Adsorption of soluble proteins to rumen bacteria and the role of adsorption in proteolysis

In vitro investigation of the ruminal digestion kinetics of different nitrogen fractions of 15N-labelled timothy forage

An in vitro method based on ¹⁵N-labelled forage nitrogen (N) was developed to study ruminal N metabolism of soluble N (SN), insoluble N (ISN) and neutral detergent insoluble N (NDIN) fractions of timothy forage. Timothy grass was grown on replicated experimental plots with one plot receiving ¹⁵N-labelled and the other unlabelled N fertilizer. Harvested grass was preserved as dried grass or as formic acid treated or untreated silage. The intact forages and their corresponding N fractions were incubated in buffered rumen fluid in vitro to determine degradation parameters based on the ¹⁵N fluxes between labelled feed N and ammonia N pools. A high percentage (25–38%) of ¹⁵N-labelled ammonia disappeared from ammonia N pool during the first 15 min of incubation. Microbial uptake of dried grass SN fraction was higher than of silage SN fractions. Fractional degradation rates of SN from formic acid treated silage, untreated silage and dried grass during the first 6 hours of incubation were 0.145, 0.125 and 0.115 /h, respectively. By the end of the incubation period (28 h), 69, 66 and 43%, of the SN fraction of formic acid treated silage, untreated silage and dried grass, respectively were recovered as ammonia. The percentage of ISN fractions degraded to ammonia N were 9, 34 and 27%, respectively. Based on the changes in ¹⁵N-labelled ammonia N pool in blank incubation and appearance of ¹⁵N to ammonia N pool from ¹⁵N-labelled NDIN fractions, it was estimated that a significant portion of microbial lysis occurred when incubations were carried out for longer than 20 hours. With dried grass the contribution of ammonia N for microbial N synthesis was greater than with silages. Use of ¹⁵N-labelled forages together with this in vitro method is a promising technique for determining soluble N degradation parameters, but it requires further development to be used for determining degradation parameters of insoluble N fractions and work with whole feeds.

Ruminal metabolism of grass silage soluble nitrogen fractions

The present study was conducted to investigate ruminal N metabolism in dairy cows using ¹⁵N-labeled N sources and dynamic models. The data summarized in this study were obtained from 2 of 4 treatments whose effects were determined in a 4 × 4 Latin square design. Soluble N (SN) isolated from timothy grass silage labeled with ¹⁵N and ammonia N (AN) labeled with ¹⁵N were administered into the rumen contents of 4 ruminally cannulated dairy cows. Ruminal N pool sizes were determined by manual evacuation of rumen contents. The excess ¹⁵N-atom% was determined in N-fractions of rumen digesta grab samples that were collected frequently between 0 to 72 h and used to determine ¹⁵N metabolism in the rumen. Calculations of area under the curve ratios of ¹⁵N were used to estimate proportions of N fractions originating from precursor N pools. A model including soluble nonammonia N (SNAN), AN, bacterial N, and protozoal N pools was developed to predict observed values of ¹⁵N atomic excess pool sizes. The model described the pool sizes accurately based on small residuals between observed and predicted values. An immediate increase in ¹⁵N enrichment of protozoal N suggests physical attachment of bacteria pool to protozoa pool. The mean proportions of bacterial N, protozoal N, and feed N in rumen solid phase were 0.59, 0.20, and 0.21, respectively. These observations suggest that protozoal N accounted for 0.25 of rumen microbial N. About 0.90 of the initial dose of AN was absorbed or taken up by microbes within 2 h. Faster ¹⁵N enrichment of bacterial N with SN than with AN treatment indicates a rapid adsorption of SNAN to microbial cells. Additionally, the recovery of ¹⁵N as microbial and feed N flow from the rumen was approximately 0.36 greater for SN than for the AN treatment, indicating that SNAN was more efficiently used for microbial growth than AN. The present study indicated that about 0.15 of microbial N flowing to the duodenum was of protozoal origin and that 0.95 of the protozoal N originated from engulfed bacterial N. The kinetic variables indicated that 0.125 of SNAN escaped ruminal degradation, which calls into question the use of in situ estimations of protein degradation to predict the flow of rumen undegradable protein.

