EFFECT OF A LYSINE-ENRICHED DIET ON L-LYSINE TRANSPORT BY THE BRUSH-BORDER MEMBRANE OF THE CHICKEN JEJUNUM

The influx of L-lysine into apical vesicles from the chicken jejunum o ccurs through two systems, one with low Michaelis constant (K-m) and f eatures of system b(0,+) and the other with relatively high K-m for L- lysine and with properties of system y(+). In the present study the ef fect of a lysine-enriched diet (Lys, containing 68 g L-lysine/kg dieta ry protein, control animals 48 g/kg) on L-lysine uptake through both t ransport systems was investigated. Results show that 1) lysine enrichm ent had no effect on either body weight or the efficiency of food util ization. 2) In Lys-fed animals, the mediated L-lysine influx was best fitted to the two-system model with y(+) and b(0,+) activity. 3) In th e presence of an Na+ gradient, total L-lysine uptake is significantly higher in Lys-fed animals than in control birds (about 40% increase). 4) Lys diet increases K(m)b(0,+) 6-fold (KSCN gradient) and 12-fold (N aSCN gradient) and maximum velocity (V-max) by 6- and 20-fold, respect ively. 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% incre ase). 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 b(0,+)- and y (+)-like transport systems are upregulated by dietary lysine but with different kinetic profiles; the high-capacity y(+)-like carrier shows a V-max increase without changes in K-m, whereas the low-capacity b(0, +)-like system shows an increase in V-max as well as in the K-m.