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Kinetics of Splanchnic 13C‐Cysteine Metabolism in Infant Pigs

Cysteine is a conditionally essential amino acid in neonates that can be synthesized from methionine by transsulfuration. We previously showed that methionine transsulfuration occurs in the GI tract of young pigs. Cysteine use by the gut epithelial cells may be important for maintenance of glutathione synthesis and cellular redox function. Our aim was to quantify the extent of gastrointestinal first‐pass cysteine metabolism in young pigs. Four‐week‐old, formula fed piglets (n=10) were given an intragastric (IG) and intravenous (IV) [1‐13C]‐cysteine infusion (IR = 15 μmol·kg−1·h−1) on two separate days in a cross‐over design. Arterial isotopic enrichments of cysteine and CO2 were measured by GC/IR‐MS and whole body fluxes are expressed as μmol·kg−1·h−1. Whole body cysteine flux (WB) was higher (P<0.01) during the IG than IV infusion (267 vs 154). Fractional (%IR) 13C‐Cys oxidation was similar during IG vs IV infusion (38.5 vs 36.2%). First‐pass fractional splanchnic extraction was 39 ± 7% of enteral 13C‐Cys intake, of which 17% was oxidized to 13CO2. The mean dietary Cys absorption during both infusion modes was similar, representing 80% of dietary intake, indicating that gut utilization was 20% of intake. We conclude that despite the higher whole body flux with enteral versus IV infusion, the fractional 13C‐Cys oxidation is similar. Gut utilization consumes 20% of the dietary Cys intake and represents half of first‐pass splanchnic use.