Peptidic soil components are a major dietary resource for the humivorous larvae of Pachnoda spp. (Coleoptera: Scarabaeidae)

Humivorous scarab beetle larvae can thrive exclusively on soil organic matter. Feeding experiments have revealed that the larva of Pachnoda ephippiata mineralizes all major humus components except the polyphenolic fraction. High proteolytic activity in the alkaline midgut fluid and an enormous ammonia production during gut passage suggested that peptidic soil components are an important dietary resource for the larva. By comparing acid-hydrolyzable amino acids in food soil and feces, we showed that a significant fraction of the peptides in soil are removed during gut passage. This agrees well with the high concentrations of free amino acids found the midgut section. Incubation experiments revealed the presence of substantial particle-associated proteolytic activity also in the hindgut, most probably due to microbial activity. High rates of ammonia formation in hindgut homogenates and the conversion of radiolabeled amino acids to acetate and propionate indicated that microbial fermentations of soil peptides play an important role in the hindgut. This was corroborated by viable counts of amino-acid-fermenting bacteria, which formed a substantial fraction of the hindgut microbiota. A complete inventory of organic and inorganic nitrogen species before, during, and after gut passage revealed the formation of nitrite and nitrate in midgut and hindgut, and a substantial nitrogen deficit in the feces, suggesting that part of the ammonia formed by mineralization is subjected to oxidation and subsequent denitrification to N2. Together, the results strongly support the hypothesis that peptidic soil components form a major energy and nutrient source for humivorous insects, supplying the animal with microbial fermentation products and essential amino acids.

Relative contributions of bacteria, protozoa, and fungi to in vitro degradation of orchard grass cell walls and their interactions

To assess the relative contributions of microbial groups (bacteria, protozoa, and fungi) in rumen fluids to the overall process of plant cell wall digestion in the rumen, representatives of these groups were selected by physical and chemical treatments of whole rumen fluid and used to construct an artificial rumen ecosystem. Physical treatments involved homogenization, centrifugation, filtration, and heat sterilization. Chemical treatments involved the addition of antibiotics and various chemicals to rumen fluid. To evaluate the potential activity and relative contribution to degradation of cell walls by specific microbial groups, the following fractions were prepared: a positive system (whole ruminal fluid), a bacterial (B) system, a protozoal (P) system, a fungal (F) system, and a negative system (cell-free rumen fluid). To assess the interactions between specific microbial fractions, mixed cultures (B+P, B+F, and P+F systems) were also assigned. Patterns of degradation due to the various treatments resulted in three distinct groups of data based on the degradation rate of cell wall material and on cell wall-degrading enzyme activities. The order of degradation was as follows: positive and F systems > B system > negative and P systems. Therefore, fungal activity was responsible for most of the cell wall degradation. Cell wall degradation by the anaerobic bacterial fraction was significantly less than by the fungal fraction, and the protozoal fraction failed to grow under the conditions used. In general, in the mixed culture systems the coculture systems demonstrated a decrease in cellulolysis compared with that of the monoculture systems. When one microbial fraction was associated with another microbial fraction, two types of results were obtained. The protozoal fraction inhibited cellulolysis of cell wall material by both the bacterial and the fungal fractions, while in the coculture between the bacterial fraction and the fungal fraction a synergistic interaction was detected.

Degradation of protein and utilization of the hydrolytic products by a predominant ruminal bacterium, Prevotella ruminicola B1(4)

Degradation and utilization of protein by Prevotella ruminicola B1(4), a proteolytic bacterium that is prominent in the rumen, was examined. In preliminary experiments, proteinaceous N sources produced faster growth rates than did NH4Cl, based on changes in optical density over time. However, ammonium chloride produced a greater maximum cell density than did proteinaceous N sources. Of the proteinaceous N sources, an enzymatic hydrolysate of soybean protein with a relative peptide size of 3 AA residues produced a greater growth rate and maximum cell density compared with the other proteinaceous N sources. Further experiments revealed that P. ruminicola B1(4) grew faster and to a greater final dry weight with soybean protein than with casein. Degradation of both proteins was low as was indicated by the slow disappearance of soluble protein, low concentrations of free AA and peptides, and the decrease in ammonia concentrations over time. Patterns of degradation did differ between the two proteins, however. Accumulation of peptides and free AA from soybean protein peaked 2 h earlier than those from casein, and concentrations of free AA and peptides from soybean protein were lower on average than those from casein. Prevotella ruminicola B1(4) preferentially utilized Asp, Ile, Leu, Lys, and Arg from soybean protein compared with casein. The relative size of peptides that accumulated from both proteins, as determined by the ratio of ninhydrin reaction after HCl hydrolysis to ninhydrin reaction before HCl hydrolysis, suggested that part of the proteolytic activity of P. ruminicola B1(4) is a dipeptidase. Our findings suggest that P. ruminicola may have a greater impact on peptide degradation than on protein degradation in the rumen.

