R Jeffcoat

Louisiana State University, Baton Rouge, Louisiana, United States

Are you R Jeffcoat?

Claim your profile

Publications (31)136.68 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Energy values of high amylose corn starches high in resistant starch (RS) were determined in vivo by two different methodologies. In one study, energy values were determined according to growth relative to glucose-based diets in rats fed diets containing RS(2), heat-treated RS(2) (RS(2)-HT), RS(3), and amylase predigested versions to isolate the RS component. Net metabolizable energy values ranged from 2.68 to 3.06 kcal/g for the RS starches, and 1.91-2.53 kcal/g for the amylase predigested versions. In a second study, rats were fed a diet containing RS(2)-HT and the metabolizable energy value was determined by bomb calorimetry. The metabolizable energy value was 2.80 kcal/g, consistent with Study 1. Thus, high amylose corn based RS ingredients and their amylase predigested equivalents have energy values approximately 65-78% and 47-62% of available starch (Atwater factor), respectively, according to the RS type (Garcia, T. A.; McCutcheon, K. L.; Francis, A. R.; Keenan, M. J.; O'Neil, C. E.; Martin, R. J.; Hegsted, M. The effects of resistant starch on gastrointestinal organs and fecal output in rats. FASEB J. 2003, 17, A335).
    Journal of Agricultural and Food Chemistry 09/2009; 57(18):8474-9. DOI:10.1021/jf900971c · 3.11 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: 1Hepatocytes were isolated by perfusion of the liver with collagenase/salt solutions and incubated in culture after attachment to plastic culture dishes for periods up to 48 h.2The cells, when incubated in serum-free culture medium in the presence of insulin, showed enhanced stearoyl-CoA desaturase activity which was not observed when 50 uM cycloheximide was included. When insulin was omitted from the medium, the cells lost 80% of their original desaturase activity.3Cells isolated from animals fed 20% (w/w) sucrose for two weeks prior to sacrifice, showed high levels of fatty acid synthesis, stearoyl-CoA desaturase activity and triacylglycerol synthesis when compared with cells isolated from animals fed a corn oil supplemented diet.4The observations are discussed in terms of the influence of stearoyl-CoA desaturase activity on hepatic lipogenesis.
    06/2008; 101(2):439 - 445. DOI:10.1111/j.1432-1033.1979.tb19737.x
  • Source
    Roger Jeffcoat
    [Show abstract] [Hide abstract]
    ABSTRACT: Many factors affect the onset of obesity including satiety control, reduced levels of physical exercise as well as hormonal and genetic parameters which influence the metabolic pathways leading to the net accumulation of triacylglycerol (TAG). The predominant fatty acid of human adipose tissue TAGs is oleic acid, reflecting primarily the composition of the diet but also the product of de novo lipogenesis. Consequently, both carbohydrates and lipids are potential sources of these stored fats. Many studies have been carried out using a variety of differing experimental protocols on healthy, obese or diabetic humans and animals in positive or neutral energy balance to establish the underlying molecular basis for obesity particularly in humans. This short review discusses the interdependence and control of the metabolism of lipids and carbohydrates as it relates to lipogenesis and proposes a unified hypothesis for obesity which brings together a number of different approaches focusing on (i) the interaction of dietary fat and carbohydrate, which typically represent approximately 80% of the daily caloric intake, and their role in the synthesis of TAGs, (ii) the biochemical pathways which control the amount of TAG produced by controlling the composition of their fatty acids via the action of stearoyl-CoA desaturase (SCD), (iii) the control of lipogenesis and SCD by dietary polyunsaturated fatty acid (PUFA) and (iv) the interaction of PUFAs with the transcription factors, peroxisome proliferator activated receptors (PPAR) alpha and gamma, which maintain the balance between oxidation and storage of lipids. The hypothesis focuses on the central role of stearoyl-CoA desaturase (SCD) and its inhibition by polyunsaturated fatty acids (PUFAs) acting via transcription factors based upon data obtained from both animal and human studies. From these observations it should be possible to determine the relevance of the hypothesis to humans and to speculate how these aspects of metabolism may impact the risk of developing related diseases such as coronary heart disease, Type 2 diabetes and certain forms of cancer.
