Studies on C3 convertases. Inhibition of C5 convertase formation by peptides containing aromatic amino acids.

The influence of various peptides containing the aromatic amino acids phenylalanine and tyrosine on the formation of the enzyme EAC1423 of the complement system from component C3 and enzyme EAC142 was investigated. Kinetic analysis of enzyme EAC1423 formation and studies on the binding of the C3b fragment of 125I-labelled component C3 to enzyme EAC142 both showed that binding of the C3b fragment of component C3 was decreased by the peptides. Kinetic studies on component-C3 turnover in the fluid phase of enzyme EAC142 failed to reveal effects of the peptides. However, an initial lag in component-C3 turnover occurred that at constant component-C3 concentration was inversely proportional to enzyme EAC142 concentration. This lag in enzyme EAC142 activity is considered as an indication that the interaction of enzyme EAC142 with component C3 possibly does not follow simple Michaelis-Menten kinetics, as was previously assumed. It is shown that the stages after enzyme EAC1423 formation are not influenced by the peptides, suggesting a high degree of specificity of the peptides for the inhibition of enzyme EAC1423 formation.