Properties and inhibition of insect integumental chitin synthetase

Chitin polymerization is catalyzed by cell-free enzyme complexes from the integument of Trichoplusia ni larvae and wing tissue of developing Hyalophora cecropia pupae obtained on extraction of homogenates for 16 hr at 5°C in 25 mM Tris-HCl buffer, pH 7.2, containing 10 mM MgCl₂, 1 mM dithiothreitol, 10 mg/ml bovine serum albumin, and 4 mg/ml digitonin. In contrast, integumental preparations from Boarmia selenaria, Earias insulana, Heliothis virescens, Oncopeltus fasciatus, Spodoptera exigua, and Tribolium castaneum exhibit little or no chitin synthetase (CS) activity. H. cecropia CS requires magnesium ions but not N-acetyl-d-glucosamine for normal activity and is almost insensitive to nikkomycin and polyoxin D. CS activity is not detected in diapausing H. cecropia pupae but synthesis of this enzyme is induced or its activity is stimulated by the molting hormone, ecdysterone, indicating possible hormonal control. T. ni CS requires magnesium ions and N-acetyl-d-glucosamine for optimal activity and is sensitive to inhibition by uridine di- and triphosphates, polyoxins B and D, and particularly nikkomycin. T. ni integumental CS activity decreases in starved larvae or those about to pupate. Both T. ni and H. cecropia CS enzymes as prepared and assayed are sensitive to captan but not to the potent insecticides diflubenzuron and BAY SIR 8514, two effective benzoylphenyl urea in vivo chitin synthesis inhibitors.