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Biomedical subjects

R G Jensen

Publications and source records attributed to R G Jensen.

At least 73 records · Page 4Linked to original sources
PubMed1985

Specificity of human milk bile salt-stimulated lipase.

A crude preparation of the bile salt-stimulated lipase in human milk did not differentiate between the primary and secondary esters of several synthetic triacylglycerols. The dioleoyl ester of 2,3-butanediol was also digested, unequivocal confirmation of the absence of positional specificity. Preferential release of 18:2 relative to 16:0 and 18:1 as compared to 18:1t (trans) was observed. The lipase did not differentiate between 16:0 and 18:1 or 16:0 and 18:0.

Bile Acids and Salts↗
PubMed1984

Lipids in human milk: a review. 1: Sampling, determination, and content.

A protocol for selection of donors of milk samples is presented. Various methods for the sampling of milk are critically reviewed and a tested procedure is described. The measurement of milk volume is reviewed. Analyses for milk lipids are presented and discussed. Results of the analyses relative to various factors affecting fat content are given. Depending upon the type of study, sampling of milk by the Egnell or similar pump is recommended. For field studies, the creamatocrit procedure for estimation of fat content is acceptable if calibrated. Satisfactory research methods are the Roese-Gottlieb, modified Folch, and dry column extractions followed by gravimetry.

Dietary Fats↗
PubMed1984

Lipids in human milk: a review. 2: Composition and fat-soluble vitamins.

Recent work on lipid classes and the fatty acid composition of milk is reviewed. At least 98% of the lipids are triacylglycerols with about 1% phospholipids and 0.3-0.4% cholesterol. Desmosterol has been identified. The major phospholipids (% of total) are: sphingomyelin (32), phosphatidylcholine (25), and phosphatidylethanolamine (28). The 12:0 content of colostrum is lower than that of mature milk. The 18:2 content of mature milk can be altered by diet; otherwise, the fatty acid composition is remarkably constant. Long-chain polyunsaturates appear to be needed for development of the infant's brain and nervous system. Data on the contents of these acids are given. The fat-soluble vitamins have been analyzed by high-performance liquid chromatography. The amounts of vitamins D and K in milk, unlike those of A and E, are lower than the quantities required to meet the U.S. Recommended Daily Allowances. Vitamin D sulfate does not appear to be a significant antirachitic component of human milk. Nutritional and analytical aspects are emphasized throughout and areas for future investigations indicated.

Cholesterol↗
PubMed1984

Methods of lipid analysis.

A protocol for sample preparation before the determination in or isolation of lipids from human milk is presented. Methods for the direct determination of lipids including the creamatocrit, spectrophotometric (turbidimetric), and enzymatic methods are discussed. Procedures for the isolation of lipids including the Roese-Gottlieb, modified Folch, and dry column extractions are compared. Finally, techniques for the separation and identification of lipid classes are described.

Female↗
PubMed1984

Photosynthesis and Ribulose 1,5-Bisphosphate Concentrations in Intact Leaves of Xanthium strumarium L.

The interacting effects of the rate of ribulose 1,5-bisphosphate (RuBP) regeneration and the rate of RuBP utilization as influenced by the amount and activation of RuBP carboxylase on photosynthesis and RuBP concentrations were resolved in experiments which examined the kinetics of the response of photosynthesis and RuBP concentrations after step changes from a rate-saturating to a rate-limiting light intensity in Xanthium strumarium. Because RuBP carboxylase requires several minutes to deactivate in vivo, it was possible to observe the effect of reducing the rate of RuBP regeneration on the RuBP concentration at constant enzyme activation state by sampling very soon after reducing the light intensity. Samples taken over longer time periods showed the effect of changes in enzyme activation at constant RuBP regeneration rate on RuBP concentration and photosynthetic rate. Within 15 s of lowering the light intensity from 1500 to 600 microEinsteins per square meter per second the RuBP concentration in the leaves dropped below the enzyme active site concentration, indicating that RuBP regeneration rate was limiting for photosynthesis. After longer intervals of time, the RuBP concentration in the leaf increased as the RuBP carboxylase assumed a new steady state activation level. No change in the rate of photosynthesis was observed during the interval that RuBP concentration increased. It is concluded that the rate of photosynthesis at the lower light intensity was limited by the rate of RuBP regeneration and that parallel changes in the activation of RuBP carboxylase occurred such that concentrations of RuBP at steady state were not altered by changes in light intensity.

