Dietary Cysteine Reduces the Methionine Requirement by an Equal Proportion in Both Parenterally and Enterally Fed Piglets

Abstract

The sulfur amino acids (SAA), methionine and cysteine, are normally supplied in a 50:50 ratio in the oral diet of pigs. In contrast, cysteine is not included in any appreciable amounts in parenteral solutions due to its instability in solution. Cysteine can replace part of the methionine requirement, but is not required when methionine is supplied at a level that meets the entire SAA requirement. However, the role of the gut on cysteine sparing has not been investigated. In the present study, the enteral and parenteral methionine requirement was determined, with excess dietary cysteine, by using the indicator amino acid oxidation (IAAO) technique. Piglets [n = 28, 2 d, 1.65 +/- 0.014 kg (SE)] were fed elemental diets containing adequate energy, phenylalanine and excess tyrosine, with varied methionine concentrations and excess cysteine [0.55 g/(kg. d)]. Diets were infused continuously via intravenous (parenteral) or gastric (enteral) catheters. Phenylalanine oxidation was determined during a primed, constant infusion of L-[1-(14)C]-phenylalanine, by measuring expired (14)CO(2) and plasma specific radioactivity (SRA) of phenylalanine. For both the parenteral and enteral groups, phenylalanine oxidation (% of dose) decreased linearly (P < 0.01) as methionine intake increased and then became low and unchanging. Using breakpoint analysis, the methionine requirement was estimated to be 0.25 and 0.18 g/(kg. d) for enteral and parenteral feeding, respectively. These data show that the parenteral methionine requirement is approximately 70% of the enteral requirement when measured in the presence of excess dietary cysteine (P < 0.05). A comparison with our previous studies in which methionine was the only source of sulfur amino acids shows that the addition of dietary cysteine reduces the methionine requirement by approximately 40% in both enterally and parenterally fed neonatal piglets. Therefore, dietary cysteine is equally effective in sparing dietary methionine whether fed enterally or parenterally.

S–carboxyethylcysteine (a constituent of Acacia seed) negatively affects casein protein utilization by rats

“…This is in agreement with the findings of Shoveller et al, [12] who observed a decrease in the concentration of some amino acids including cysteine with increase in methionine intake.…”

“…Numerous studies have also shown that dietary cysteine can replace part of the methionine requirement in orally fed animals. Estimates of this cysteine sparing effect on methionine ranged from 40 -70 % in growing pigs, 50 -55 % in chicks, 17 -90 % in humans and 50 -65 % in rats [12]. Dietary cysteine has also been demonstrated to affect other metabolic pathways such as the control of the expression of 3-phosphoglycerate dehydrogenase in rat liver [31].…”

“…It has also been shown that the addition of dietary cysteine reduced the methionine requirement by approximately 40 % in both enterally and parenterally fed neonatal piglets [12] while methionine has been reported to spare cysteine in the infants supplied a generous amount of dietary methionine [13]. Polat [14] reported significant increase in the weight of tilapia fish fed soy bean meal (SBM) supplemented with 0.5 % methionine but supplementations above this level of methionine were not significantly different from the control diet.…”

Methionine transmethylation and transsulfuration in the piglet gastrointestinal tract

“…The difference in this case was attributed to GIT metabolism, because TPN bypasses the gut and results in gut atrophy. A subsequent study showed that feeding an excess of dietary cystine significantly decreased the methionine requirement consistent with the methionine-sparing effect of cysteine (22). However, even with excess dietary cystine, the methionine requirement in TPN-fed pigs was still Ϸ28% lower than enterally fed pigs, again implying that the GIT represents 28% of body methionine metabolism.…”

“…However, subsequent metabolic studies with stable isotopic tracers also have indirectly suggested that a substantial fraction of the dietary methionine undergoes transmethylation and remethylation during first-pass splanchnic metabolism in adult humans (20,21). More recent studies show that the whole-body methionine requirement and circulating homocysteine concentrations are significantly higher in piglets fed enterally than parenterally, demonstrating that splanchnic methionine metabolism is nutritionally significant (22,23).…”