Size and composition of lymph chylomicrons following feeding corn oil or its fatty acid methyl esters

Male rats with thoracic duct cannulae were intubated with corn oil or fatty acid methyl esters and the lymph was collected over the next 2-72 h. The apoprotein (apo) composition of the chylomicrons, isolated by conventional ultracentrifugation, was determined by sodium dodecyl sulfate - polyacrylamide - glycerol gel electrophoresis and isoelectric focusing. The lipid content and composition was assessed by gas--liquid chromatography. The particle size was obtained by calculation and confirmed by electron microscopy. The study demonstrates that both the monoacylglycerol (corn oil feeding) and the phosphatidic acid (methyl ester feeding) pathways of triacylglycerol biosynthesis yield chylomicrons with closely similar apoprotein profiles representing apo B-48, apo A-IV, apo E, apo A-I, and the apo C components. A protein band corresponding to apo B-100 was occasionally observed as a minor component of the chylomicrons from both groups of animals. The chylomicrons from corn oil feeding had about two times larger diameters than those from methyl ester feeding. There were no significant differences in the composition of the apoproteins, although the smaller particles had two times higher apoprotein/triacylglycerol ratios. It was calculated that the amount of apo B per lipid particle for the ester fed rats ranged from one to eight molecules and was closely correlated with the particle size. The corn oil fed rats yielded about three molecules apo B per lipid particle regardless of the particle size. It is concluded that the pathway of intestinal triacylglycerol biosynthesis has a significant effect on the apoprotein mass and to a lesser extent on the apoprotein and lipid composition of the chylomicrons. The phosphatidic acid pathway produces smaller particles and transfers to the bloodstream twice as much apoprotein per gram of fat than the monoacylglycerol pathway, which yields the larger particles. Possible variations in the site and rate of biosynthesis of the triacylglycerols could not be entirely excluded as contributing factors.     

Stereochemical course of intestinal absorption and transport of mustard-seed oil triacylglycerols in the rat

Male rats with thoracic duct cannulae were intubated with mustard-seed oil or the corresponding fatty acid methyl esters and the lymph was collected over 0-24 h. The chylomicron and very low density lipoprotein fractions were obtained by conventional ultracentrifugation. The triacylglycerols and glycerophospholipids were isolated and the positional distribution and molecular association of fatty acids were determined by stereospecific and chromatographic methods. The oleic, linoleic, and linolenic acids were recovered in the lymph in the proportion in which they occurred in the fat fed, while eicosenoic, erucic, and lignoceric acids were rejected to about the same extent by the two pathways of intestinal triacylglycerol biosynthesis. It is shown that the lymph triacylglycerols arising via the monoacylglycerol or the phosphatidic acid pathway possess structures that are closely similar to each other and to that of the original mustard-seed oil. It is proposed that this is a result of comparable fatty acid and positional specificity of the acyltransferases associated with the acylglycerol synthesis in the animal and plant tissues and the wide range of fatty acid chain lengths in the mustard-seed oil.     

Phospholipid methylation in canine myocardium: kinetic characteristics and the effect of endotoxin administration

The kinetic characteristics and the effect of endotoxin administration on the enzymatic methylation of phospholipids in dog heart microsomes were studied using S-adenosyl-L-[methyl-3H]methionine as a methyl donor. Kinetic studies in control dogs reveal that the stepwise methylation of phosphatidylethanolamine to phosphatidylcholine was catalyzed by three different enzymes. Methyltransferase I catalyzed the methylation of phosphatidylethanolamine to phosphatidyl-N-monomethylethanolamine, had a very low Km (approximately 1.5 microM) for S-adenosylmethionine, and a pH optimum of 6.5, and it was stimulated by Mg2+ and Ca2+. Methyltransferase II catalyzed the methylation of phosphatidyl-N-monomethylethanolamine to phosphatidyl-N,N-dimethylethanolamine, had a low Km (8-12 microM) for S-adenosylmethionine, and a pH optimum of 8.5, and it was stimulated by low concentrations (less than 1 mM) of Ca2+ but was unaffected by Mg2+. Methyltransferase III catalyzed the formation of phosphatidylcholine from phosphatidyl-N,N-dimethylethanolamine, had a high Km (approximately 33 microM) for S-adenosylmethionine, and a pH optimum of 9.5, and it was unaffected by Mg2+ or Ca2+. Experiments with trypsin digestion indicate that methyltransferases I and III were partially embedded while methyltransferase II was completely exposed to the surface of the membrane. Endotoxin administration (2 and 4 hr) decreased the Km and Vmax by 30 to 36% and 24 to 37.7%, respectively, for S-adenosylmethionine. Since the enzymatic methylation of phospholipids has been implicated to play an important role in the regulation of membrane structure and function, the endotoxin-induced decreases in the Km and Vmax of phospholipid-methylating enzymes in dog heart microsomes may contribute to the development of myocardial dysfunction in endotoxin shock.     

