Quantification of circulating peptides and assessment of peptide uptake across the gastrointestinal tract of sheep

Regulation of Milk Protein Synthesis by Free and Peptide-Bound Amino Acids in Dairy Cows

Milk protein (MP) synthesis in the mammary gland of dairy cows is a complex biological process. As the substrates for protein synthesis, amino acids (AAs) are the most important nutrients for milk synthesis. Free AAs (FAAs) are the main precursors of MP synthesis, and their supplies are supplemented by peptide-bound AAs (PBAAs) in the blood. Utilization of AAs in the mammary gland of dairy cows has attracted the great interest of researchers because of the goal of increasing MP yield. Supplying sufficient and balanced AAs is critical to improve MP concentration and yield in dairy cows. Great progress has been made in understanding limiting AAs and their requirements for MP synthesis in dairy cows. This review focuses on the effects of FAA and PBAA supply on MP synthesis and their underlying mechanisms. Advances in our knowledge in the field can help us to develop more accurate models to predict dietary protein requirements for dairy cows MP synthesis, which will ultimately improve the nitrogen utilization efficiency and lactation performance of dairy cows.

Omasal Flow of Soluble Proteins, Peptides, and Free Amino Acids in Dairy Cows Fed Diets Supplemented with Proteins of Varying Ruminal Degradabilities

Three ruminally and duodenally cannulated cows were assigned to an incomplete 4 x 4 Latin square with four 14-d periods and were fed diets supplemented with urea, solvent soybean meal, xylose-treated soybean meal (XSBM), or corn gluten meal to study the effects of crude protein source on omasal canal flows of soluble AA. Soluble AA in omasal digesta were fractionated by ultrafiltration into soluble proteins greater than 10 kDa (10K), oligopeptides between 3 and 10 kDa (3-10K), peptides smaller than 3 kDa (small peptides), and free AA (FAA). Omasal flow of total soluble AA ranged from 254 to 377 g/d and accounted for 9.2 to 15.9% of total AA flow. Averaged across diets, flows of AA in 10K, 3-10K, small peptides, and FAA were 29, 217, 50, and 5 g/d, respectively, and accounted for 10.3, 71.0, 17.5, and 1.6% of the total soluble AA flow. Cows with diets supplemented with solvent soybean meal had higher flows of Met, Val, and total AA associated with small peptides than those whose diets were supplemented with XSBM, whereas supplementation with corn gluten meal resulted in higher total small peptide-AA flows than did XSBM. Averaged across diets, 27, 75, and 93% of soluble AA in 10K, 3-10K, and peptides plus FAA flowing out of the rumen were of dietary origin. On average, 10% of the total AA flow from the rumen was soluble AA from dietary origin, indicating a substantial escape of dietary soluble N from ruminal degradation. Omasal concentrations and flows of soluble small peptides isolated by ultrafiltration were substantially smaller than most published ruminal small peptide concentrations and outflows measured in acid-deproteinized supernatants of digesta.

Effects of intraruminal propionate supplementation on nitrogen utilisation by the portal-drained viscera, the liver and the hindlimb in lambs fed frozen rye grass

