Platelet activation by 2-(4-bromo-2,3-dioxobutylthio)adenosine 5'-diphosphate is mediated by its binding to a putative ADP receptor, aggregin

A functional mobA gene for molybdopterin cytosine dinucleotide cofactor biosynthesis is required for activity and holoenzyme assembly of the heterotrimeric nicotine dehydrogenases of Arthrobacter nicotinovorans

Two Arthrobacter nicotinovorans molybdenum enzymes hydroxylate the pyridine ring of nicotine. Molybdopterin cytosine dinucleotide (MCD) was determined to be a cofactor of these enzymes. A mobA gene responsible for the formation of MCD could be identified and its function shown to be required for assembly of the heterotrimeric molybdenum enzymes.

ADP-induced platelet activation

Platelet activation is central to the pathogenesis of hemostasis and arterial thrombosis. Platelet aggregation plays a major role in acute coronary artery diseases, myocardial infarction, unstable angina, and stroke. ADP is the first known and an important agonist for platelet aggregation. ADP not only causes primary aggregation of platelets but is also responsible for the secondary aggregation induced by ADP and other agonists. ADP also induces platelet shape change, secretion from storage granules, influx and intracellular mobilization of Ca2+, and inhibition of stimulated adenylyl cyclase activity. The ADP-receptor protein mediating ADP-induced platelet responses has neither been purified nor cloned. Therefore, signal transduction mechanisms underlying ADP-induced platelet responses either remain uncertain or less well understood. Recent contributions from chemists, biochemists, cell biologists, pharmacologists, molecular biologists, and clinical investigators have added considerably to and enhanced our knowledge of ADP-induced platelet responses. Although considerable efforts have been directed toward identifying and cloning the ADP-receptor, these have not been completely successful or without controversy. Considerable progress has been made toward understanding the mechanisms of ADP-induced platelet responses but disagreements persist. New drugs that do not mimic ADP have been found to inhibit fairly selectively ADP-induced platelet activation ex vivo. Drugs that mimic ADP and selectively act at the platelet ADP-receptor have been designed, synthesized, and evaluated for their therapeutic efficacy to block selectively ADP-induced platelet responses. This review examines in detail the developments that have taken place to identify the ADP-receptor protein and to better understand mechanisms underlying ADP-induced platelet responses to develop strategies for designing innovative drugs that block ADP-induced platelet responses by acting selectively at the ADP-receptor and/or by selectively interfering with components of ADP-induced platelet activation mechanisms.

Immunoaffinity method to identify aggregin, a putative ADP-receptor in human blood platelets

The ADP-receptor on the surface of human platelets and cells of megakaryocytic lineage has been classified as P2T purinergic receptor for which ADP is an agonist and ATP is an antagonist. Although it is one of the earliest identified of the important cellular receptors, it has neither been purified nor cloned. We have developed an immunoaffinity method for rapidly identifying the platelet ADP-receptor and this method can be extended to the purification of the receptor. A polyclonal antibody to glutamate dehydrogenase (GDH) covalently modified by 5'-p-fluorosulfonylbenzoyladenosine (FSBA) recognized neither FSBA nor glutamate dehydrogenase. Immunoblot of the gel obtained by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of solubilized FSBA-labeled platelets showed the presence of a protein band at 100 kDa and this band was absent in the immunoblots of platelets that were preincubated with ADP and ATP or covalently modified by the chemically reactive ADP-affinity analogs, 2- and 8-(4-bromo-2,3-dioxobutylthio)adenosine-5'-diphosphate (2- and 8BDB-TADP) and 2-(3-bromo-2-oxopropylthio)adenosine-5'-diphosphate (2-BOP-TADP), prior to treatment with FSBA. FSBA as well as 2- and 8-BDB-TADP and 2-BOP-TADP have been previously shown to inhibit ADP-induced platelet responses by selectively and covalently modifying aggregin (100 kDa), an ADP-receptor in intact human blood platelets. The results show that polyclonal antibody to FSBA-labeled GDH is capable of recognizing FSBA-labeled aggregin on platelets and, thus, could be used to purify aggregin by immunoaffinity column chromatography. The immunoaffinity method was found to be far more sensitive than the radiochemical methods to identify aggregin previously developed in our laboratory. Since FSBA is also capable of reacting with enzymes that require ATP for their catalytic function, the polyclonal antibody may be used to identify and purify other P2-type purinergic receptors that require binding of ATP before eliciting cellular responses.