Methionine transmethylation and transsulfuration in the piglet gastrointestinal tract

Abstract

Methionine is an indispensable sulfur amino acid that functions as a key precursor for the synthesis of homocysteine and cysteine. Studies in adult humans suggest that splanchnic tissues convert dietary methionine to homocysteine and cysteine by means of transmethylation and transsulfuration, respectively. Studies in piglets show that significant metabolism of dietary indispensable amino acids occurs in the gastrointestinal tissues (GIT), yet the metabolic fate of methionine in GIT is unknown. We show here that 20% of the dietary methionine intake is metabolized by the GIT in piglets implanted with portal and arterial catheters and fed milk formula. Based on analyses from intraduodenal and intravenous infusions of [1-(13)C]methionine and [(2)H(3)]methionine, we found that the whole-body methionine transmethylation and remethylation rates were significantly higher during duodenal than intravenous tracer infusion. First-pass splanchnic metabolism accounted for 18% and 43% of the whole-body transmethylation and remethylation, respectively. Significant transmethylation and transsulfuration was demonstrated in the GIT, representing approximately 27% and approximately 23% of whole-body fluxes, respectively. The methionine used by the GIT was metabolized into homocysteine (31%), CO(2) (40%), or tissue protein (29%). Cystathionine beta-synthase mRNA and activity was present in multiple GITs, including intestinal epithelial cells, but was significantly lower than liver. We conclude that the GIT consumes 20% of the dietary methionine and is a significant site of net homocysteine production. Moreover, the GITs represent a significant site of whole-body transmethylation and transsulfuration, and these two pathways account for a majority of methionine used by the GITs.

Fig. 1.

Time course of mean plasma tracer enrichments during IV (Upper) and ID (Lower) infusion of [1-¹³C]methionine (filled circles) and [methyl-²H₃]methionine (open circles). Enrichments expressed as mole percent excess (MPE). Means ± SE (n = 16).

Fig. 2.

Schematic representation of the metabolic fate of enteral and systemic methionine kinetics in the GIT (n = 16). Dietary methionine intake (A); arterial methionine flux through the GIT (B); portal methionine outflow (C); unidirectional, first-pass uptake of dietary methionine by the GIT (D); dietary methionine not metabolized by the GIT in first pass (E); unidirectional uptake of arterial methionine by the GIT (F); arterial methionine not metabolized by the GIT (G); recycled methionine derived from proteolysis (H); and methionine that is converted to homocysteine (I), CO₂ (J), and protein (K) by the GIT. Rates expressed as μmol·kg⁻¹·h⁻¹. Dietary intake includes [1-¹³C] and [methyl-²H₃]methionine tracers. Detailed explanation of equations and assumptions used for calculating the methionine kinetic in the GIT are included in supporting information (SI) Appendix.

Fig. 3.

The absolute rates and relative proportions of dietary methionine intake, transmethylation, and transsulfuration in the whole body (filled bars) and GIT (open bars). Rates expressed as μmol·kg⁻¹·h⁻¹. Percentages above filled bars represent whole-body rates relative to dietary methionine intake, whereas those above open bars represent GIT rates relative to the respective whole-body rates. Means ± SE (n = 16).

Fig. 4.

CBS mRNA abundance and enzymatic activity in liver and selected GITs. (A) The relative amounts of CBS mRNA product derived from 100 ng of RNA of representative tissue samples from liver (L), jejunum (J), stomach (St), pancreas (P), and intestinal epithelial cells (IEC) after RT-PCR and separation on agarose gel. (B) Abundance of CBS mRNA expressed relative to 18S RNA in liver, jejunum, stomach, pancreas, and IEC (n = 4). (C) CBS enzymatic activity determined in liver, jejunum, stomach, pancreas, and IEC (n = 4). (D) Relative changes in CBS enzymatic activity determined in liver tissue and intestinal epithelial cells in the absence and presence of 1.0 mM SAM (n = 3). Means ± SEM.