The influence of propionate supplementation on the splanchnic metabolism of amino acids (AA) and other N compounds (urea-N and NH3-N) and the supply of AA and NH3-N to the hindlimb was investigated in growing lambs. Six rumen-cannulated and multicatheterised lambs (32·2kg) were fed frozen rye grass at 690kJ metabolisable energy intake/d per kg average metabolic body weight. They were infused intraruminally with a salt solution (control) or with propionate solutions at 0·23mol/l (P1) or 0·41mol/l (P2) infused at a maximal rate of 1·68 (sd 0·057) ml/min according to a repeated Latin square design. The propionate infusion did not increase the net portal appearance of total AA (TAA)-N but increased that of some branched-chain AA (valine and to a lesser extent isoleucine). Simultaneously, the propionate treatment (especially P2) induced an increased TAA utilisation by the liver. This was due mainly to an increased (+79%;P<0·07) utilisation of the essential AA and particularly the branched-chain AA. A stimulation of protein synthesis in the liver is hypothesised since (1) propionate stimulated insulin secretion and (2) utilisation of non-essential AA were less influenced by the propionate treatment in the liver (except for alanine), suggesting that the AA utilised by the liver were directed towards protein synthesis rather than towards oxidation or urea synthesis. At the splanchnic level, the propionate treatment did not have any effect on the TAA, non-essential AA and essential AA, except for a net splanchnic release that was decreased for leucine (P<0·02) and methionine (P<0·01) and increased for threonine (P<0·05). The propionate treatment did not have any effect on the hindlimb uptake of AA (essential and non-essential). As a consequence, even though the propionate treatment induced some major alterations in the splanchnic metabolism of AA, there were no changes in the net AA balance in the hindlimb (and hence probably on muscle growth). The role of the splanchnic tissues in the regulation of the AA supply to the peripheral tissues (such as muscle) therefore appears to be prominent in the regulation of muscle growth. Whether the peripheral tissues regulate their own supply by interacting with the splanchnic tissues (and especially the liver) or the liver is the only regulator of the AA supply to the muscle remains in doubt.

Aminopeptidase N Gene Expression and Abundance in Caprine Mammary Gland is Influenced by Circulating Plasma Peptide

This study examined the localization and the effect of circulating peptides on the expression of aminopeptidase N (EC 3.4.11.2) in caprine mammary gland. Four lactating goats in mid to late lactation were used in a crossover design and were subjected to 2 dietary treatments. Abomasal infusion of casein hydrolysate was used to increase the concentration of peptide-bound amino acid in the circulation. Samples of mammary gland tissue from each goat were taken by biopsy at the end of each treatment period to measure gene and protein expression of aminopeptidase N in the tissue. There were no measurable effects on feed intake and milk production for any of the treatments. Western blot analysis showed that aminopeptidase N is located on the basolateral side of parenchymal cells and not on the apical membranes. Abomasal infusion of casein hydrolysate caused a marked change in the profile of arterial blood free amino acids and peptide-bound amino acids smaller than 1500 Da. Abundance of aminopeptidase N mRNA and protein increased by 51 and 58%, respectively, in casein hydrolysate-infused goats compared with the control treatment. It was concluded that aminopeptidase N is one candidate actively involved in the mammary gland to support protein synthesis and milk production. In accordance with the nutritional conditions in the current experiment, it is suggested that aminopeptidase N expression is partly controlled by the metabolic requirements of the gland and postabsorptive forms of amino acids in the circulation.

Effect of the methodology on peptide amino acid concentrations in blood and plasma of sheep

Different methodologies for the measurement of peptide amino acid (PAA) in blood and plasma were compared in sheep. Preparation of blood and plasma samples consisted of a deproteinization, either chemical with sulfosalicylic acid (0.04 g for 1 ml of sample) or physical by ultrafiltration (10,000-MW cut-off filters), with or without a subsequent ultrafiltration through a 3,000-MW cut-off filter. Peptide concentrations were determined by quantification of amino acid concentrations before and after acid hydrolysis of samples. Free amino acid concentrations were similar by all the method used (about 2.5 and 2.7 mM, for blood and plasma respectively). Peptide concentrations were higher with chemical deproteinization (10.6 and 4.2 mM, for blood and plasma respectively) than with physical deproteinization (5.7 and 3.3 mM, for blood and plasma respectively). When the deproteinized samples were further treated to remove material of molecular weight above than 3 kDa, peptide concentrations were significantly reduced, which indicates inefficiencies in the ability of the deproteinizing procedures in removing all the proteinaceous materials. Concentration of small PAA (< 3 kDa) in blood was about 1.5-fold that in plasma, mainly due to peptide Gly and Glu derived from the hydrolysis of the erythrocyte glutathione. The choice of a methodology for quantifying circulating peptides is discussed.