Enzymatic transformation of PGH2 to PGF2 alpha catalyzed by glutathione S-transferases

Glutathione S-transferases (GSTs) purified from both rat liver cytosol and microsomes catalyzed the direct reduction of PGH2 to PGF2 alpha. As much as 40% of the substrate was transformed into a prostanoid whose Rf value corresponded to that of PGF2 alpha. The identification of the reaction product as PGF2 alpha was confirmed by TLC and reverse-phase HPLC as well as by mass spectral analysis. In the absence of GSTs, PGH2 was found to be primarily converted to PGE2 and PGD2. Also, PGF2 alpha formation was completely abolished by decylglutathione, a potent inhibitor of both peroxidase and transferase activity associated with GSTs. These results indicate that the direct reduction of endoperoxide moiety of PGH2 to form PGF2 alpha is an enzymatic process. Interestingly, selenium-dependent glutathione peroxidase (Se-GSH-Px) showed very little PGF2 alpha formation from PGH2. However, this enzyme was very active in the reduction of PGG2 to PGH2. In contrast, GSTs were very poor in the conversion of PGG2 to PGH2. Therefore, it is possible that the relative tissue distribution of Se-GSH-Px and GSTs might play an important role in the tissue specific synthesis of PGF2 alpha.     

The induction of a specific form of cytochrome P-450 (P-450j) by fasting

In previous work we have demonstrated that liver microsomal N-nitrosodimethylamine demethylase (NDMAd) activity is increased in rats by fasting, and we have postulated that this is due to the induction of a specific form of cytochrome P-450. This communication provides evidence for such a hypothesis. Fasting for 24 and 48 h caused 59 and 116% increases, respectively, in NDMAd activity in male rats, and fasting for 48 h caused a 63% increase in female rats. These increases were accompanied by corresponding increases of cytochrome P-450j (P-450ac) determined by immunoblotting. Fasting for 24 and 48 h also increased the mRNA for P-450j by 153 to 250%, as determined by hybridization with a cDNA probe of this cytochrome. The results suggest that fasting affects the gene expression of P-450j.     

Fourier transform infrared investigation of the Escherichia coli methionine aporepressor

This study represents the first physicochemical analysis of the recently cloned methionine repressor protein (Met aporepressor) from Escherichia coli. Infrared spectrometry was used to investigate the secondary structure and the hydrogen-deuterium exchange behavior of the E. coli Met aporepressor. The secondary structure of the native bacterial protein was derived by analysis of the amide I mode. The amide I band contour was found to consist of five major component bands (at 1625, 1639, 1653, 1665, and 1676 cm-1) which reflect the presence of various substructures. The relative areas of these component bands are consistent with a high alpha-helical content of the peptide chain secondary structure in solution (43%) and a small amount of beta-sheet structure (7%). The remaining substructure is assigned to turns (10%) and to unordered (or less ordered) structures (40%). The temperature dependence of the infrared spectra of native Met aporepressor in D2O medium over the temperature interval 20-80 degrees C indicates that there are two discrete thermal events: the first thermal event, centered at 42 degrees C, is associated with the hydrogen-deuterium exchange of the hard-to-exchange alpha-helical peptide bonds accompanied by a partial denaturation of the protein, while the second event, centered around 50 degrees C, represents the irreversible thermal denaturation of the protein.     

Distribution of heparinase covalently immobilized to agarose: Experimental and theoretical studies

An immobilized enzyme reactor has been developed to remove heparin, the anticoagulant that is required in all extracorporeal devices for patients undergoing open-heart surgery or kidney dialysis. The device uses the enzyme heparinase (EC 4.2.2.7), which is covalently linked to agarose with cyanogen bromide. A critical parameter in the development of a model for the degradation of heparin catalyzed by immobilized heparinase is the radial concentration profile of the enzyme within the agarose matrix. Experimental determinations of bound enzyme con centrations have been conducted previously for several enzyme systems using radioactive or fluorescent labels. For the development of the heparinase reactor it is necessary to use catalytically but not electrophoretically pure enzyme, and thus it is not possible to use the labeling techniques. To obtain information about the bound enzyme distribution, an experimental study of the intrinsic binding kinetics of heparinase to cyanogen bromide-activated agarose was conducted. The binding reaction was studied as a function of both the concentration of heparinase and the gel-reactive group. At conditions of functional group excess, the binding kinetics were pseudo first order in heparinase concentration with a rate constant equal to 0.12 C(c[triple chemical bond]n) (h(-1)), where C(c[triple chemical bond]n) is the gel-reactive group concentration. The reactive group concentration remained constant within the 2-4-h experiments. Competitive binding between heparinase and the protein contaminants was unimportant. A model was formulated for the immobilization procedure based on the diffusion of heparinase within the porous network and the binding kinetics as determined above. The model predicted the immobilization of heparinase to be kinetically controlled and the enzyme to distribute uniformly within the agarose matrix. These experimental techniques could be applied to predict the immobilized enzyme distribution for different enzyme systems that are not electrophoretically pure.     