    Medical Hypotheses 02/2007; 68(5):1159-71. DOI:10.1016/j.mehy.2006.06.009 · 1.07 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The objective of the present study was to investigate the effects of starches with differing rates of hydrolysis on exposure to pancreatin in vitro on postprandial carbohydrate metabolism in healthy subjects and in subjects with type 2 diabetes. Two test starches, prepared from uncooked native granular starch products, and naturally enriched with 13C, were consumed in a randomized crossover design by eight healthy and thirteen type 2 diabetic subjects. One starch was characterized in vitro as being rapidly hydrolysed (R, 94% after 180 min), and the other was more slowly hydrolysed (S, 51% after 180 min). Each subject consumed 50 g of each test starch. In addition, the type 2 diabetic subjects consumed 89.7 g of the S starch on a separate occasion. Blood samples were taken at 10 min intervals for 3 h, and at 20 min intervals for a further 3 h during a 6 h postprandial period. Breath 13CO2 enrichment was measured at the same time points, and indirect calorimetry was performed for seven 20 min sessions immediately before and during the 6 h postprandial period. With the R starch, plasma glucose concentrations and serum insulin concentrations rose faster and the maximum glucose change was approximately 1.8 times that for the S starch, averaged across both subject groups. The areas under the curves for glucose and insulin were, respectively, 1.7 and 1.8 times higher for the R starch compared with the S starch, averaged across both subject groups. The rate of 13CO2 output and the proportion of 13C recovered in breath after consumption of the R starch was similar for both subject groups. The results provide evidence that starches which have different rates of hydrolysis in vitro result in different patterns of glycaemia and insulinaemia in both healthy adults and in diet-controlled type 2 diabetic subjects. Data from the hydrolysis of novel starch products in vitro, therefore, are useful in predicting glycaemic responses in vivo.
    British Journal Of Nutrition 12/2003; 90(5):853-64. DOI:10.1079/BJN2003972 · 3.34 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Antibiotics are being proposed for the treatment of cardiovascular disease. In the past, antibiotics were advocated for the control of hypercholesterolemia. We have therefore investigated the relation between colonic bacterial activity and serum lipids. In a four-phase randomized crossover study, we fed a different starch supplement during each 2-week phase to 24 healthy subjects. In two phases, supplements containing resistant starches were fed that reach the colon and are largely fermented by colonic bacteria. Fecal starch recovery therefore reflects the metabolic activity of colonic microflora. The control treatments were conventional starches. Blood lipid levels were obtained at the start and 4-day fecal collections at the end of each phase. Resistant starch supplements increased fecal starch excretion by 3.8 +/- 1.2 g/d more than conventional starches (P = .006). Mean starch excretion was related positively to pretreatment serum high-density lipoprotein (HDL) cholesterol (r = -.57, P = .003) and negatively to low-density lipoprotein (LDL) cholesterol (r = -.57, P = .004), apolipoprotein B:AI (r = -.56, P = .005), and fecal output of fusobacteria (r = -.73, P = .003) and bacteroides (r = -.72, P = .003). The ratio of fusobacteria to total anaerobes was also related to pretreatment LDL cholesterol (r = .56, P = .037). Differences in starch excretion between healthy subjects, as a measure of bacterial activity, accounted for 32% of the variation in pretreatment LDL cholesterol. The activity of colonic microflora therefore appears to influence serum lipid levels. Alterations of bacterial number and activity may provide an additional strategy to control serum lipid risk factors for cardiovascular disease.
