Protein Metabolism II ANS 520

Fate of Rumen Ammonia 1. Bacterial protein synthesis 2. Absorbed from reticulorumen and omasum NH3 passes from rumen by diffusion into portal blood. (High concentration to low) Form of ammonia dependent on pH of rumen NH3 + H+ NH4+ Less absorption at more acid pH 3. At pH of rumen, no NH3 lost as gas

Fate of Absorbed Ammonia 1. Transported to liver by portal vein 2. Converted to urea via urea cycle in liver NH3 Urea Urea cycle 3. Urea released into blood 4. If capacity of urea cycle in liver is exceeded Ammonia toxicity Over consumption of urea

Fate of Blood Urea 1. Excreted into urine 2. Recycled to digestive tract, g N/d Saliva – Related to concentration of urea in blood Sheep: 0.5 to 1.0 Cattle: 1.0 to 7.6 Diffusion into GIT Sheep: 2 to 5 Cattle: 25 to 40

Urea Diffusion into Rumen Rumen wall Blood urea Urea NH3 Bacterial population Total N transferred is greater when high N diets are fed. 2. Percentage of diet N transferred is greater when low N diet are fed

Urea Diffusion into Rumen Rumen wall Urea transporter Blood urea Urea High [NH3] inhibits NH3 Bacterial population

Adjustments to Low Protein Intake Kidney Blood urea Urea Urine urea Urea is predominant form of N in urine Reabsorption of urea by kidney increased when ruminants fed low N diets Conserves nitrogen in the body Greater portion recycled to digestive tract Sheep fed the same diet tend to reabsorb more urea than cattle

Nitrogen Recycling - Cattle Marini et al. JAS 2003

Sources of Nitrogen Recycled to GIT Urea flowing back into digestive tract Rumen Saliva Diffusion from blood Lower digestive tract (large intestine, colon, cecum) Endogenous protein secretions into GIT Mucins Enzymes Sloughing of tissue Turnover of microbial cells in rumen & reticulum

Significance of Recycled Nitrogen Source of N for microbes when protein consumption is limited Wild species Protein intake during winter is very low Rumen deficient of nitrogen for microbial activity Slowly degraded feed proteins Recycling provides nitrogen for microbial growth Infrequent feeding of supplemental protein Programs to reduce supplemental nitrogen Difficult to make ruminants severely protein deficient

Urea Nitrogen - Cattle Marini et al. JAS 2003

Amino Acid Synthesis Ammonia Fixation 1. Glutamine synthetase/glutamate synthase Glutamine synthetase Glu + NH3 + ATP Gln Glutmate synthase -ketoglutarate + glutamine + NADPH2 2 Glu High affinity for NH3 - Concentrates NH3 in cells – Uses ATP Because of N recycling this reaction may not be that important

Amino Acid Synthesis Ammonia Fixation 2. Glutamic dehydrogenase -ketoglutarate + NH3 + NADH Glu Low affinity for NH3 – High concentration of enzyme in rumen bacteria – Does not use ATP Probably predominant pathway 3. Other AA can be synthesized by transamination reactions with glutamic acid Estimates of NH3 requirements range from 5 (culture) to 20 mg/100 ml (in situ digestion)

Sources of Amino Acids for Host Animal 1. Microbial proteins Quantity determined by: Fermentability of the feed Quantity of feed consumed c) Nitrogen available to microorganisms 2. Undegraded feed proteins (UIP) Quantity will vary in relation to: a) Degradability of feed proteins b) Quantity of feed proteins consumed

Nutritional Value of Microbial Proteins 1996 NRC for Beef Microbial protein 80% digestible in the intestine UIP 80% digestible in the intestine 2001 NRC for Dairy and Level 1 CNCPS Digestibility of RUP (UIP) is variable in Dairy NRC UIP 80% digestible in Level 1 CNCPS

Metabolizable Protein Model Tissue proteins NH3 Blood urea Urine Amino acid pools Energy NH3 Metabolizable Microbial protein protein Protein Protein from diet Rumen Intestine Feces A B C

Protein Metabolism of Ruminants Concept of Metabolizable Protein Metabolizable protein (MP) = Absorbed amino acids or = Digestible fraction of microbial proteins + digestible fraction of undegraded feed proteins Digestible protein (amino acids) available for metabolism Concept is similar to Metabolizable energy

Less Extensively Degraded Protein Protein Metabolism in the Rumen Less Extensively Degraded Protein Feed Rumen Intestine Microbes Digestion Metabolizable protein Undegraded feed

Extensively Degraded Protein Protein Metabolism in the Rumen Extensively Degraded Protein Feed Rumen Intestine Microbes Digestion NH3 Metabolizable protein Undegraded feed

Metabolizable Protein Supply to Host Animal Metabolizable protein (MP): Microorganisms – Digestible proteins Undegraded feed proteins – Digestible proteins Microorganisms g/d = 0.13 (TDN intake, g/d) (0.8) (0.8) Microbes 80% true protein that is 80% digested Feed g/d = (Feed protein) (Portion undegraded) (0.8) Feed proteins 80% digested

Absorption of Amino Acids Amino acids and small peptides absorbed by active transport (specific for groups of AA) From intestines Portal blood Transport of amino acids into cells is similar process From blood Cells Active transport, requires energy

Utilization of Absorbed Amino Acids Via portal vein to liver Used for synthesis of proteins in liver Metabolized (deaminated) - Used for energy – Carbon for glucose Escape the liver Carried by blood to body tissues Used for synthesis of tissue proteins, milk, fetal growth, wool Metabolized - Used for energy

Requirements for Absorbed Amino Acids Metabolizable Protein (MP) Protein (amino acid) requirements Maintenance Growth Lactation Pregnancy Wool

Protein Metabolism Concept of Net Protein Net protein = protein gained in tissues, milk, or fetal growth = NP Metabolizable protein is used with less than 100% efficiency Net protein = (MP - Metabolic loss) As a quantity, net protein is less than metabolizable protein

Net Protein Required for Production Amino Acids Proteins Milk kg/d = (Milk yield, kg/d) (% protein in milk) Growth g/d = SWG (268 - (29.4 (RE/SWG))) SWG = Shrunk weight gain, kg/d RE = Retained energy, Mcal/d RE obtained from net energy equations.

Protein Metabolism Metabolic Loss Protein synthesis and metabolism of amino acids draw from the same pool Proteins Amino acids Metabolism Metabolic loss results from continuous catabolism from amino acid pools Continuous turnover of tissue proteins adds to amino acid pools in tissues

Amino Acid (MP) Requirements Maintenance (three fractions) Protein required to support zero gain or production 1. Metabolism Metabolized Urine Milk Amino acids Feces Wool (Synthesis) GIT Scurf (Degradation) Pregnancy Tissue proteins = Endogenous urinary N 2. Proteins lost from body surface (hair, skin, secretions) = Scurf proteins 3. Proteins lost from undigested digestive secretions and fecal bacteria = Metabolic fecal N