzpyrene and hydroxybenzpyrene metabolism.

The kinetics of benzypyrene (BP) metabolism were examined in liver microsomes, and in accordance with the results of Hansen and Fouts [9] exhibited curcilinear Lineweaver-Burk plots. The problem was exacerbated in microsomes of 3-methylcholanthrene (MC-ms) treated rats. The Km for BP, measuring hydroxybenzypyrene (OHBP) appearance was about 0.3 muM in MC-treated adult rats and about 1.0 muM in untreated rats. These values were obtained using a substrate range of 0.2-2.0 muM benzpyrene, 20 mug of microsomal protein/ml and a 3 min assay time. With longer assay times and with higher microsomal protein concentrations curvilinear reciprocal plots were obtained. This was found to be due to a combination of three factors, namely non-specific binding of BP to the microsomes, rapid depletion of substrate, and further metabolism of hydroxy products. At 100 mug microsomal protein/ml about 50% of added BP was non-specifically bound to the microsomes in the range of 0.2-2.0 muM BP. Addition of albumin to the medium (1 mg/ml) greatly enhanced the BP hydroxylase activity but only slightly increased the amount of BP remaining in the medium after sedimentation of the microsomes by centrifugation. 3-OHBP, one of the phenolic products of BP metabolism was found to be metabolized to a non-fluorescent products(s); the Km for this compound was similar to that for BP. Differences were seen in the Vmax rates of BP disappearance and OHBP appearance. Disappearance of BP is several fold faster than OHBP appearance and has a larger Km. The latter may be due to the need to use higher amounts of protein and to allow depletion of enough substrate to make measurements significantly reproducible or the higher Km may reflect a composite value for different routes of BP metabolism.

Animals↗
PubMed1976

Inhibition of nadph-driven microsomal lipid peroxidation by cytosol factor(s). Effect of a fat-free, high carbohydrate diet.

Male Swiss mice were given free access to a fat-free, high carbohydrate diet. The liver cytosol fraction from these mice contained a heat-sensitive factor that markedly inhibited microsomal, ferric pyrophosphate stimulated, NADPH-driven lipid peroxidation. The diet-induced factor was apparently incorporated into the microsomes after 12 days of continuous feeding, since lipid peroxidation by these microsomes was strongly diminished. The factor disappeared from the cytosol after 24 h of fasting and reappeared after refeeding the mice with the fat-free, high carbohydrate diet.

Animals↗
PubMed1976

Hepatic organelle interaction. IV. Mechanism of succinate enhancement of formaldehyde accumulation from endoplasmic reticulum N-dealkylations.

Further evidence for organelle interaction during drug metabolism by the liver is presented. The apparent stimulation by succinate of formaldehyde accumulation in the medium, which was reported to occur with liver slices and homogenates as well as with mitochondria plus microsomes, has been shown to be the result of succinate inhibition of mitochondrial aldehyde dehydrogenase. The mechanism of succinate inhibition is shown to be by reverse electron transport, and an increase in the NADH to NAD+ ratio in the mitochondria; the aldehyde dehydrogenase requires the oxidized form of the pyridine nucleotide as its cofactor. Studies on in vitro N-demethylation by liver microsomes and endoplasmic reticulum segments which cosediment with the mitochondria indicate that formaldehyde produced by the mixed function oxidase is handled differently from formaldehyde added to the medium. The latter is mainly retained in the medium containing 5 mM semicarbazide, while the generated formaldehyde is more than 50% consumed by the mitochondria. Electron microscopy has indicated that the microsomes and the endoplasmic reticulum fragments have a tendency to align themselves close to the mitochondria when present in the same medium. Consequently, it is possible that formaldehyde released to the medium adjacent to the mitochondria, as by N-demethylation, would be exposed to semicarbazide for shorter periods than that added directly to the medium. In agreement with this suggestion, complexing of formaldehyde with semicarbazide was observed spectroscopically not to be an extremely rapid reaction even at 37 degrees C. This is believed to be the reason for the greater extent of consumption of formaldehyde generated by the endoplasmic reticulum.

Aldehyde Oxidoreductases↗
PubMed1974

Production of membrane whorls in rat liver by some inhibitors of protein synthesis.

Inhibitors of protein synthesis capable of differential effects on nascent peptide synthesis on membrane-bound and free polyribosomes were employed to investigate the structure and function of cellular membranes of liver. The formation of membranous whorls in the cytoplasm and distension of nuclear membranes were induced by inhibitors of protein synthesis (i.e., cycloheximide and emetine) which predominantly interfere with nascent peptide synthesis on membrane-bound polyribosomes in situ. Other inhibitors of protein synthesis such as puromycin and fusidic acid, which inhibit nascent peptide synthesis on both free and membrane-bound polyribosomes, and chloramphenicol, which inhibits mitochondrial protein synthesis, did not induce these alterations. Cycloheximide, puromycin, and chloramphenicol produce some common cellular lesions as reflected by similar alterations in morphology, such as swelling of mitochondria, degranulation of rough endoplasmic reticulum, and aggregation of free ribosomes. The process of whorl formation in the cytoplasm, the incorporation of [(3)H]leucine and of [(3)H]choline into endoplasmic reticulum and the total NADPH-cytochrome c reductase activity of the endoplasmic reticulum were determined. During maximum formation of membranous whorls, [(3)H]leucine incorporation into cytoplasmic membranes was inhibited, while [(3)H]choline incorporation into these structures was increased; maximum inhibition of protein synthesis and stimulation of choline incorporation into endoplasmic reticulum, however, preceded whorl formation. Cycloheximide decreased the activity of NADPH-cytochrome c reductase of rough endoplasmic reticulum, but increased NADPH-cytochrome c reductase activity of smooth endoplasmic reticulum. In addition, cycloheximide decreased the content of hemoprotein in both the microsomal and mitochondrial fractions of rat liver, and the activities of mixed function oxidase and of oxidative phosphorylation were impaired to different degrees. Succinate-stimulated microsomal oxidation was also inhibited. The possible mechanisms involved in the formation of membranous whorls, as well as their functions, are discussed.

Animals↗