    Metabolism 03/1999; 48(2):264-8. DOI:10.1016/S0026-0495(99)90045-8 · 3.61 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: To assess the effects on fecal bulking, fecal short chain fatty acid (SCFA) production, blood lipids and glycemic indices of two different forms of resistant starch (RS2 and RS3) from a high-amylose cornstarch. Twenty-four healthy subjects (12 men; 12 women) consumed four supplements taken for 2 weeks in random order separated by 2-week washout periods. The supplements were a low-fiber (control) and supplements providing an additional 30 g dietary fiber as wheat bran (high-fiber control) or the equivalent amount of resistant starch analyzed gravimetrically as dietary fiber from RS2 or RS3. Four-day fecal collections and 12-hour breath gas collections were obtained at the end of each period. Fasting blood was taken at the beginning and end of each period. Glycemic indices of supplements were also assessed. The wheat bran supplement increased fecal bulk 96+/-14 g/day compared with the low-fiber control (p<0.001) with the mean for both resistant starches also being greater (22+/-8 g/day) than the low-fiber control (p=0.013). On the resistant starch phases, the mean fecal butyrate:SCFA ratio, which has been suggested to have positive implications for colonic health, was significantly above the low-fiber control by 31+/-14% (p=0.035). Resistant starches did not alter serum lipids, urea or breath H2 or CH4. No significant differences in glycemic index were seen between the RS and control supplements. The potential physiological benefits of the resistant starches studied appear to relate to colonic health in terms of effects on fecal bulk and SCFA metabolism.
    Journal of the American College of Nutrition 01/1999; 17(6):609-16. DOI:10.1080/07315724.1998.10718810 · 1.68 Impact Factor
  • M Foot · R Jeffcoat · M D Barratt · N J Russell
    [Show abstract] [Hide abstract]
    ABSTRACT: The relationship between the delta 9-desaturase activity of the psychrophilic bacterium Micrococcus cryophilus grown at different temperatures and the physical state of its membrane lipids as measured by ESR spectroscopy has been studied. Arrhenius plots of desaturase activity were biphasic with a discontinuity at a temperature which depended upon the bacterial growth temperature. Changes in the desaturase activation energy, which increased as the growth temperature was lowered, are discussed in the context of membrane lipid fluidity adaptation to changing environmental temperature. The fluidity of membranes and isolated lipids was measured using nitroxide-labeled fatty acids. The spectra of 2-(10-carboxydecyl)-2-hexyl-4,4-dimethyl-3-oxazolidinoxyl in membranes indicated that there were two lipid environments within the membrane whose relative proportions were dependent both on temperature of measurement and on bacterial growth temperature. In contrast, 2-(3-carboxypropyl)-4,4-dimethyl-2-tridecyl-3-oxazolidinoxyl spectra showed a single lipid environment and plots of log order parameter (S3) vs 1/T were biphasic with inflexion temperatures which were closely related to the bacterial growth temperature. As with membranes, plots of log S3 vs 1/T for total lipids, phosphatidylglycerol and cardiolipin, but not phosphatidylethanolamine, were biphasic and showed inflexions which correlated well with bacterial growth temperature. These results are interpreted as being consistent with a location for the desaturase within the bulk lipid of the membrane rather than in association with specific lipid types.
    Archives of Biochemistry and Biophysics 08/1983; 224(2):718-27. · 3.04 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The interaction of rat mammary gland medium-chain thioesterase with yeast fatty acid synthetase has been investigated. Medium-chain thioesterase interacts with yeast fatty acid synthetase causing premature chain termination of the fatty acids synthesized from acetyl-CoA and malonyl-CoA. This effect is most marked under conditions of rate-limiting malonyl-CoA availability. Immobilized yeast fatty acid synthetase specifically binds rat mammary gland medium-chain thioesterase. This interaction has been used to purify medium-chain thioesterase to near homogeneity from samples of rat mammary gland cytosol. The stoichiometry of binding of medium-chain thioesterase to yeast fatty acid synthetase has been investigated. Yeast fatty acid synthetase binds 5.7 +/- 1 mol medium-chain thioesterase/mol yeast fatty acid synthetase. It is concluded that yeast fatty acid synthetase has a medium-chain thioesterase binding site.
    European Journal of Biochemistry 08/1983; 134(1):27-32. DOI:10.1111/j.1432-1033.1983.tb07526.x · 3.58 Impact Factor
  • Source
    M Foot · R Jeffcoat · N J Russell
    [Show abstract] [Hide abstract]
    ABSTRACT: The delta 9-desaturase of the psychrophilic bacterium Micrococcus cryophilus is shown to be a membrane-bound enzyme that is probably linked to a cyanide- (and azide-) sensitive respiratory chain with oxygen as the final acceptor. It has a pH optimum of 8.7 and contains an essential thiol group, but has no special ion requirements. The desaturase activity of washed membranes could not be increased by adding supernatant or NADH and NADPH, possibly owing to the endogenous generation of reduced cofactors by the membranes. The substrate for the desaturase is not acyl-CoA and is probably not acyl-acyl-carrier protein. Evidence is presented that the substrate in vivo is saturated phospholipid and a scheme for the possible routes of incorporation of exogenous stearic acid into oleoyl-phospholipid is presented.