The effect of condensed tannins in Lotus pedunculatus on the solubilization and degradation of ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39; Rubisco) protein in the rumen and the sites of Rubisco digestion

Three experiments were undertaken to determine the effect of condensed tannin (CT) in Lotus pedunculatus (45-55 g extractable CT/kg DM) on the digestion of the principal leaf protein, ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39; Rubisco; fraction 1 leaf protein). In two of the experiments Lotus pedunculatus was fed to sheep, with one group receiving a continuous intraruminal infusion (per fistulum) of PEG (molecular weight 3500) to bind and inactivate the CT (PEG group). The other group, which did not receive PEG, was termed the control sheep (CT acting). Expt 3 involved in vitro incubations of Lotus pedunculatus in buffered rumen fluid, with and without PEG added. In all experiments the results have been interpreted in terms of the effects of CT on Rubisco solubilization and degradation. Disappearance of N and Rubisco from Lotus pedunculatus suspended in polyester bags in the rumen was used as a measure of solubilization. Degradation was defined as the disappearance of Rubisco from in vitro incubations of Lotus pedunculatus in rumen fluid. In Expt 1, CT reduced the digestion of Rubisco in the rumen from 0.96 to 0.72 of intake (P < 0.01). Rubisco digestion in the small intestine was 0.27 of intake in control sheep and 0.04 of intake in PEG sheep. In Expt 2, PEG had no effect on the loss of Rubisco from Lotus pedunculatus contained in polyester bags which were incubated in the rumen, hence CT did not affect the solubilization of Rubisco. Observations in Expt 1 were confirmed by in vitro incubations in Expt 3, where PEG addition substantially increased the rate of degradation of plant protein to NH3. Addition of PEG decreased the period of time taken to degrade 50% of the Rubisco from about 13.8 h to about 3.0 h. It was concluded that the action of CT reduced the digestion of Rubisco in the rumen of sheep fed on fresh Lotus pedunculatus, and that this was primarily due to the ability of CT to slow its degradation by rumen micro-organisms, without affecting its solubilization. Both fresh-minced, and freeze-dried and ground lotus were used for in sacco and in vitro incubations; however, fresh-minced lotus was more suitable for the evaluation of protein solubilization and degradation in fresh forages.

Effects of Sainfoin ( Onobrychis viciifolia Scop.) Condensed Tannins on Growth and Proteolysis by Four Strains of Ruminal Bacteria

Sainfoin leaf condensed tannins inhibited growth and protease activity in Butyrivibrio fibrisolvens A38 and Streptococcus bovis 45S1 but had little effect on Prevotella ruminicola B 1 4 or Ruminobacter amylophilus WP225. Tannins bound to cell coat polymers in all strains. Morphological changes in B. fibrisolvens and S. bovis implicated the cell wall as a target of tannin toxicity.

Binding and degradation of lectins by components of rumen liquor

The binding of 15 different plant lectins to feed particles and microbes in rumen liquor, and their degradation were studied in vitro. The rate of degradation assessed from the label released when radioactive iodine-labelled lectins were incubated with rumen liquor conflicted with the rates calculated from measurements of the survival of the antigenic structure (immuno-rocket electrophoresis) or the biological function (haemagglutination) of the lectins. Thus solubilization of the radioactive label indicated that Concanavalin A (Con A), but not the soyabean agglutinin, SBA, or kidney bean phytohaemagglutinin, PHA-E3L, was stable to rumen proteolysis. In contrast, both SBA and PHA-E3L were shown by immuno-rocket electrophoresis or haemagglutination tests to be highly resistant to breakdown, while the degradation of Con A proceeded at a constant slow rate under the same conditions. This was in accord with the previously established general stability of lectins in the gut of single-stomach animals. Of the 15 lectins, SBA, favin (Vicia faba lectin) and Con A were bound by hay and the particle fraction of rumen liquor. This was, in part, specific and reversible in the presence of appropriate sugars. Most pure bacterial strains preferentially bound lectins with specificity for glucose/mannose (favin and Con A), while rumen fungi reacted with SBA. The level of binding was low with other lectins. However, inter-strain differences of lectin-binding were found in Selenomonas ruminantium and Ruminococcus flavefaciens. Clearly, as some lectins were not fully degraded in the rumen, they could be expected to depress the utilization of the diet not only in single-stomach animals but, possibly, also in ruminants.

