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Absorption of VFA 70% of VFA absorbed from rumen-reticulum 60 to 70% of remainder absorbed from omasum Papillae are important – provide surface area Absorption from rumen is by passive diffusion Concentration in portal vein less than rumen VFA concentrations Rumen 50 - 150 mM Portal blood 1 - 2 mM Peripheral blood 0.5 - 1 mM Absorption increases at lower pH H+ + Ac- HAc Undissociated acids diffuse more readily At pH 5.7 to 6.7 both forms are present, however most is dissociated At higher pH, 1 equiv of HCO3 enters the rumen with absorption of 2 equiv of VFA VFA Absorption Absorption of AcRumen Ac- AcHAc Portal blood H+ Metabolism HCO3H2O H2CO3 + CO2 CO2 Metabolism HAc HAc Carbonic anhydrase VFA Absorption Rate of absorption: Butyrate > Propionate > Acetate Absorption greater with increasing concentrations of acids in the rumen Absorption increases at lower rumen pH Absorption greater in grain fed animals Faster fermentation – More VFA produced Lower pH Growth of papillae Metabolism of VFA by GIT Half or more of butyrate converted to - hydroxybutyric acid in rumen epithelium. 5% of propionate converted to lactic acid by rumen epithelium. Some acetate is used as energy by tissues of gut. VFA and metabolites carried by portal vein to liver. Tissue Metabolism VFA VFA GIT tissues Liver Body tissues Use of VFA Energy Carbon for synthesis Long-chain fatty acids Glucose Amino acids Other Utilization of Acetate in Metabolism 1. Acetate (As energy) Acetate Acetyl CoA Energy Krebs cycle 2 carbons 2 CO2 (10 ATP/mole) 2. Acetate (Carbon for synthesis of fatty acids – in adipose) Acetate Acetyl CoA Fatty acids H+NADPH NADP+ Lipids Glycerol Pentose PO4 shunt CO2 Glucose Utilization of Butryate in Metabolism Butyrate (As energy) Butyrate Butyrl CoA B-hydroxybutyrate Krebs cycle Energy (27 ATP/mole) Acetyl CoA 2 CO2 Some butyrate also used as a primer for short-chain fatty acids Utilization of Propionate in Metabolism Propionate Propionate Propionyl CoA Methylmalonyl CoA CO2 Glucose Succinyl CoA Vit B12 Krebs cycle Energy (18 ATP/mole) 2 CO2 Utilization of VFA in Metabolism Summary Acetate Energy Carbon source for fatty acids Adipose Mammary gland Not used for net synthesis of glucose Propionate Energy Precursor of glucose Butyrate Energy Carbon source for fatty acids - mammary Effect of VFA on Endocrine System Propionate Increases blood glucose Stimulates release of insulin Butryate Not used for synthesis of glucose Stimulates release of insulin Stimulates release of glucagon Increases blood glucose Acetate Not used for synthesis of glucose Does not stimulate release of insulin Glucose Stimulates release of insulin Energetic Efficiency of VFA in Metabolism ATP/mole Energy in ATP (kcal/mole) % Heat of combustion Acetate 10 Propionate 18 Butyrate 27 76.0 136.8 205.2 36.3 37.2 39.1 Glucose 288.8 42.9 38 Energetic Efficiency of VFA Fermentation and Metabolism Cellulose 10 Glucose (6730 kcal) Starch VFA (5240 kcal 60A 30P 10B Absorbed as glucose (6730 kcal) ATP (1946 kcal) 28.9% ATP (2888 kcal) 42.9% Lower Energy Value of Roughage Compared with Grain - Less digested - Lignin limits digestibility of digestible fiber - Greater energy lost from fermentation CH4 Heat - Increased rumination Rumen contractions Chewing - More bulk in digestive tract Comparative Prices of Corn and Alfalfa Hay NEg Mcal/kg $/ton DM $/Mcal NEg Corn 1.55 121.75 0.0864 Alfalfa hay 0.68 75.00 0.1213 Requirements for Glucose Ruminants 1. Nervous system Energy and source of carbon 2. Fat synthesis NADPH Glycerol 3.Pregnancy Fetal energy requirement 4. Lactation Milk sugar - lactose Sources of Glucose Carbon Ruminants Ruminants dependent