    Biochemical Journal 03/1983; 209(2):345-53. DOI:10.1042/bj2090345 · 4.78 Impact Factor
  • Biochemical Society Transactions 11/1980; 8(5):534. DOI:10.1042/bst0080534 · 3.24 Impact Factor
  • Source
    Roger Jeffcoat · Peter A. Roberts · Anthony T. James
    [Show abstract] [Hide abstract]
    ABSTRACT: The replacement of dietary starch by sucrose results in an increase in hepatic lipogenesis in the rat. When corn oil (4% by weight or 9% of the energy content of the diet) was included with the sucrose (20% by weight, 20% of the energy content) the lipogenic effect of the sucrose was completely suppressed. In contrast, when beef tallow replaced the corn oil, the induced activity caused by the sucrose was reduced by only approximately 20%. No significant differences were observed between males and females. These diets containing sucrose supplemented with either 4% (w/w) corn oil or 4% (w/w) beef tallow, were then used to ascertain whether or not the effects on hepatic lipogenesis were reflected in changes in the amount of fat deposited during growth from 4--24 weeks of age. It was shown that the percentage body fat was only statistically different (P less than 0.05) when animals fed sucrose-supplemented diets were compared with animals fed diets supplemented with sucrose and beef tallow. However, there were no significant differences in total carcass weight of these rats. The results are discussed in terms of the relative contribution of liver and adipose tissue to total lipogenesis and the factors which control the lipogenic activity in the two tissues.
    European Journal of Biochemistry 12/1979; 101(2):447-53. DOI:10.1111/j.1432-1033.1979.tb19738.x · 3.58 Impact Factor
  • R Jeffcoat · P A Roberts · J Ormesher · A T James
    [Show abstract] [Hide abstract]
    ABSTRACT: 1. Hepatocytes were isolated by perfusion of the liver with collagenase/salt solutions and incubated in culture after attachment to plastic culture dishes for periods up to 48 h. 2. The cells, when incubated in serum-free culture medium in the presence of insulin, showed enhanced stearolyl-CoA desaturase activity which was not observed when 50 muM cycloheximide was included. When insulin was omitted from the medium, the cells lost 80% of their original desaturase activity. 3. Cells isolated from animals fed 20% (w/w) sucrose for two weeks prior to sacrifice, showed high levels of fatty acid synthesis, stearolyl-CoA desaturase activity and triacylglycerol synthesis when compared with cells isolated from animals fed a corn oil supplemental diet. 4. The observations are discussed in terms of the influence of stearoyl-CoA desaturase activity on hepatic lipogenesis.
    European Journal of Biochemistry 12/1979; 101(2):439-45. · 3.58 Impact Factor
  • R Jeffcoat · A P Dunton · A T James
    [Show abstract] [Hide abstract]
    ABSTRACT: Microsomes prepared from the livers of 4-week-old rats were, after extraction with 0.1 M potassium phosphate buffer, pH 7.4, unable to catalyse either the delta6 desaturation of alpha-linolenic acid (9c.12c.15c., 18 : 3) into 6c.9c.12c.15c., 18 : 4 or the delta5 desaturation of eicosatrienoic acid (8c.11c.14c., 20 : 3) into arachidonic acid (5c.8c.11c.14c., 20 : 4). Both these enzymes only showed full activity after incubation of the microsomes with either the 100 000 X g supernatant fraction or with purified bovine catalase. Bovine serum albumin, while capable of restoring 50% of the delta5 desaturase activity has no effect on the delta6 desaturase. In contrast the delta9 desaturase activity of microsomes was never completely lost after extraction with buffer but could be stimulated by optimum concentrations of both bovine serum albumin and catalase. The significance of the different responses of the three desaturases to the cytoplasmic components is discussed.
