AREGU December 46/6

Soriano-Garcı́a, Juan F., Mònica Torras-Llort, Miquel Moretó, and Ruth Ferrer. Regulation of L-methionine and L-lysine uptake in chicken jejunal brush-border membrane by dietary methionine. Am. J. Physiol. 277 (Regulatory Integrative Comp. Physiol. 46): R1654–R1661, 1999.—In the chicken intestine, L-methionine is transported by systems that are specific for neutral amino acids (Land B-like) and by systems that can also transport cationic amino acids (y1m and b0,1-like). These four uptake pathways have been investigated in brush-border membrane vesicles from the jejunum of chickens fed a diet enriched with 0.4% L-methionine. Methionine supplementation from the 1st to the 6th wk of age has no effect on body weight or on the efficiency of food utilization. The kinetic analysis of L-methionine influx across the transport systems specific for neutral amino acids shows, for system L, no dietary effect on the Michaelis constant (Km) and a 30% reduction in maximal velocity (Vmax); for system B it shows a decrease in Km (30%) and in Vmax (51%). Transport systems shared by cationic and neutral amino acids show no dietary effect on b0,1 activity and a significant reduction in y1m Vmax, similar for L-methionine and L-lysine, both in the absence and in the presence of Na1 (L-methionine, 30 and 26% reduction; L-lysine, 19 and 28% reduction, respectively). The downregulation induced by L-methionine supplementation may be an adaptive response to reduce the risk of intoxication by dietary excess of L-methionine. These results support the view that the toxicity of the supplemented substrate can be an important factor in the regulation of amino acid transport by dietary content.