Absorptive metabolism

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In ruminants, the pattern of AA in the proteins arriving at the duodenum differ substantially from the dietary proteins ingested due to extensive degradation and synthesis in the rumen by bacteria and protozoa (Table 19.2). One might predict that the ability to increase milk protein yield would be a simple function of matching duodenal supply of individual AA with that required for milk synthesis. This can be achieved either by increasing the microbial protein synthesis and rumen outflow or by feeding rumen undegradable proteins that have a balanced AA composition with respect

Table 19.2. Profile (%) of essential amino acids in milk, rumen bacteria and protozoa, and plant and animal feed proteins. (Adapted from NRC, 2001.)

Rumen

Rumen

Maize gluten

Soybean

Amino acid

Milk

bacteria

protozoa

meal

meal

Fishmeal

Arginine

7.2

10.4

9.3

7.1

16.2

13.1

Histidine

5.5

4.2

3.6

4.7

6.1

6.4

Isoleucine

11.4

11.6

12.7

9.1

10.1

9.2

Leucine

19.5

15.9

15.8

37.2

17.2

16.2

Lysine

16.0

16.6

20.6

3.7

13.9

17.2

Methionine

5.5

5.1

4.2

5.2

3.2

6.3

Phenylalanine

10.0

10.1

10.7

14.1

11.6

9.0

Threonine

8.9

11.4

10.5

7.5

8.7

9.4

Tryptophan

3.0

2.7

2.8

1.2

2.8

2.4

Valine

13.0

12.4

9.7

10.3

10.2

10.8

to milk protein. Although simple, this concept implies two major assumptions. First, that the duodenal flow of AA is adequately predicted. The flow of AA to the duodenum is a complex mixture of microbial protein, rumen undegradable feed proteins and endogenous proteins (NRC, 2001). Recent schemes have been refined to provide duodenal flow of essential AA (CNCPS, 2000; NRC, 2001). However, the accuracy of such predictions is critically dependent on knowledge of the chemical composition of dietary ingredients, which is usually not available on a regular basis. The second assumption is that the AA profile at the site of absorption reflects the profile delivered to the mammary gland. As we will demonstrate below, this is almost never the case.

In the first instance, the AA profile of intestinal outflow is altered, even before reaching the blood circulation. During passage across the intestinal wall, AA can be incorporated into intestinal proteins (constitutive or secretions) or catabolized by the tissues. In addition, bacteria in the small intestine can degrade them. Although the gastrointestinal tract comprises only 4-8% of whole body protein mass, protein synthesis by these tissues accounts for 20-35% of whole body protein synthesis in ruminants (Lobley et ai, 1980; Lapierre et ai, 1999). In growing sheep, small intestinal use of luminal-derived essential AA for protein synthesis and catabolism ranges from 17% for valine to 39% for histi-dine (MacRae et al., 1997a). Total use of AA by the whole gut is even greater with more than 80% of essential AA (except phenylalanine and histidine) derived from the blood circulation (MacRae et ai, 1997a). Does this high activity lead to catabolism of AA across the gut wall and reduce net supply of AA to other tissues? For non-essential AA, there are clear indications that catabolism occurs across the gut wall. Stoll et ai (1999) calculated that 70% of the C02 produced by the pig gut was derived from oxidation of AA, primarily from glutamate, glutamine and aspartate. Negative net fluxes across the mesenteric-drained viscera would also indicate substantial catabolism of glutamine in sheep (Gate et ai, 1999), and of aspartate and glutamate in dairy cows (Berthiaume et ai, 2001).

Is there any indication that the gut oxidizes essential AA? Direct measurements of AA catabolism across the ruminant gut are scarce. In ruminants, catabolism of most of the essential AA is believed to be restricted mainly to the liver and kidney, except for the branched-chain AA (isoleucine, leucine and valine) for which the catabolic enzymes are widely distributed in ruminant tissues (Goodwin et ai, 1987). Measurement of AA oxidation by the gut of dairy cows has been limited to leucine, and here 16-24% of absorbed leucine was oxidized (Lapierre et ai, 1999). Preliminary data in growing sheep indicate that gut oxidation of lysine, methionine, and phenylalanine accounted for 29%, 7% and 5%, respectively, of whole body oxidation of these AA (Lobley and Lapierre, 2001). Those estimates were based on measurement of arterial-derived AA and did not include an estimate of oxidation of AA derived from the gut lumen (i.e. during absorption). In the pig, oxidation of systemic (arterial) lysine and threonine are negligible, although there was catabolism of luminal-derived lysine that accounted for 30% of whole-body lysine oxidation (van Goudoever et ai, 2000).