on gluconeogenesis for major portion of glucose Sources of glucose in metabolism 1. Propionate 2. Amino acids 3. Lactic acid 4. Glycerol 5. Carbohydrate digestion in intestine Absorption of glucose from intestine Glucose Synthesis Acetate Ketone Acetyl CoA Bodies Fatty Butyrate acids Amino acids Citrate Glycerol Acetyl CoA Lactate CO2 2 CO2 Pyruvate Oxaloacetate PEP Glucose Succinate Proteins Amino acids Propionate Conservation of Glucose Ruminants 1. Glucose not extensively used for synthesis of long-chain fatty acids in adipose of ruminants - Not clear why glucose carbon is not used - Glycerol is needed for synthesis of triglycerides - Comes from glucose - Acetate supplies carbon for fatty acid synthesis 2. Low hexokinase activity in the liver 3. Ruminants have low blood glucose concentrations - Low concentrations of glucose in RBC Consequences of Inadequate Glucose in Metabolism 1. Low blood glucose 2. High blood ketones 3. High blood concentrations of long-chain fatty acids (NEFA) Causes fatty liver and/or ketosis in lactating cows and pregnancy toxemia in pregnant ewes Pregnancy Toxemia Pregnant Ewes • • • • • • • • During the last month of pregnancy Ewes with multiple fetuses Inadequate nutrition of ewe High demands for glucose by fetuses Low blood glucose and insulin Mobilization of body fat Increase in nonesterified fatty acids in blood Increased ketone production by liver Fatty Acid Metabolism Relation to Glucose Diet fat Adipose Diet CHOH CO2 Acetate Malonyl CoA LCFA NEFA Acetate CO2 Glycerol LCFA acyl CoA 2 CO2 Triglycerides Carnitine FA acyl carnitine Malonyl CoA inhibits CO2 (Mitochondria) Ketones Low Blood Glucose and Insulin • Increased release of nonesterified fatty acids from adipose. • Less synthesis of fatty acids Reduced malonyl CoA • Reduced sensitivity of carnitine palmitoyltransferase-1 to malonyl CoA Increased transfer of fatty acids into mitochondria for oxidation • Increased ketone production Fatty Acid Oxidation FA acyl carnitine Carnitine CoA FA acyl CoA Acetyl CoA CO2 Acetoacetyl CoA Acetoacetate (Mitochondria) 3-OH butyrate Low Milk Fat Cows fed high grain diets: Reduced milk fat percentage Early theory Low rumen pH Shift from acetate to propionate production Increased blood insulin Decrease in blood growth hormone More recent theory Increased production of trans fatty acids in the rumen Trans fatty acids reduce milk fat synthesis Long-Chain Fatty Acid Synthesis Ruminants Synthesis is primarily in adipose or mammary gland – Limited synthesis in the liver Ruminants conserve glucose supply – Glucose not extensively used for long chain fatty acid synthesis Most of carbon is supplied by acetate Some butyrate used in mammary gland Glucose metabolism supplies some of NADPH needed for fatty acid synthesis Long-Chain Fatty Acid Synthesis Lactic acid, Propionate, Amino acids Glucose Ruminants limit use of glucose Acetyl-CoA carboxylase Acetyl CoA NADPH Acetate Fatty acids Triglycerides NADP Glycerol-3-P Glu-6-P dehydrogenase Gly-3-P dehydrogenase Glucose Long-Chain Fatty Acid Synthesis Glucose NADPH NADP Pyruvate Malate Fatty acids Malate dehydrogenase Pyruvate NADP Oxaloacetate NADPH Acetyl CoA Oxaloacetate Citrate Mitocondria Acetyl CoA Citrate lyase Citrate Cytosol Acetate Long-Chain Fatty Acid Synthesis Citrate Citrate Isocitrate NADP Isocitrate NADPH dehydrogenase a-Ketoglutarate Mitochondria Cytosol Supplies about half of NADPH for fatty acid synthesis Long-Chain Fatty Acid Synthesis Butyrate • Can be used in mammary gland as primer for synthesis of fatty acids • Shorter chain acids Methylmalonyl (propionate) • Is used as primer for synthesis of fatty acids in sheep fed high-grain diets • Branched-chain acids