Sites of digestion in beef steers fed bermudagrass hay and supplemented with high-nitrogen feeds alone or mixed with tallow

Five crossbred beef steers (329 kg) were used in a 5 x 5 Latin square experiment with 14-d periods to determine the effects of supplementation with high-nitrogen (N) feeds alone or mixed with tallow on sites of digestion with a basal diet of bermudagrass hay. Hay was 1.93% nitrogen, 75% neutral detergent fibre and fed at 1.83% of body weight (dry matter; DM). Supplements were basal (B; 105 g DM): 81.8% dried molasses product (DMP) and 18.2% calcium carbonate (CC); soybean meal (S; 942 g DM): 88.0% soybean meal, 9.8% DMP and 2.2% CC; S mixed with 9.8% tallow (SF; 1041 g DM); corn gluten and blood meals (CB; 662 g DM): 62.5% corn gluten meal, 20.8% blood meal, 13.6% DMP and 3.0% CC; CB mixed with 13.2% tallow (CBF; 757 g DM). Total N intake was 117, 185, 187, 174 and 172 g/d, and duodenal N flow was 121, 148, 143, 162 and 169 g/d for B, S, SF, CB and CBF, respectively, being lower for B than for other treatments and higher for supplements with the corn gluten and blood meal mix than for soybean meal (P less than 0.05). Duodenal microbial N flow was 39, 51, 49, 38 and 45 g/d for B, S, SF, CB and CBF, respectively, being greater (P less than 0.05) for supplements with soybean meal than with corn gluten and blood meals. Duodenal flow of feed N was greater (P less than 0.05) with than without high-N feeds and for supplemental corn gluten and blood meals than for soybean meal (78, 90, 86, 117 and 116 g/d for B, S, SF, CB and CBF, respectively). In conclusion, mixing of tallow and high-N feeds did not affect the extent of ruminal N disappearance, and soybean meal supplementation increased duodenal N flow less than did supplementation with corn gluten and blood meals. Increased duodenal N flow with soybean meal was associated with about equal elevations of ruminal outflow of microbial and feed N, whereas the corn gluten-blood meal mix affected the intestinal protein supply by increasing ruminal escape of feed protein.

Metabolism of peptides and amino acids during in vitro protein degradation by mixed rumen organisms

In vitro inoculum enriched with particle-associated organisms was prepared using rumen contents from a cow fed a 60% forage, 40% concentrate diet. Treatment of in vitro inoculum with cetyltrimethylammonium bromide was used to release intracellular free amino acids from mixed rumen organisms. Addition of 10 mM tosylarginine methyl ester, a competitive inhibitor of trypsin, decreased degradation rate and intracellular free amino acids in incubations containing either casein or serum albumin. Extracellular peptides increased rapidly to a maximum at 60 min in casein incubations but were not different from zero in albumin incubations. Accumulation of intracellular free amino acids was maximal at 60 min in casein and albumin incubations; the concentration observed with albumin was about one-fourth that with casein. Ammonia production from intact casein was slightly greater than that from acid and enzymatically hydrolyzed casein and about 80% greater than that from albumin. Ammonia production and appearance of extracellular free amino acids lagged behind accumulation of intracellular free amino acids. Results suggest that formation and metabolism of extracellular peptides are important in controlling the rate of protein degradation by mixed rumen organisms.

Significance of microflora in proteolysis in the colon

Protease activities in human ileal effluent and feces were compared by using a variety of native and diazotized protein substrates. In many cases the diazotized proteins had altered susceptibilities to hydrolysis compared with the native proteins. Proteolytic activity was significantly greater than (P less than 0.001) in small intestinal effluent than in feces (319 +/- 45 and 11 +/- 6 mg of azocasein hydrolyzed per h per g, respectively). Moreover, fecal proteolysis was qualitatively different in that ileal effluent did not hydrolyze the highly globular protein bovine serum albumin, whereas all fecal samples tested degraded this substrate. Inhibition experiments provided further evidence that fecal protease activity differed from that in the small intestine. Physical disruption of fecal bacteria released large quantities of proteases, indicating that the lysis of bacteria in the colon may contribute to the extracellular proteolytic activity in feces. Protease inhibition studies with washed fecal bacteria showed that they produced serine, cystine, and metalloproteases, and experiments with synthetic p-nitroanilide substrates indicated that low levels of trypsin- and chymotrypsin-like activities were associated with whole cells. An elastase-like enzyme was bound to the outer membranes of some fecal bacteria.