S-guanylation of human serum albumin is a unique posttranslational modification and results in a novel class of antibacterial agents.

8-Nitroguanosine 3',5'-cyclic monophosphate (8-nitro-cGMP) is a nitric oxide metabolite and an important second messenger. 8-Nitro-cGMP reacts with sulfhydryl groups forming a novel posttranslational modification, namely, S-guanylation. In this work, we found, by using a quantitative competition enzyme-linked immunosorbent assay procedure, that S-guanylated human serum albumin (S-cGMP-HSA) is a component of normal plasma, and that hemodialysis patients decrease its concentration, on an average, from 68 to 34 nM. End-stage renal disease is often accompanied by septicemia, and we found that S-cGMP-HSA possesses an in vitro antibacterial effect with half maximal inhibitory concentration of approximately 2 μM against Escherichia coli American Type Culture Collection. Our findings indicate that S-cGMP-HSA can be regarded as an endogenous antibacterial agent in healthy conditions and as a useful new class of antibacterial agents with a circulation time sufficient for in vivo biological activity. The clinical development of S-cGMP-HSA as a safe and strong antibacterial agent arisen from endogenous posttranslational modification would be expected.