In addition to these direct measurements, some clues to AA metabolism across the gut can be gleaned by interpretation of the ratio of AA disappearing from the small intestine (difference between ileal and duodenal flows) to that appearing from the mesenteric-drained viscera (MDV: blood draining only the small intestine) and from the portal-drained viscera (PDV: draining the whole gut). The gastrointestinal vascular drainage system in ruminants is complex, however, and this may give rise to a confused picture of AA catabolism when the disappearance across the small intestine and mesenteric or portal fluxes are measured in isolation. Only a few studies are available where these have been measured simultaneously (e.g. sheep, MacRae et ai, 1997b; dairy cows, Berthiaume et ai, 2001). In both species, recoveries in the MDV of most essential AA (EAA) disappearing from the small intestines are close to 100% or even greater (Table 19.3). Those data seemed to suggest that catabolism of AA across the intestine wall did not occur. However, substantial secretions from the pancreas and gall bladder (bile) are secreted into the small intestines beyond

Table 19.3. Relative net fluxes of amino acids across the mesenteric-drained viscera (MDV), the portal-drained viscera (PDV) and small intestinal disappearance (SID) in sheep and dairy cows.

Sheep3_ _Dairy cowb

Table 19.3. Relative net fluxes of amino acids across the mesenteric-drained viscera (MDV), the portal-drained viscera (PDV) and small intestinal disappearance (SID) in sheep and dairy cows.

Sheep3_ _Dairy cowb

Amino acid

MDV: SID

PDV: MDV

MDV:SID

PDVMDV

Histidine

1.27

0.75

Isoleucine

1.11

0.55

1.02

0.61

Leucine

1.02

0.64

0.92

0.68

Lysine

1.03

0.56

0.76

0.72

Methionine

-

-

1.01

0.66

Phenylalanine

1.12

0.68

1.00

0.76

Threonine

0.85

0.69

1.15

0.38

Valine

0.76

0.57

1.11

0.46

aFrom MacRae et al. (1997b). bFrom Berthiaume et al. (2001).

aFrom MacRae et al. (1997b). bFrom Berthiaume et al. (2001).

cannula placement and are reabsorbed before reaching the ileum. This will not affect net apparent disappearance in the small intestine, but as the AA necessary for this purpose are partly extracted from the non-MDV blood supply, these endogenous secretions will add 'extra' AA to the MDV net flux. Therefore, the high recovery of AA into the MDV relative to small intestinal disappearance does not suggest, but would not preclude, any catabolism of AA across the small intestine.

Further information can be gained from the ratio of AA flows at the PDV (a mix of MDV flow and those from the forestomachs and hind gut) compared to the MDV (only the small intestines). The net flows of AA from the PDV average only 55-75% of MDV flows (Table 19.3; Seal and Parker, 1996; MacRae et al., 1997b; Berthiaume et al., 2001). There are two possible explanations for this apparent loss of EAA from the MDV to the PDV. The first involves catabolism of EAA by the digestive tissues, excluding the small intestines, and the second involves a complex loop of endogenous protein secretion and reabsorption. The limited data seem to suggest that most EAA, with the exception of the branched-chained AA and perhaps lysine (Lobley and Lapierre, 2001), are not oxidized across the ruminant gut. On the other hand, endogenous secretions measured at the duodenum, originating from saliva, gastric juices and sloughed off epithelial cells (Tamminga et al., 1995), represent 15-25% of duodenal nitrogen flow (Larsen et al., 2000; Ouellet et al., 2002). These secretions, therefore, could account for a large fraction of the 'apparent loss' of AA (25-45%) between the PDV and MDV net fluxes.

Does this flow of endogenous protein into the gut lumen result in preferential net use (loss) of specific AA? When the relative disappearance of essential AA across the small intestines was compared with PDV appearance in dairy cows, there were proportionally greater losses of valine and threonine (Berthiaume et al., 2001). Similarly, numerical decreases have been observed for threonine and valine in sheep (Remond et al., 2000). In cattle fed grass pellets, threonine represented a lower proportion (relative to leucine) in both the MDV and the PDV net appearances when compared to rumen microbial protein (Seal and Parker, 1996). Intestinal secretions are dominated by mucopolysaccharides, which are rich in threonine and valine (Mukkur et al., 1985). Mucins tend to be poorly reabsorbed (Tamminga et al., 1995), thus further distorting the pattern of essential AA available to the animal.

It is clear that gut metabolism has a significant impact on the availability of nonessential AA to the animal. The situation is less clear for essential AA, although catabolism of certain AA (including branched-chain AA) cannot be ignored nor the impact of endogenous secretions on the pattern of AA supplied into blood circulation.

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