    Biochimica et Biophysica Acta 02/1978; 528(1):28-35. · 4.66 Impact Factor
  • Source
    R Jeffcoat · A T James
    FEBS Letters 02/1978; 85(1):114-8. DOI:10.1016/0014-5793(78)81260-5 · 3.34 Impact Factor
  • R. Jeffcoat · A.P. Dunton · A.T. James
    [Show abstract] [Hide abstract]
    ABSTRACT: Microsomes prepared from the livers of 4-week-old rats were, after extraction with 0.1 M potassium phosphate buffer, pH 7.4, unable to catalyse either the Δ6 desaturation of α-linolenic acid (9c.12c.15c., 18:3) into 6c.9c.12c.15c., 18:4 or the Δ5 desaturation of eicosatrienoic acid (8c.11c.14c., 20 : 3) into arachidonic acid (5c.8c.11c.14c., 20:4). Both these enzymes only showed full activity after incubation of the microsomes with either the 100 000 × g supernatant fraction or with purified bovine catalase. Bovine serum albumin, while capable of restoring 50% of the Δ5 desaturase activity had no effect on the Δ6 desaturase. In contrast the Δ9 desaturase activity of microsomes was never completely lost after extraction with buffer but could be stimulated by optimum concentrations of both bovine serum albumin and catalase. The significance of the different responses of the three desaturases to the cytoplasmic components is discussed.
    Biochimica et Biophysica Acta (BBA) - Lipids and Lipid Metabolism 01/1978; 528(1-528):28-35. DOI:10.1016/0005-2760(78)90049-8
  • R Jeffcoat · A T James
    [Show abstract] [Hide abstract]
    ABSTRACT: In this paper we present further evidence for the close control of fatty acid synthetase and stearoyl-CoA desaturase. Furthermore, we have established that whereas dietary palmitic acid may influence the activity of this desaturase but not of fatty acid synthetase, dietary linoleic acid appears to control both these enzymes. Finally, we have studied the influence of dietary fat and carbohydrate on the activities of the delta6 and delta5 desaturases. The former is only slightly affected by these dietary components. The delta5 desaturase activity is stimulated as the dietary fat content rises but is unaffected by dietary carbohydrate. The control of these enzymes is therefore independent of the control of fatty acid synthetase and stearoyl-CoA desaturase. From the data presented, the magnitude of the controlling effect of polyunsaturated fatty acids on fatty acid synthetase and stearoyl-CoA desaturase activity is determined and its relevance to lipogenesis in man based on daily intake of carbohydrate and linoleic acid is discussed.
    Lipids 07/1977; 12(6):469-74. DOI:10.1007/BF02535444 · 2.35 Impact Factor
  • R Jeffcoat · M R Pollard
    [Show abstract] [Hide abstract]
    ABSTRACT: Unwashed rat liver microsomes were used to study the inhibition of the delta6 and delta9 desaturases by cyclopropenoid fatty acids with the ring structure about the 9,10 or 6,7 carbon atoms. The 9,10 cyclopropenoid acid (sterculic acid) is shown to be an effective inhibitor of only delta9 desaturase and then only in the presence of MgCl2 and coenzyme A (presumably due to the formation of sterculoyl-CoA). Two 6,7 cyclopropenoid acids of different chain lengths showed no marked inhibition of either the delta6 or delta9 desaturase. By the use of [3H]-sterculic acid, it has been shown that under conditions of high inhibition of the delta9 desaturase the inhibitor is not covalently attached to the enzyme at any point. This disproves older ideas on the mechanism of inhibition that assumed reaction between the cyclopropenoid ring and sulphydryl groups on the enzymes.
