Sulfur amino acid deficiency upregulates intestinal methionine cycle activity and suppresses epithelial growth in neonatal pigs

Abstract

We recently showed that the developing gut is a significant site of methionine transmethylation to homocysteine and transsulfuration to cysteine. We hypothesized that sulfur amino acid (SAA) deficiency would preferentially reduce mucosal growth and antioxidant function in neonatal pigs. Neonatal pigs were enterally fed a control or an SAA-free diet for 7 days, and then whole body methionine and cysteine kinetics were measured using an intravenous infusion of [1-(13)C;methyl-(2)H(3)]methionine and [(15)N]cysteine. Body weight gain and plasma methionine, cysteine, homocysteine, and taurine and total erythrocyte glutathione concentrations were markedly decreased (-46% to -85%) in SAA-free compared with control pigs. Whole body methionine and cysteine fluxes were reduced, yet methionine utilization for protein synthesis and methionine remethylation were relatively preserved at the expense of methionine transsulfuration, in response to SAA deficiency. Intestinal tissue concentrations of methionine and cysteine were markedly reduced and hepatic levels were maintained in SAA-free compared with control pigs. SAA deficiency increased the activity of methionine metabolic enzymes, i.e., methionine adenosyltransferase, methionine synthase, and cystathionine beta-synthase, and S-adenosylmethionine concentration in the jejunum, whereas methionine synthase activity increased and S-adenosylmethionine level decreased in the liver. Small intestine weight and protein and DNA mass were lower, whereas liver weight and DNA mass were unchanged, in SAA-free compared with control pigs. Dietary SAA deficiency induced small intestinal villus atrophy, lower goblet cell numbers, and Ki-67-positive proliferative crypt cells in association with lower tissue glutathione, especially in the jejunum. We conclude that SAA deficiency upregulates intestinal methionine cycle activity and suppresses epithelial growth in neonatal pigs.

Fig. 1.

Tracer infusion and blood sampling protocol.

Fig. 2.

Redox potential values (E_h) from cysteine-cystine (Cys/CySS) couple in plasma (A) and tissue (jejunum, ileum, and liver; C) and plasma cysteine concentration (B) in piglets continuously enterally fed a control or a sulfur amino acid (SAA)-free diet for 7 days. Values are means ± SE of 9 control and 7 SAA-free animals. **P < 0.01; ***P < 0.001 vs. control.

Fig. 3.

Intestinal cross sections stained with hematoxylin and eosin (A) and morphological parameters (B–G) of jejunum, ileum, and colon of piglets continuously enterally fed a control or an SAA-free diet for 7 days. Values are means ± SE of 9 control and 7 SAA-free animals. *P < 0.05; **P < 0.01; ***P < 0.001 vs. control.

Fig. 4.

A: intestinal cross sections immunostained for Ki-67. Magnification ×100. B: proliferative cell distribution (i.e., Ki-67-positive cells) in crypts of jejunum, ileum, and colon of piglets continuously enterally fed a control or an SAA-free diet for 7 days. Values are means ± SE of 9 control and 7 SAA-free animals. *P < 0.05 vs. control.

Fig. 5.

Enzyme activity of methionine adenosyltransferase (MAT; A), methionine synthase (MS; B), and cystathionine β-synthase (CBS; C) in jejunum, ileum, and liver of piglets continuously enterally fed a control or an SAA-free diet for 7 days. Values are means ± SE of 9 control and 7 SAA-free animals. *P < 0.05; **P < 0.01; ***P < 0.001 vs. control.