Effect of a lysine-enriched diet onl-lysine transport by the brush-border membrane of the chicken jejunum.

The influx ofl-lysine into apical vesicles from the chicken jejunum occurs through two systems, one with low Michaelis constant ( K m) and features of system b0,+ and the other with relatively high K m forl-lysine and with properties of system y+. In the present study the effect of a lysine-enriched diet (Lys, containing 68 g l-lysine/kg dietary protein, control animals 48 g/kg) onl-lysine uptake through both transport systems was investigated. Results show that 1) lysine enrichment had no effect on either body weight or the efficiency of food utilization. 2) In Lys-fed animals, the mediatedl-lysine influx was best fitted to the two-system model with y+and b0,+ activity. 3) In the presence of an Na+ gradient, totall-lysine uptake is significantly higher in Lys-fed animals than in control birds (about 40% increase). 4) Lys diet increases K mb0,+6-fold (KSCN gradient) and 12-fold (NaSCN gradient) and maximum velocity ( V max) by 6- and 20-fold, respectively. The effects of Lys enrichment on the y+-like system are only observed on the V max and in the presence of a Na+ gradient (30% increase). 5) Na+ is involved in the activation of the transport process in the Lys-fed chickens, but there is no correlation between external Na+concentration and l-lysine influx. In conclusion, both b0,+- and y+-like transport systems are upregulated by dietary lysine but with different kinetic profiles; the high-capacity y+-like carrier shows a V maxincrease without changes in K m, whereas the low-capacity b0,+-like system shows an increase in V max as well as in the K m.