    Lipids 07/1977; 12(6):480-5. DOI:10.1007/BF02535446 · 2.35 Impact Factor
  • Source
    R Jeffcoat · P R Brawn · R Safford · A T James
    [Show abstract] [Hide abstract]
    ABSTRACT: 1. Rat liver microsomal stearoyl-CoA desaturase activity was shown to be stimulated by both bovine serum albumin and a basic cytoplasmic protein from rat liver. 2. Partially purified desaturase is unaffected by either of these two proteins. 3. Bovine serum albumin appears to exert its effect on the crude system by protecting the desaturase substrate, stearoly-CoA, from the action of endogenous thiolesterases. 4. By using partially purified enzyme preparations, it was possible to establish the substate specificity of the delta9-fatty acyl-CoA desaturase with the C14, C15, C16, C17, C18 and C19 fatty acyl-CoA substrates. Maximum enzyme activity was shown with stearoyl-CoA decreasing with both palmitoyl-CoA and nonadecanoyl-CoA, as reported previously for free fatty acids. 5. Both cytochrome b5 and NADH-cytochrome b5 reductase (EC 1.6.2.2) are required for these studies and a method is described for the purification of homogeneous preparations of detergent-isolated cytochrome b5 from rat liver. 6. From amino acid analyses, a comparison was made of the hydrophobicity of the membrane portion of cytochrome b5 with the hydrophobicity reported for stearoyl-CoA desaturase. The close resemblance of the two values suggested that unlike cytochrome b5 and its reductase, the stearoyl-CoA desaturase may be largely buried in the endoplasmic reticulum.
    Biochemical Journal 03/1977; 161(2):431-7. DOI:10.1042/bj1610431 · 4.78 Impact Factor
  • R Jeffcoat · P R Brawn · A T James
    [Show abstract] [Hide abstract]
    ABSTRACT: 1. The influence of bovine serum albumin and soluble rat liver proteins on the activity of rat liver microsomal delta9 and delta6 desaturases has been studied. 2. In the absence of bovine serum albumin, the delta9 desaturase which converts stearoyl-CoA into oleoyl-CoA, shows a non-linear correlation between enzyme activity and protein concentration. 3. Optimum concentrations of bovine serum albumin have three main effects on the enzyme activity: (i) establishes a linear relationship between enzyme activity and protein concentration, (ii) stimulates the enzyme activity 2--3-fold and (iii) raises the optimum substrate concentration from 10 to 100 muM. 4. A highly purified soluble liver protein of molecular weight 24 000 also stimulated the enzyme activity and brought about a linear relationship between enzyme activity and protein concentration. 5. It was concluded that the non-linear kinetics were due to limiting amounts of substrate binding protein in the microsomal preparations. 6. The delta6 desaturase which converts linoleoyl-CoA into gamma-linolenoyl-CoA was also stimulated by bovine serum albumin and soluble liver proteins. 7. The significance of the fatty acid-binding proteins is discussed.
    Biochimica et Biophysica Acta 05/1976; 431(1):33-44. · 4.66 Impact Factor
  • R. Jeffcoat · P.R. Brawn · A.T. James
    [Show abstract] [Hide abstract]
    ABSTRACT: 1.1. The influence of bovine serum albumin and soluble rat liver proteins on the activity of rat liver microsomal Δ9 and Δ6 desaturases has been studied.2.2. In the absence of bovine serum albumin, the Δ9 desaturase which converts stearoyl-CoA into oleoyl-CoA, shows a non-linear correlation between enzyme activity and protein concentration.3.3. Optimum concentrations of bovine serum albumin have three main effects on the enzyme activity: (i) establishes a linear relationship between enzyme activity and protein concentration, (ii) stimulates the enzyme activity 2–3 fold and (iii) raises the optimum substrate concentration from 10 to 100 μM.4.4. A highly purified soluble liver protein of molecular weight 24000 also stimulated the enzyme activity and brought about a linear relationship between enzyme activity and protein concentration.5.5. It was concluded that the non-linear kinetics were due to limiting amounts of substrate binding protein in the microsomal preparations.6.6. The Δ6 desaturase which converts linoleoyl-CoA into γ-linolenoyl-CoA was also stimulated by bovine serum albumin and soluble liver proteins.7.7. The significance of the fatty acid-binding proteins is discussed.
    Biochimica et Biophysica Acta (BBA) - Lipids and Lipid Metabolism 04/1976; 431(1-431):33-44. DOI:10.1016/0005-2760(76)90257-5

Publication Stats

562 Citations
136.68 Total Impact Points

Institutions

  • 2009
    • Louisiana State University
      • School of Human Ecology
      Baton Rouge, Louisiana, United States
  • 1977–2008
    • Unilever
      • Bioscience
      Londinium, England, United Kingdom
  • 1999
    • St. Michael's Hospital
      Toronto, Ontario, Canada
  • 1974
    • University of Oxford
      • Department of Biochemistry
      Oxford, England, United Kingdom