Immobilized heparinase: In vitro reactor model

Heparinase immobilized to agarose has previously been shown to be useful in degrading heparin and thereby preventing thromboembolytic complications when this anticoagulant has been used in extracorporeal perfusions. The current study examined the kinetics of this immobilized enzyme. When heparinase is covalently bound to 8% agarose, the partition coefficient of heparin in the catalytic particle is 0.36 +/- 0.048 (N = 10). The immobilized enzyme has a K(m) of 0.15 +/- 0.03 mg/mL and an activation energy of 10.3 +/- 0.57 kcal/gmol (N = 5). These values are statistically indistinguishable from the values for the free enzyme. The immobilized enzyme showed a pH activity optimum between 7.0 and 7.4, compared to the optimum pH of 6.5 for the soluble enzyme. The activity optimum of immobilized heparinase with respect to salt concentration was between 0 and 0.1M. A reactor containing immobilized heparinase recirculating internally at 1300 mL/min behaved as a continuously stirred tank reactor (CSTR) when solutions at a flow rate of 120 mL/min were passed through the device. The residence time distribution was determined using blue dextran (molecular weight 2 x 10(6) daltons), which is sterically excluded from the agarose catalyst. A model of the heparinase reactor based on ideal CSTR behavior and the immobilized enzyme kinetic parameters was developed. It accurately predicted experimental conversions over a range of catalyst volumes, enzyme loadings, and substrate concentrations to within 7% in most cases and with a maximum deviation of 13%.     

Neuroanatomy of morphine-modulating peptides

Antisera against two mammalian peptides related to the molluscan cardioexcitatory peptide Phe-Met-Arg-Phe-NH2 were used to locate immunoreactive neurons in the rat brain, nerve fibres and terminals in the spinal cord, sympathetic ganglion cells and adrenal chromaffin cells. Immunoreactivity for the newly characterised octa- and octadecapeptide was detected in nerve cell bodies in the hypothalamic area, including parts of the dorsomedial, periventricular and paraventricular nuclei, and in the nucleus tractus solitarii. Nerve terminals in the superficial laminae of the spinal cord were also immunoreactive for these peptides, while the sensory ganglia were nonreactive. Some principal ganglion cells in the superior cervical ganglia exhibited bright immunofluorescence for the peptides, and a few adrenal medullary cells were immunoreactive. The presence of these peptides in the substantia gelatinosa of the spinal cord suggests that they may be involved in sensory neurotransmission, especially in the mechanisms mediating pain. In the hypothalamo-hypophysial system these peptides may be involved in the regulation of hormonal systems. They may also act as co-transmitters in the sympathetic nervous system.     

Tungstate, a molybdate analog inactivating nitrate reductase, deregulates the expression of the nitrate reductase structural gene

Nitrate reductase (NR, EC 1.6.6.1) from higher plants is a homodimeric enzyme carrying a molybdenum cofactor at the catalytic site. Tungsten can be substituted for molybdenum in the cofactor structure, resulting in an inactive enzyme. When nitratefed Nicotiana tabacum plants were grown on a nutrient solution in which tungstate was substituted for molybdate, NR activity in the leaves decreased to a very low level within 24 hours while NR protein accumulated progressively to a level severalfold higher than the control after 6 days. NR mRNA level in molybdate-grown plants exhibited a considerable day-night fluctuation. However, when plants were treated with tungstate, NR mRNA level remained very high. NR activity and protein increased over a 24-hour period when nitrate was added back to N-starved molybdate-grown plants. NR mRNA level increased markedly during the first 2 hours and then decreased. In the presence of tungstate, however, the induction of NR activity by nitrate was totally abolished while high levels of NR protein and mRNA were both induced, and the high level of NR mRNA was maintained over a 10-hour period. These results suggest that the substitution of tungsten for molybdenum in NR complex leads to an overexpression of the NR structural gene. Possible mechanisms involved in this deregulation are discussed.