Journal Article↗
PubMed1983

Determination of lipase specificity.

Specificity of lipases is controlled by the molecular properties of the enzyme, structure of the substrate and factors affecting binding of the enzyme to the substrate. Types of specificity are as follows. I. Substrate: (a) different rates of lipolysis of TG, DG, and MG by the same enzyme; (b) separate enzymes from the same source for TG, DG and MG. II. Positional: (a) primary esters; (b) secondary esters; and (c) all three esters or nonspecific hydrolysis. III. Fatty acid, preference for similar fatty acids. IV. Stereospecificity: faster hydrolysis of one primary sn ester as compared to the other. V. Combinations of I-IV. Lipases with these specificities are: Ia, pancreatic; Ib, postheparin plasma. IIa, pancreatic; IIb, Geotrichum candidum, for fatty acids with cis-9-unsaturation, and IIc, Candida cylindracea. III, G. candidum for unsaturates. IV. sn-1, postheparin plasma and sn-3 human and rat lingual lipases. V. Rat lingual lipase. Methods for determination involve digestion of natural fats of known structure and synthetic acylglycerols followed by analysis of the lipolysis products. All of the types of specificity have been detected with use of synthetic acylglycerols. Detection of stereospecificity requires enantiomeric acylglycerols which are difficult to synthesize, so other methods have been developed. These involve the generation of 1,2-(2,3)DG and resolution of the enantiomers. Trioleoylglycerol or racemic TG can be used as substrates. If the lipase is stereospecific, then either the sn-1,2- or 2,3-enantiomer will predominate. The relative amounts of the enantiomers can be determined by measurement of specific rotation, and nuclear magnetic resonance spectra. The DG can also be separated by conversion to phospholipids and hydrolysis with phospholipases A-2 or C. Applications of these procedures are discussed and data on the specificity of various lipases presented.

Animals↗
PubMed1983

Desmosterol in human milk.

Milk samples were collected from mothers at 2, 6, 12 and 16 weeks postpartum. Desmosterol was found to be present in all the milk samples. Identification of desmosterol was based on retention times with two gas liquid chromatography (GLC) columns and verified by GC-mass spectrometry. The concentration of desmosterol in breast milk increased significantly (P less than .05) from 0.6 mg/100 ml at 2 weeks to 1.3 mg/100 ml at 16 weeks postpartum. Desmosterol was not significantly correlated with total lipid, total cholesterol or free cholesterol in the milk.

Desmosterol↗
PubMed1983

Detection and determination of lipase (acylglycerol hydrolase) activity from various sources.

Methods for the detection and determination of lipases (acylglycerol hydrolases) and preparation of assays are reviewed including substrates, conditions and screening. Some newer methods for the determination of lipase activity are discussed. Several of these are: (a) titrimetry, (b) colorimetry of Cu soaps of free fatty acids (FFA), (c) colorimetry of chromophores in the acyl chain of FFA or in glycerol, (d) radioassay, (e) gas liquid chromatography, (f) enzymatic treatment of FFA and measurement of the resulting products, and (g) direct immunological determination of the lipase. Examples and sensitivities are given and advantages and disadvantages are described.

Chromatography, Gas↗
PubMed1983

Bile salt-stimulated lipase in human milk from 2 to 16 weeks postpartum.

Our objectives were to determine the change in bile salt-stimulated lipase (BSSL) activity in mature milk with time postpartum and to determine if BSSL activity was related to total lipid in the milk. Milk samples were collected from 12 mothers at 2, 6, 12 and 16 weeks postpartum. A significant (P less than .05) decrease in BSSL activity with time was observed. The average values expressed as mumol free fatty acids released/min/ml milk were 5.3, 4.5, 4.1 and 3.6 at 2, 6, 12 and 16 weeks postpartum. Total lipid in the milk increased significantly (P less than .05) from 4.1 g/dl at 2 weeks to 5.4 g/dl at 16 weeks postpartum but was not significantly correlated with BSSL activity.

