ABSTRACT
In quinoproteins where the quinone is covalently bound, polymerization is limited and glycine-dependent redox cycling constitutes a sensitive method for detection of the protein-bound quinones. Three aspects of the people research-quinone catalyzed redox cycling in nitro-blue tetrazolium/glycinate, quinoprotein detection, and the characterization of the redox-active pyrroloquinoline quinone (PQQ)-like material in biological fluidsare is discussed. In the case of the glycine-dependent redox cycling of PQQ, a Schiffs base adduct is formed, which can hydrolyze to a reactive quinoneimine, or become stabilized as a redox-inert oxazole. The rate of redox cycling in glycinate-nitro-blue tetrazolium-borate, therefore, measures free PQQ-like material in quinoprotein-free samples. The glycine-dependent redox cycling of PQQ is inhibited by divalent cations because of complex formation that probably occurs with the Schiffs base adduct. The redox-cycling activity, present in other biological samples including plasma, lysates of blood cells, brain homogenates, cerebrospinal fluid, synovial fluid and bile is under further investigation, but it appears likely that it represents free PQQ.
