Microbial activation of two serine enzymes and prophenoloxidase in the plasma fraction of hemolymph of the silkworm, Bombyx mori

Elicitors, such as zymosan and cell wall fractions of Gram-negative and Gram-positive bacteria, initiated the activation of two serine enzymes in silkworm plasma. One of them is an esterase capable of hydrolyzing a synthetic substrate for tryptic activity, N-α-benzoyl-l-arginine ethyl ester and the other is prophenoloxidase activating enzyme (PPAE) which converts prophenoloxidase into the active enzyme. Activities of the esterase and PPAE appeared sequentially during the incubation of plasma with the elicitor and were short-lived after activation. These serine enzymes were insensitive to diisopropylfluorophosphate before activation but became sensitive upon activation, suggesting that they may be present as zymogens in plasma. Both Gram-negative and Gram-positive bacterial cell wall preparations lost almost completely their abilities to initiate the activation of the enzymes after treatment with egg white lysozyme. Peptidoglycan isolated from Gram-positive bacteria initiated the activation of the enzymes in plasma at a concentration of a few ng/ml. These results together with the inability of lipopolysaccharides to activate the serine enzymes, strongly suggest that the active principle of bacterial cell walls is peptidoglycan.