Bile Acids and Salts↗
PubMed1983

Photosynthesis and Activation of Ribulose Bisphosphate Carboxylase in Wheat Seedlings : Regulation by CO(2) and O(2).

Photosynthetic carbon assimilation in plants is regulated by activity of the ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase. Although the carboxylase requires CO(2) to activate the enzyme, changes in CO(2) between 100 and 1,400 microliters per liter did not cause changes in activation of the leaf carboxylase in light. With these CO(2) levels and 21% O(2) or 1% or less O(2), the levels of ribulose bisphosphate were high and not limiting for CO(2) fixation. With high leaf ribulose bisphosphate, the K(act)(CO(2)) of the carboxylase must be lower than in dark, where RuBP is quite low in leaves. When leaves were illuminated in the absence of CO(2) and O(2), activation of the carboxylase dropped to zero while RuBP levels approached the binding site concentration of the carboxylase, probably by forming the inactive enzyme-RuBP complex.The mechanism for changing activation of the RuBP carboxylase in the light involves not only Mg(2+) and pH changes in the chloroplast stroma, but also the effects of binding RuBP to the enzyme. In light when RuBP is greater than the binding site concentration of the carboxylase, Mg(2+) and pH most likely determine the ratio of inactive enzyme-RuBP to active enzyme-CO(2)-Mg(2+)-RuBP forms. Higher irradiances favor more optimal Mg(2+) and pH, with greater activation of the carboxylase and increased photosynthesis.

Journal Article↗
PubMed1982

The lipolytic triad: human lingual, breast milk, and pancreatic lipases: physiological implications of their characteristics in digestion of dietary fats.

The characteristics of human lingual, breast milk, and pancreatic lipases as related to the digestion of dietary fats in infants are discussed. The activity and specificity of these enzymes and structure of the dietary fats largely determine the rates of lipolysis, the types of digestion products formed, and the rates of absorption. Also possibly influenced are micelle formation, intestinal health, breast milk jaundice, and the absorption of other nutrients. In premature infants, the action of lingual and breast milk lipase are particularly important in the absorption of dietary fatty acids.

Bile Acids and Salts↗
PubMed1982

Changes in the lipids of human milk from 2 to 16 weeks postpartum.

Changes in total lipid, fatty acids, total cholesterol, free cholesterol, and lipid phosphorus in mature milk with time were investigated. Milk samples were collected from 10 mothers at 2, 6, 12, and 16 weeks postpartum. During 1 day, each mother donated two complete breast expressions. Expressions were taken using an electric breast pump 1 h after the previous a.m. and p.m. nursing. For each mother, the a.m. and p.m. samples were pooled for analysis. It was observed that the amount of total lipid increased significantly (p less than 0.05) from 3.9 g/100 ml at 2 weeks to 5.2 g/100 at 16 weeks postpartum. The total fatty acid composition remained uniform during the investigation. Average total cholesterol and free cholesterol in the milk were 10.3 mg/100 ml and 8.3 mg/100 ml, respectively. These concentrations did not change significantly with time postpartum. Average lipid phosphorus was 3.9 mg/100 ml and also remained constant throughout. We conclude that the fatty acid pattern, lipid phosphorus, total cholesterol, and free cholesterol of mature milk to 16 weeks postpartum remains relatively constant while total lipid concentration increases.

Cholesterol↗
PubMed1982

Measurement and preservation of the in vivo activation of ribulose 1,5-bisphosphate carboxylase in leaf extracts.

