Amino acid utilization and isotope discrimination of amino nitrogen in nitrogen metabolism of rat liverin vivo

Abstract

Urea and plasma protein differ in natural 15N abundance up to 10%. The origin of this difference is the branched nitrogen metabolism in the liver. One main branch is the protein synthesis pathway, the other the urea synthesis pathway. By this branching 15N of precursor amino acids is depleted in urea while it is enriched in protein. With the 15N abundance of precursor amino acids, which may be taken from jejunum tissue, utilization of amino acids in liver metabolism can be calculated from isotope discrimination in either pathway. This was investigated by feeding different proteins to rats. When feeding high quality protein (whey protein) utilization of amino acids in liver metabolism at requirement intake was better than at zero protein intake (> 85% vs. 70%). From this we conclude that the pattern of amino acids available from the metabolic pool at zero protein intake is characterized by an imbalance. This endogenous imbalance can be complemented by exogenous dietary amino acids so that nitrogen excretion may even be smaller than the so-called "obligatory" losses of intakes not exceeding requirement. Thus, the quality of dietary protein is reflected not only by N balance. It also may be quantified by analysis of isotope discrimination in nitrogen metabolism of the liver. In addition, the quality of amino acid pattern available from the metabolic pool is indicated by this method.

The effect of dietary water soluble carbohydrate to nitrogen ratio on nitrogen partitioning and isotopic fractionation of lactating goats offered a high-nitrogen diet

“…The current study provided little support for relationship between N partitioning and N isotopic fractionation, in contrast to the report of Sick et al (1997) with rats. Several possible causes exist for the absence of the expected relationship.…”

“…This is consistent with findings from Sponheimer et al (2003), who suggested that the main reason for the enrichment of 15 N in faeces is the presence of enriched endogenous material. The majority of N in milk and plasma exists as true protein, which has been reported to be enriched in 15 N (Sick et al, 1997;Cheng et al, 2010). In contrast, urea is the main N source of urine and is reported to be depleted in 15 N (Steele and Daniel, 1978).…”

Relationship between efficiency of nitrogen utilization and isotopic nitrogen fractionation in dairy cows: contribution of digestion v. metabolism?

“…Another possible explanation is that rumen available N only takes into account degradable feed N and not urea-N recycled to the rumen, which has been demonstrated to be affected by dietary CP level (Sarraseca et al, 1998). As plasma urea (Sutoh et al, 1993;Sick et al, 1997) and therefore urine (Cheng et al, 2011;Cantalapiedra-Hijar et al, 2015) are consistently depleted in 15 N relative to diet, urea recycled to the rumen and taken up by rumen bacteria would favor both 15 N depletion of rumen bacteria cells and rumen N use efficiency. Hence, the positive relationship between Δ 15 N of rumen bacteria and MNE found in this experiment (Figure 3a) could also be explained by the link between endogenous urea-N capture by rumen microorganisms (resulting in 15 N depletion of rumen bacteria) and the AA flowing to the duodenum (Al-Dehneh et al, 1997).…”

“…This is in line with previous findings using indirect approaches in dairy cows (Cantalapiedra-Hijar et al, 2015) fed diets similar to those of this experiment. Previous studies found that the liver is an important site for isotopic N fractionation (Sick et al, 1997;Cantalapiedra-Hijar et al, 2015), because hepatic enzymes involved in transamination and deamination have been shown to react preferentially with AA containing 14 N over 15 N (Macko et al, 1986;Weiss et al, 1988). As the measured metabolic efficiency of AA utilization (milk CP yield/apparently digested AA in the small intestine) was quite variable in our study (CV = 40%), a large variation in liver AA catabolism and thus in AA transamination/deamination Isotopic nitrogen fractionation in ruminants was expected to cause differences in isotopic N fractionation.…”

“…As of its role in AA catabolism, the liver has been proposed as a key organ contributing to this isotopic N fractionation (Sick et al, 1997;Cantalapiedra-Hijar et al, 2015). In addition, previous findings in ruminants suggest an important role of rumen bacteria in the natural 15 N enrichment of animal proteins relative to diet (Δ 15 N animal-diet ; Sutoh et al, 1987 and1993;Wattiaux and Reed, 1995).…”