Inhibition of the Third Component of the Complement System by Derivatives of Aromatic Amino Acids
Authors: Basch Ross S. 
Published: January 2023

Summary
It is clear that a wide variety of aromatic organic compounds, including the derivatives of the aromatic amino acids, are capable of inhibiting the cation independent phase of complement action. The activity of the compounds tested was not completely restricted to a single step. Both the formation of the IA reactive cell and its subsequent stabilization were affected. However, the kinetic flow data, the static inhibition tests and the similarity of the structural requirements for inhibition of the specific step with the requirements for the inhibition of the whole system all lead to the conclusion that the primary site of action is at the formation of the thermostable intermediate.
The mechanism of this inhibition is less clear. The pattern of inhibition produced by the various analogs suggests the inhibition of a proteolytic enzyme. The step inhibited is extremely sensitive to changes in temperature. This also suggests an enzymatic mechanism. The kinetic data presented in Figures 7 and 8, although not providing direct proof, are compatible with the idea that a component in the heated guinea pig serum is a substrate in an enzymatic reaction. Furthermore, the data obtained in the presence of acetyl-L-tyrosine indicate that this compound, and presumably the other aromatic ring containing compounds, are acting as competitive inhibitors. The simplest explanation for these observations is that some component present in the EAC′1,4,2,3c complex is an enzyme, presumably proteolytic in nature, which acts upon a substrate present in heated guinea pig serum and which is competitively inhibited by aromatic ring compounds.
Conclusive identification of either the presumptive enzyme or its putative substrate is not yet possible. The inhibition of immune adherence by the aromatic amino acid derivatives make it clear that these compounds can and do react with C′3c. Furthermore the rank order of these compounds in inhibiting IA is similar to their rank order as inhibitors of whole C′3. If C′3c alone is required for IA, then these data suggest that it is also the enzyme required for the formation of the thermostable intermediate.
When this work was begun, the complexity of the reaction leading to the formation of the thermostable intermediate was not fully appreciated. The recent work of Nilsson and Muller-Eberhard (10) and Nelson (as reported by Linscott (11)) has made it clear that several components are required to complete this step in addition to C′3c and C′3b. The description of the heated guinea pig serum as C′3b was made for purposes of convenience rather than as a declaration of conviction. Where the newly discovered components interact in the experiments described here is unknown.
Despite all of these uncertainties of interpretation, the data presented here clearly show that a group of aromatic amino acid derivatives react with the component responsible for immune adherence and also inhibit the formation of the thermostable intermediate found during the action of whole C′3. This report does not unequivocally demonstrate the presence of an enzymatic step in the production of these cells, nor does it delineate the mechanism of the reaction. It does suggest that an enzymatic step exists; it further suggests what components are involved and where further study would seem most profitable.