Photosynthetic carbon fixation is regulated in the chloroplast by the amount of ribulose 1,5-bisphosphate carboxylase which is activated. The activated carboxylase was preserved in detached leaves (barley, maize, soybean, spinach, wheat) for 90 min when stored on ice. With leaf extracts stored at 2 degrees C, the amount of activated enzyme, representing that originally in the leaf, as well as the fully activated enzyme, formed by incubation of leaf extracts with Mg(2+) and bicarbonate, both slowly declined in activity. However, for each activity this decline was proportional such that the ratio (percent activation) appeared constant. No change was observed in activation of the enzyme during the brief time of leaf homogenization. Optimal conditions (Mg(2+), incubation time) for measurement of leaf activation of ribulose bisphosphate carboxylase vary depending on the plant.3-Phosphonoproprionate, a positive effector of the purified ribulose bisphosphate carboxylase and a metabolically inert analog of 2-phosphoglycolate, was used to examine what metabolic effectors might do to enzyme activation during leaf homogenization and preparation of the extract at 2 degrees C. Activation under these conditions was not altered by 3-phosphonoproprionate. When 3-phosphonoproprionate was brushed on attached leaves or taken up by the transpiration stream of detached leaves, a considerable increase in activation of the carboxylase was measured.

Journal Article↗
PubMed1982

Photosynthetic CO(2) Fixation at Air Levels of CO(2) by Isolated Spinach Chloroplasts.

A system has been developed for the study of photosynthetic CO(2) fixation by isolated spinach chloroplasts at air levels of CO(2). Rates of CO(2) fixation were typically 20 to 60 micromoles/milligrams chlorophyll per hour. The rate of fixation was linear for 10 minutes but then declined to less than 10% of the initial value by 40 minutes. Ribulose 1,5-bisphosphate (RuBP) levels remained unchanged during this period, indicating that they were not the cause for the decline. The initial activity of the RuBP carboxylase in the chloroplast was high for 8 to 10 minutes and then declined similar to the rate of CO(2) fixation, suggesting that the decline in CO(2) fixation may have been caused by deactivation of the enzyme.In contrast, the addition of 50 micromolar 3-phosphoglycerate or 50 micromolar glycerate doubled the initial rate of CO(2) fixation without increasing the activity of the carboxylase. Addition of 4 millimolar MgCl(2) plus 2 millimolar pyrophosphate had no effect on the initial rate of CO(2) fixation nor its decline but did stop deactivation of the chloroplast carboxylase during the decline.The decline in photosynthesis with isolated chloroplasts, where RuBP is high, may be due to deactivation of the chloroplast RuBP carboxylase; however, measurements of this parameter may not always indicate this. Care must be taken when interpreting measurements of the ;activated' RuBP carboxylase as more forms are measured than the enzyme . CO(2) . Mg(2+) . RuBP form directly involved in carboxylation or oxygenation.

Journal Article↗
PubMed1981

Light limitation of photosynthesis and activation of ribulose bisphosphate carboxylase in wheat seedlings.

In limiting light the activation of ribulose-1,5-bisphosphate (RuP(2)) carboxylase [3-phospho-D-glycerate carboxylyase (dimerizing), EC 4.1.1.39] in leaf extracts of 7- to 8-day-old wheat seedlings changed proportionally with the photosynthetic rate of the intact plants. Higher rates of photosynthesis, induced by increasing irradiances, were accompanied by an increase in activation of the leaf RuP(2) carboxylase, while RuP(2) levels remained unchanged. The degree of activation varied from 20% to 60% of full activation at irradiances of 225-1650 muE/m(2).s (photosynthetically active radiation; E = einstein, 1 mol of photons). Between 225 muE/m(2).s and darkness, activation approached 50% while RuP(2) levels dropped more than 90%. During steady-state photosynthesis, levels of the substrate RuP(2) were 250-300 nmol/mg of chlorophyll in the leaves and were similar at all irradiances above 225 muE/m(2).s (25% of light saturation). When velocities of the carboxylase in leaf extracts were corrected for CO(2) levels estimated to exist within the leaf, they compared favorably with the photosynthetic rates of the intact seedlings. Comparison of CO(2) exchange rate, RuP(2) level, and activation of the carboxylase indicates that light limitation of photosynthesis can be due to two factors: the availability of RuP(2) in dark to dim light and activation of the RuP(2) carboxylase in dim light and higher irradiances.

Journal Article↗