Effect of the methodology on circulating peptides determination and consequences on net flux measurements across the gastrointestinal tract of sheep

Two methodologies for the measurement of peptide amino acids (PAA) in blood were compared to evaluate their effects on the measurement of the net flux of peptides across the gastrointestinal tract of sheep. These methods consisted of a chemical deproteinization of blood samples with sulfosalicylic acid (1.6 M, 0.1 ml for 1 ml of sample) or perchloric acid (1 M, 1 ml for 1 ml of sample) followed by ultrafiltration through a 3,000-Da cut-off filter (SSA + UF3 kD) or gel filtration through a Sephadex G-15 column (1,500-Da cut-off filter; PCA + G-15), respectively, prior to PAA analysis. Peptide concentrations as determined by amino acid concentrations before and after hydrolysis of samples were slightly greater with the SSA + UF3 kD (991 microM) than with the PCA + G-15 (605 microM) methodology. However, both methodologies gave similar net portal-drained viscera flux data in sheep fed on alfalfa pellets with histidine as the only significant uptake of peptide amino acid.

Vascular sources of phenylalanine, tyrosine, lysine, and methionine for casein synthesis in lactating goats

The contribution to casein biosynthesis of peptides derived from blood was examined in late lactation goats (254 to 295 d in milk). Ratios of mammary uptake of free amino acids (AA) in blood to output of AA in milk protein and ratios of the enrichments of Phe, Tyr, Met, and Lys at isotopic plateau in secreted milk casein to the free AA in arterial and mammary vein blood were monitored during the last 5 h of a 30-h continuous i.v. infusion of [1-13C]Phe, [2H4]Tyr, [5-13CH3]Met, and [2-15N]Lys on two occasions: before (control) and on d 6 of an i.v. infusion of Phe (6 g/d). During the control, uptakes of free Phe and Met were less than their output in milk. This result was comparable with the labeling kinetic results, suggesting that vascular peptides contributed 5 to 11% of Phe and 8 to 18% of Met. Free Tyr and Lys uptakes during the control were sufficient for milk output; however, the labeling kinetics indicated that 13 to 25% of the Tyr and 4 to 13% of the Lys were derived from peptides. Infusion of Phe increased the uptake of free AA but reduced the contribution of peptides toward Phe (0 to 3%) and Tyr (8 to 14%) supply for casein synthesis. Whole body hydroxylation of Phe to Tyr increased from 10 to 18% with the infusion of Phe; within the mammary gland, this conversion was lower (3 to 5%). Results suggest that the mammary utilization of peptides containing Phe and Tyr appears to depend on the supply of free AA in blood.

Current concepts of amino acid and protein metabolism in the mammary gland of the lactating ruminant

Milk protein responses to protein nutrition are typically poor and, in part, may be due to the low efficiency (approximately 25 to 30%) of converting dietary N into milk. Posthepatic availability of amino acids (AA) is not limited, yet only approximately 30% is converted into milk. The poor capture of AA by the mammary gland may relate to the imbalanced and uncoordinated timing of nutrient delivery to the gland. The infusion of essential AA improves the efficiency of utilization (0.31); however, further catabolism of AA within the mammary gland suggests that AA transport is not a major limitation. These losses may serve ancillary or functional roles, but mammary oxidation of some AA occurs only when AA extraction exceeds the stoichiometric requirements for milk protein synthesis. Intracellular substrate supply may be more limiting than is the appartus for protein synthesis. Studies utilizing isotope labeling and conducted in vitro and in vivo now suggest that circulating peptides and proteins can serve as sources of perhaps all AA for casein synthesis, but the source of these remains elusive. Constitutive protein and casein turnover contribute significantly (42 to 72%) to mammary protein synthesis. All AA are extensively channeled through an intermediary protein pool or pools that have rapid turnover rates. The AA are then incorporated into casein, which appears to be fixed in association with protein turnover. The mammary gland is a major controller of its metabolism, and the mechanisms of AA extraction and conversion into milk protein are linked to secretion events. Blood flow may be a key point of regulation whereby mechanisms sense and respond to nutrient supply and balance to the gland via alterations in hemodynamics.