Competition between selenomethionine and methionine absorption in the intestinal tract of green sturgeon (Acipenser medirostris)

Abstract

l-Selenomethionine (SeMet) is a dominant form of selenium (Se) found in organisms at all levels of aquatic food chains and a key source of Se bioaccumulation and ecotoxicity. In mammals, intestinal absorption of SeMet is at least partly via the Na(+)-dependent neutral amino acid transporter. The mechanism of SeMet absorption and competitive effects of other dietary components on SeMet absorption in fish are unknown. Thus the in vitro uptake rates of l-methionine (Met) and the competitive effect of SeMet on Met absorption, an indicator that SeMet uses the same nutrient transporter(s) for absorption, in the various regions of the green sturgeon (Acipenser medirostris) intestine were investigated using intact tissues (a modified everted sleeve method). Intestinal tissue was incubated in Ringer's solution containing 0-10mmolL(-1) Met or SeMet (n=5 for each substrate's concentration and intestinal region), respectively, as well as constant tracer levels of isotope-labeled Met. The data indicate that SeMet uptake was mediated by the same transporter(s) as Met and that the absorption kinetics were similar for both substrates. When there were differences in absorption they appeared to be mostly due to higher permeability (passive uptake) of the tissue for Met than for SeMet, particularly in the pyloric caeca (PC) and distal intestine (DI). Maximum rates of absorption, on the other hand, tended to be higher for SeMet than Met in the mid intestine and DI, whereas differences in affinity for the transporters varied between these tissues but were very similar in the PC. These differences may be due to differences in regional intestinal characteristics such as amount of mucus secreted and degree of tissue contraction, and/or substrate differences regarding solubility in and movement through the mucus, influence on tissue contraction, permeability through membranes or between cells, intracellular metabolism, as well as basolateral transport. Interestingly, an increasing proximal-to-distal gradient for rates of methionine and putative SeMet absorption was observed in green sturgeon which differs from the declining gradient usually observed for substrate absorption in other fish species and mammals.