Studies of L-canavanine incorporation into insectan lysozyme

L-Canavanine is incorporated into the lysozyme synthesized, in response to administration of bacterial cell wall materials, by canavanine-treated larvae of the tobacco hornworm Manduca sexta (Sphingidae). Maximum canavanine incorporation into M. sexta lysozyme occurs when the larvae are provided 1 mg of canavanine g-1 fresh body weight. Analysis of canavanine-containing lysozyme purified from these insects reveals that 21% of the arginine residues are replaced by canavanine; this residue substitution results in a loss of 49.5% of the catalytic activity. When the larvae are provided 0.5 mg of canavanine g-1, 16.5% of the arginine residues are substituted by canavanine and 39.5% of the catalytic activity is lost. Canavanine is also incorporated into the lysozyme induced by canavanine-treated pupae of the giant silk moth Hyalophora cecropia (Saturnidae). In contrast, replacement of 17% of the arginine in H. cecropia lysozyme by canavanine fails to affect the catalytic activity. We have determined the primary structure of M. sexta lysozyme and compared it with the primary structure of H. cecropia lysozyme which has been described elsewhere. M. sexta lysozyme has an arginine at positions 23, 42, and 107. H. cecropia contains serine, lysine, and lysine, respectively, at these locations. The ability of incorporated canavanine to inhibit M. sexta lysozyme activity selectively may result from the fact that replacement of any one of the 3 arginine residues at position 23, 42, or 107 by canavanine causes the loss of catalytic activity.