Characterisation of l-alanine and glycine absorption across the gut of an ancient vertebrate

This study utilised an in vitro technique to characterise absorption of two amino acids across the intestinal epithelium of Pacific hagfish, Eptatretus stoutii. Uptake of l-alanine and glycine conformed to Michaelis–Menten kinetics. An uptake affinity (K _m; substrate concentration required to attain a 50% uptake saturation) of 7.0 mM and an uptake capacity (J _max) of 83 nmol cm⁻² h⁻¹ were described for l-alanine. The K _m and J _max for glycine were 2.2 mM and 11.9 nmol cm⁻² h⁻¹, respectively. Evidence suggested that the pathways of l-alanine and glycine absorption were shared, and sodium dependent. Further analysis indicated that glycine uptake was independent of luminal pH and proline, but a component of uptake was significantly impaired by 100-fold excesses of threonine or asparagine. The presence of a short-term (24 h) exposure to waterborne glycine, similar in nature to that which may be expected to occur when feeding inside an animal carcass, had no significant impact on gastrointestinal glycine uptake. This may indicate a lack of cross talk between absorptive epithelia. These results are the first published data to describe gastrointestinal uptake of an organic nutrient in the oldest extant vertebrate and may provide potential insight into the evolution of nutrient transport systems.