Abstract
An ion-dependent tyrosine transporter from Leptinotarsa decemlineata midgut was further characterized using brush border membrane vesicles. Tyrosine transport is not stimulated by Li +, Rb +, Cs +, Ca 2+, or Mg 2+. K + paired with SCN − stimulates tyrosine uptake, but K + paired with less permeant PO 4 2− or gluconate anions does not stimulate tyrosine uptake. KSCN stimulates initial tyrosine uptake in a concentration-dependent manner, reaching a maximum at 20 mM. Tyrosine transport is slower when moving against a positive electrical gradient than against a negative or neutral electrical gradient. In the presence of an inward K + gradient, tyrosine is not concentrated in BBMV above the equilibrium concentration. Tyrosine transport is highest at pH 6.4 and 7.4, and significantly lower at pH 9.4. This ion-dependent tyrosine transporter is insensitive to the amino acid analogs methylamino-isobutyric acid and 2-aminobicyclo-[2,2,11-heptane-2-carboxylic acid. None of eight plant lectins tested inhibited initial tyrosine uptake. The Bacillus thuringiensis Cry3A ∂-endotoxin, active against L. decemlineata in vivo, did not affect initial tyrosine uptake rate.
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