Lipoprotein lipase and cholesterol transfer activities of lean and obese Zucker rats.

Adult female lean and obese Zucker rats maintained under standard conditions were used for the estimation of plasma, liver and white adipose tissue (WAT) activity of lipoprotein lipase, plasma and liver hepatic lipase and plasma lecithin-cholesterol acyltransferase. No differences in plasma or tissue levels of lipoprotein lipase between lean and obese rats were detected, but the larger WAT size of the obese rats resulted in higher lipase activity per unit of rat weight. Hepatic lipase levels in plasma were higher in the obese, but in liver, the higher activity was found in lean rats. No significant differences were found for lecithin-cholesterol acyltransferase activity, except when the levels in the HDL fraction were expressed per unit of protein weight, showing lower activity in the obese rats. In conclusion, the essentially maintained enzyme activities in obese rat tissues suggest that they cannot explain the deficient lipoproteins processing of obese rats, and, consequently their dislipidaemia.     

Rate and equilibrium constants of the tetramerization process of phosphorylase b. Influence of AMP and Mg2+

The association-dissociation equilibrium of phosphorylase b at different enzymatic concentrations has been studied at 25 degrees C in this paper, pointing out how this equilibrium is affected by both AMP and Mg2+ concentrations. It has also been proved that association of phosphorylase b in the presence of AMP and Mg2+ follows second-order and first-order rate laws in the direction of tetramerization and dimerization, respectively. Rate constants have also been calculated and their dependence upon protein, AMP and Mg2+ concentrations studied thoroughly.     

A new method for assaying rat liver microsomal 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity and its application in a study of the effect of dietary cholesterol on this effect of dietary cholesterol on this enzyme.

A new method suitable for measuring rat liver 3-hydroxy-3-methylglutaryl-CoA reductase activity is described and its advantages over methods previously available are discussed. An accurate time course was measured for the inhibition of liver microsomal 3-hydroxy-3-methylglutaryl-CoA reductase activity by dietary cholesterol; this enzyme was affected 1 1/4 h after the rats began to consume a cholesterol-rich diet. In this experiment there was no correlation between concentrations of microsomal cholesterol ester and the activity of 3-hydroxy-3-methylglutary-CoA reductase.     

Influence of fasting and cholesterol feeding on the cholesterol synthesis in rats in vivo.

An inhibition of the cholesterol synthesis located between lanosterol and cholesterol could be shown in fasting rats in vivo with glucose, palmitate, acetate, and mevalonate as tracer substances. The same type of inhibition of the cholesterol synthesis was observed after cholesterol feeding. No indication was obtained for any other inhibiting effect located between acetyl-coenzyme A and lanosterol by these in vivo experiments.     

The mechanism of the phosphoryl transfer catalyzed by Yersinia protein-tyrosine phosphatase: a computational and isotope effect study.

In order to evaluate various mechanistic proposals that have been made regarding the mechanism of the first step of the reaction catalyzed by protein-tyrosine phosphatases, new experimental data have been obtained, and some existing data have been carefully reevaluated. New kinetic isotope effect data for the uncatalyzed hydrolysis of p-nitrophenyl phosphate allow a better evaluation of previously reported data from enzymatic reactions with this substrate. The interpretation, and misinterpretation, of pH rate studies is considered. The pathway of this reaction has been modeled computationally and is found to be generally consistent with experimental studies, except for the extent of proton transfer to the leaving group.     

Photo-induced charge transfer. A critical test of the mechanism and range of biological electron transfer processes.

The vibronic coupling theory of electron tunneling between biomolecules requires that all such tunnelings involve vibronic coupling, finds temperature dependence to tunneling at finite temperatures, and predicts relatively short tunneling distances. This theory might be expected to apply to most electron transfers involved in the membrane-bound electron transfer reactions of photosynthesis and oxidative phosphorylation. This paper calculates the properties of a weak charge-transfer optical absorption band, whose predicted characteristics are a direct and simple consequence of the model that describes vibronically coupled tunneling. The new absorption band provides the basis for a critical experimental test of the constructs and parameters of the tunneling theory. If the tunneling theory is valid, the oscillator strength of such bands will be the most reliable measure of the tunneling matrix element and of the distance between the sites exchanging an electron.     

Using the dual isotope method to assess cecal amino acid absorption of goat whey protein in rats,a pilot study

Measurement of ileal amino acids (AA) bioavailability is recommended to evaluate protein quality. A dual isotope tracer method, based on plasma isotopic enrichment ratios, has been proposed to determine true digestibility in humans. In a pilot study, we aimed to evaluate whether this method could be implemented in rats to determine AA bioavailability based on isotopic enrichment ratios measured in cecal digesta or plasma samples. Goat milk proteins were intrinsically labeled with ¹⁵N and ²H. Wistar rats were fed a meal containing the doubly labeled goat whey proteins and a tracer dose of ¹³C-spirulina. Blood samples were collected 0, 1 h and 3 h after meal ingestion from the tail vein. The rats were euthanized 4 h (n?=?6) or 6 h (n?=?6) after meal to collect plasma and intestinal contents. True orocecal protein digestibility and AA bioavailability were assessed by means of ¹⁵N and ²H enrichment in cecum content and compared with absorption indexes determined at the plasma or cecum level using isotopic ratios. Plasma kinetics of isotopic enrichment could not be completed due to the limited quantity of plasma obtained with sequential blood collection. However, the absorption indexes determined from cecal ¹⁵N or ²H/¹³C ratios gave coherent values with true orocecal AA bioavailability. This dual isotope approach with measurements of isotopic ratios in digestive content could be an interesting strategy to determine true AA bioavailability in ileal digesta of rats.

     

Modifications of the envelope of Chlamydia psittaci during its developmental cycle: freeze-fracture study of complementary replicas.

Examination of complementary replicas obtained by freeze-fracture of Chlamydia psittaci revealed, at the level of the plasma membrane, a progressive differentiation of "crate-like formations," which likely correspond to transmembranal pores. Recognition of "early" and "late" stages observed in the intermediate bodies permitted detailed study of the developmental cycle of this chlamydia.     

Lack of a direct correlation between cholesterol synthesis and cell renewal in liver and intestine in the rat

An attempt was made to investigate whether the synthesis of cholesterol is correlated with the synthesis of DNA in the rat. This study was carried out in vivo on two organs in which the cell renewal is known to be different: the liver and the intestine. Various experimental conditions were realized which modify the rate of cholesterol synthesis in these organs. Measurements of the incorporation of [14C] acetate into the unsaponifiable lipids and of [3H] thymidine into the nuclear DNA give respectively an index for the intensity of cholesterol- and DNA syntheses. Radio activities were measured 70 min after subcutaneous injection of the two labelled precursors. The results showed that the intensity of cholesterol synthesis in the intestine and in the liver can change without proportional variations of DNA synthesis in these organs. Thus, it is not possible to establish a simple and direct correlation between the two synthesis.