Abstract
The brush-border membrane vesicles were isolated from the midgut epithelial cells of the last instar cabbage armyworm larvae (Mamestra brassicae). It was observed that 14C-UDP-N-acetylglucosamine (UDP-AGA) bound fast to the vesicles at the optimum pH of 7.0. Almost 90% of the binding was inhibited by polyoxin D. Chitin was synthesized by the vesicles in vitro in the presence of chitodextrin as a primer. These facts suggested that much of the binding was based on the 14C-UDP-AGA-chitin synthetase binding, whose activity was stronger at 20°C than 25°C. On the other hand, the chitin synthesis by the vesicles progressed much faster at higher temperatures, indicating that the turnover of the binding would be faster at higher temperatures. The polyoxin D-treated vesicles retained the binding activity at the optimum pH 7.5, although UDP-ACA's binding activity became weak about one tenth of the original. The results led to a conclusion that the brush-border membrane vesicles had two substances bound to UDP-AGA, one would be chitin synthetase and the other a UDP-AGA transporter. However, the amount of UDP-AGA bound to the polyoxin D-treated vesicles was too small to conclude that the binding was based on the UDP-AGA transporter.
